What is .NET?
Answer: .NET is a general-purpose software development platform, similar to Java. At its core is a virtual machine that turns intermediate language (IL) into machine code. High-level language compilers for C#, VB.NET and C++ are provided to turn source code into IL. C# is a new programming language, very similar to Java. An extensive class library is included, featuring all the functionality one might expect from a contempory development platform - windows GUI development (Windows Forms), database access (ADO.NET), web development (ASP.NET), web services, XML etc.
When was .NET announced?
Answer: Bill Gates delivered a keynote at Forum 2000, held June 22, 2000, outlining the .NET 'vision'. The July 2000 PDC had a number of sessions on .NET technology, and delegates were given CDs containing a pre-release version of the .NET framework/SDK and Visual Studio.NET.
What versions of .NET are there?
Answer: The final versions of the 1.0 SDK and runtime were made publicly available around 6pm PST on 15-Jan-2002. At the same time, the final version of Visual Studio.NET was made available to MSDN subscribers.
.NET 1.1 was released in April 2003, and was mostly bug fixes for 1.0.
.NET 2.0 was released to MSDN subscribers in late October 2005, and was officially launched in early November.
What operating systems does the .NET Framework run on?
Answer: The runtime supports Windows Server 2003, Windows XP, Windows 2000, NT4 SP6a and Windows ME/98. Windows 95 is not supported. Some parts of the framework do not work on all platforms - for example, ASP.NET is only supported on XP and Windows 2000/2003. Windows 98/ME cannot be used for development.
IIS is not supported on Windows XP Home Edition, and so cannot be used to host ASP.NET. However, the ASP.NET Web Matrix web server does run on XP Home.
The .NET Compact Framework is a version of the .NET Framework for mobile devices, running Windows CE or Windows Mobile.
The MONO project has a version of the .NET Framework that runs on Linux.
What tools can we use to develop .NET applications?
Answer: There are a number of tools, described here in ascending order of cost:
· The .NET Framework SDK free and includes command-line compilers for C++, C#, and VB.NET and various other utilities to aid development.
· Sharp Develop is a free IDE for C# and VB.NET.
· Microsoft Visual Studio Express editions are cut-down versions of Visual Studio, for hobbyist or novice developers. There are different versions for C#, VB, web development etc. Originally the plan was to charge $49, but MS has decided to offer them as free downloads instead, at least until November 2006.
· Microsoft Visual Studio Standard 2005 is around $300, or $200 for the upgrade.
· Microsoft Visual Studio Professional 2005 is around $800, or $550 for the upgrade.
· At the top end of the price range are the Microsoft Visual Studio Team Edition for Software Developers 2005 with MSDN Premium and Team Suite editions.
What is the CLI? Is it the same as the CLR?
Answer: The CLI (Common Language Infrastructure) is the definition of the fundamentals of the .NET framework - the Common Type System (CTS), metadata, the Virtual Execution Environment (VES) and its use of intermediate language (IL), and the support of multiple programming languages via the Common Language Specification (CLS). The CLI is documented through ECMA .
The CLR (Common Language Runtime) is Microsoft's primary implementation of the CLI. Microsoft also have a shared source implementation known as ROTOR , for educational purposes, as well as the .NET Compact Framework for mobile devices. Non-Microsoft CLI implementations include Mono and DotGNU Portable.NET .
What is the CTS, and how does it relate to the CLS?
Answer: CTS = Common Type System. This is the full range of types that the .NET runtime understands. Not all .NET languages support all the types in the CTS.
CLS = Common Language Specification. This is a subset of the CTS which all .NET languages are expected to support. The idea is that any program which uses CLS-compliant types can interoperate with any .NET program written in any language. This interop is very fine-grained - for example a VB.NET class can inherit from a C# class.
What is IL?
Answer: IL = Intermediate Language. Also known as MSIL (Microsoft Intermediate Language) or CIL (Common Intermediate Language). All .NET source code (of any language) is compiled to IL during development. The IL is then converted to machine code at the point where the software is installed, or (more commonly) at run-time by a Just-In-Time (JIT) compiler.
What is C#?
Answer: C# is a new language designed by Microsoft to work with the .NET framework. In their "Introduction to C#" whitepaper, Microsoft describe C# as follows:
"C# is a simple, modern, object oriented, and type-safe programming language derived from C and C++. C# (pronounced “C sharp”) is firmly planted in the C and C++ family tree of languages, and will immediately be familiar to C and C++ programmers. C# aims to combine the high productivity of Visual Basic and the raw power of C++."
Compare C#,C++ and Java
Answer: C#'s most important features compared with those of C++ and Java
| Feature | C# | C++ | Java |
| Inheritance | Single class inheritance, multiple interface implementation | Multiple class inheritance | Single class inheritance, multiple interface implementation |
| The notion of interface | Through the "interface" keyword | Through abstract class | Through the "interface" keyword |
| Memory management | Managed, using a garbage collector | Manual | Managed, using a garbage collector |
| Pointers | Yes, but only in the rarely-used unsafe mode. References are used, instead. | Yes, a very commonly used feature. | Not at all. References are used, instead. |
| Form of Compiled Source Code | .NET intermediate language (IL) | Executables. | Byte code. |
| One common base class | Yes | No | Yes |
What does 'managed' mean in the .NET context?
Answer: The term 'managed' is the cause of much confusion. It is used in various places within .NET, meaning slightly different things.
Managed code: The .NET framework provides several core run-time services to the programs that run within it - for example exception handling and security. For these services to work, the code must provide a minimum level of information to the runtime. Such code is called managed code.
Managed data: This is data that is allocated and freed by the .NET runtime's garbage collector.
Managed classes: This is usually referred to in the context of Managed Extensions (ME) for C++. When using ME C++, a class can be marked with the __gc keyword. As the name suggests, this means that the memory for instances of the class is managed by the garbage collector, but it also means more than that. The class becomes a fully paid-up member of the .NET community with the benefits and restrictions that brings. An example of a benefit is proper interop with classes written in other languages - for example, a managed C++ class can inherit from a VB class. An example of a restriction is that a managed class can only inherit from one base class.
What is reflection?
Answer: All .NET compilers produce metadata about the types defined in the modules they produce. This metadata is packaged along with the module (modules in turn are packaged together in assemblies), and can be accessed by a mechanism called reflection. The System.Reflection namespace contains classes that can be used to interrogate the types for a module/assembly.
Using reflection to access .NET metadata is very similar to using ITypeLib/ITypeInfo to access type library data in COM, and it is used for similar purposes - e.g. determining data type sizes for marshaling data across context/process/machine boundaries.
Reflection can also be used to dynamically invoke methods, or even create types dynamically at run-time .
What is an assembly?
Answer: An assembly is sometimes described as a logical .EXE or .DLL, and can be an application (with a main entry point) or a library. An assembly consists of one or more files (dlls, exes, html files etc), and represents a group of resources, type definitions, and implementations of those types. An assembly may also contain references to other assemblies. These resources, types and references are described in a block of data called a manifest. The manifest is part of the assembly, thus making the assembly self-describing.
An important aspect of assemblies is that they are part of the identity of a type. The identity of a type is the assembly that houses it combined with the type name. This means, for example, that if assembly A exports a type called T, and assembly B exports a type called T, the .NET runtime sees these as two completely different types. Furthermore, don't get confused between assemblies and namespaces - namespaces are merely a hierarchical way of organising type names. To the runtime, type names are type names, regardless of whether namespaces are used to organise the names. It's the assembly plus the typename (regardless of whether the type name belongs to a namespace) that uniquely indentifies a type to the runtime.
Assemblies are also important in .NET with respect to security - many of the security restrictions are enforced at the assembly boundary.
Assemblies are the unit of versioning in .NET .
How can I produce an assembly?
Answer: The simplest way to produce an assembly is directly from a .NET compiler. For example, the following C# program:
public class SKP
{
public SKP() { System.Console.WriteLine( "Hello from SKP" ); }
}
can be compiled into a library assembly (dll) like this:
csc /t:library skp.cs
You can then view the contents of the assembly by running the "IL Disassembler" tool that comes with the .NET SDK.
Alternatively you can compile your source into modules, and then combine the modules into an assembly using the assembly linker (al.exe). For the C# compiler, the /target:module switch is used to generate a module instead of an assembly.
What is the difference between a private assembly and a shared assembly?
Answer:
· Location and visibility: A private assembly is normally used by a single application, and is stored in the application's directory, or a sub-directory beneath. A shared assembly is normally stored in the global assembly cache, which is a repository of assemblies maintained by the .NET runtime. Shared assemblies are usually libraries of code which many applications will find useful, e.g. the .NET framework classes.
· Versioning: The runtime enforces versioning constraints only on shared assemblies, not on private assemblies.
How do assemblies find each other?
Answer: By searching directory paths. There are several factors which can affect the path (such as the AppDomain host, and application configuration files), but for private assemblies the search path is normally the application's directory and its sub-directories. For shared assemblies, the search path is normally same as the private assembly path plus the shared assembly cache.
How does assembly versioning work?
Each assembly has a version number called the compatibility version. Also each reference to an assembly (from another assembly) includes both the name and version of the referenced assembly.
The version number has four numeric parts . Assemblies with either of the first two parts different are normally viewed as incompatible. If the first two parts are the same, but the third is different, the assemblies are deemed as 'maybe compatible'. If only the fourth part is different, the assemblies are deemed compatible. However, this is just the default guideline - it is the version policy that decides to what extent these rules are enforced. The version policy can be specified via the application configuration file.
Remember: versioning is only applied to shared assemblies, not private assemblies.
How can I develop an application that automatically updates itself from the web?
Answer: For .NET 1.x, use the Updater Application Block . For .NET 2.x, use ClickOnce.
What is an application domain?
Answer: An AppDomain can be thought of as a lightweight process. Multiple AppDomains can exist inside a Win32 process. The primary purpose of the AppDomain is to isolate applications from each other, and so it is particularly useful in hosting scenarios such as ASP.NET. An AppDomain can be destroyed by the host without affecting other AppDomains in the process.
Win32 processes provide isolation by having distinct memory address spaces. This is effective, but expensive. The .NET runtime enforces AppDomain isolation by keeping control over the use of memory - all memory in the AppDomain is managed by the .NET runtime, so the runtime can ensure that AppDomains do not access each other's memory.
One non-obvious use of AppDomains is for unloading types. Currently the only way to unload a .NET type is to destroy the AppDomain it is loaded into. This is particularly useful if you create and destroy types on-the-fly via reflection.
How does an AppDomain get created?
Answer: AppDomains are usually created by hosts. Examples of hosts are the Windows Shell, ASP.NET and IE. When you run a .NET application from the command-line, the host is the Shell. The Shell creates a new AppDomain for every application.
AppDomains can also be explicitly created by .NET applications.
Can we write my own .NET host?
Answer: Yes.
What is garbage collection?
Answer: Garbage collection is a heap-management strategy where a run-time component takes responsibility for managing the lifetime of the memory used by objects. This concept is not new to .NET - Java and many other languages/runtimes have used garbage collection for some time.
Is it true that objects don't always get destroyed immediately when the last reference goes away?
Answer: Yes. The garbage collector offers no guarantees about the time when an object will be destroyed and its memory reclaimed.
Why doesn't the .NET runtime offer deterministic destruction?
Answer: Because of the garbage collection algorithm. The .NET garbage collector works by periodically running through a list of all the objects that are currently being referenced by an application. All the objects that it doesn't find during this search are ready to be destroyed and the memory reclaimed. The implication of this algorithm is that the runtime doesn't get notified immediately when the final reference on an object goes away - it only finds out during the next 'sweep' of the heap.
Furthermore, this type of algorithm works best by performing the garbage collection sweep as rarely as possible. Normally heap exhaustion is the trigger for a collection sweep.
Is the lack of deterministic destruction in .NET a problem?
Answer: It's certainly an issue that affects component design. If you have objects that maintain expensive or scarce resources (e.g. database locks), you need to provide some way to tell the object to release the resource when it is done. Microsoft recommend that you provide a method called Dispose() for this purpose. However, this causes problems for distributed objects - in a distributed system who calls the Dispose() method? Some form of reference-counting or ownership-management mechanism is needed to handle distributed objects - unfortunately the runtime offers no help with this.
Should I implement Finalize on my class? Should I implement IDisposable?
Answer: This issue is a little more complex than it first appears. There are really two categories of class that require deterministic destruction - the first category manipulate unmanaged types directly, whereas the second category manipulate managed types that require deterministic destruction. An example of the first category is a class with an IntPtr member representing an OS file handle. An example of the second category is a class with a System.IO.FileStream member.
For the first category, it makes sense to implement IDisposable and override Finalize. This allows the object user to 'do the right thing' by calling Dispose, but also provides a fallback of freeing the unmanaged resource in the Finalizer, should the calling code fail in its duty. However this logic does not apply to the second category of class, with only managed resources. In this case implementing Finalize is pointless, as managed member objects cannot be accessed in the Finalizer. This is because there is no guarantee about the ordering of Finalizer execution. So only the Dispose method should be implemented. (If you think about it, it doesn't really make sense to call Dispose on member objects from a Finalizer anyway, as the member object's Finalizer will do the required cleanup.)
Note that some developers argue that implementing a Finalizer is always a bad idea, as it hides a bug in your code (i.e. the lack of a Dispose call). A less radical approach is to implement Finalize but include a Debug.Assert at the start, thus signalling the problem in developer builds but allowing the cleanup to occur in release builds.
Do We have any control over the garbage collection algorithm?
Answer: A little. For example the System.GC class exposes a Collect method, which forces the garbage collector to collect all unreferenced objects immediately.
Also there is a gcConcurrent setting that can be specified via the application configuration file. This specifies whether or not the garbage collector performs some of its collection activities on a separate thread. The setting only applies on multi-processor machines, and defaults to true.
How can we find out what the garbage collector is doing?
Answer: Lots of interesting statistics are exported from the .NET runtime via the '.NET CLR ' performance counters. Use Performance Monitor to view them.
What is the lapsed listener problem?
Answer: The lapsed listener problem is one of the primary causes of leaks in .NET applications. It occurs when a subscriber (or 'listener') signs up for a publisher's event, but fails to unsubscribe. The failure to unsubscribe means that the publisher maintains a reference to the subscriber as long as the publisher is alive. For some publishers, this may be the duration of the application.
This situation causes two problems. The obvious problem is the leakage of the subscriber object. The other problem is the performance degradation due to the publisher sending redundant notifications to 'zombie' subscribers.
There are at least a couple of solutions to the problem. The simplest is to make sure the subscriber is unsubscribed from the publisher, typically by adding an Unsubscribe() method to the subscriber.
What is serialization?
Answer: Serialization is the process of converting an object into a stream of bytes. Deserialization is the opposite process, i.e. creating an object from a stream of bytes. Serialization/Deserialization is mostly used to transport objects (e.g. during remoting), or to persist objects (e.g. to a file or database).
Does the .NET Framework have in-built support for serialization?
Answer: There are two separate mechanisms provided by the .NET class library - XmlSerializer and SoapFormatter/BinaryFormatter. Microsoft uses XmlSerializer for Web Services, and SoapFormatter/BinaryFormatter for remoting. Both are available for use in your own code.
Suppose we want to serialize instances of my class. Should we use XmlSerializer, SoapFormatter or BinaryFormatter?
Answer: It depends. XmlSerializer has severe limitations such as the requirement that the target class has a parameterless constructor, and only public read/write properties and fields can be serialized. However, on the plus side, XmlSerializer has good support for customising the XML document that is produced or consumed. XmlSerializer's features mean that it is most suitable for cross-platform work, or for constructing objects from existing XML documents.
SoapFormatter and BinaryFormatter have fewer limitations than XmlSerializer. They can serialize private fields, for example. However they both require that the target class be marked with the [Serializable] attribute, so like XmlSerializer the class needs to be written with serialization in mind. Also there are some quirks to watch out for - for example on deserialization the constructor of the new object is not invoked.
The choice between SoapFormatter and BinaryFormatter depends on the application. BinaryFormatter makes sense where both serialization and deserialization will be performed on the .NET platform and where performance is important. SoapFormatter generally makes more sense in all other cases, for ease of debugging if nothing else.
Can we customize the serialization process?
Answer: Yes. XmlSerializer supports a range of attributes that can be used to configure serialization for a particular class. For example, a field or property can be marked with the [XmlIgnore] attribute to exclude it from serialization. Another example is the [XmlElement] attribute, which can be used to specify the XML element name to be used for a particular property or field.
Serialization via SoapFormatter/BinaryFormatter can also be controlled to some extent by attributes. For example, the [NonSerialized] attribute is the equivalent of XmlSerializer's [XmlIgnore] attribute. Ultimate control of the serialization process can be acheived by implementing the the ISerializable interface on the class whose instances are to be serialized.
Why is XmlSerializer so slow?
Answer: There is a once-per-process-per-type overhead with XmlSerializer. So the first time you serialize or deserialize an object of a given type in an application, there is a significant delay. This normally doesn't matter, but it may mean, for example, that XmlSerializer is a poor choice for loading configuration settings during startup of a GUI application.
Why do we get errors when we try to serialize a Hashtable?
Answer: XmlSerializer will refuse to serialize instances of any class that implements IDictionary, e.g. Hashtable. SoapFormatter and BinaryFormatter do not have this restriction.
XmlSerializer is throwing a generic "There was an error reflecting MyClass" error. How do we find out what the problem is?
Answer: Look at the InnerException property of the exception that is thrown to get a more specific error message.
What are attributes?
Answer: There are at least two types of .NET attribute. The first type refer to as a metadata attribute - it allows some data to be attached to a class or method. This data becomes part of the metadata for the class, and (like other class metadata) can be accessed via reflection. An example of a metadata attribute is [serializable], which can be attached to a class and means that instances of the class can be serialized.
[serializable] public class SKP{}
The other type of attribute is a context attribute. Context attributes use a similar syntax to metadata attributes but they are fundamentally different. Context attributes provide an interception mechanism whereby instance activation and method calls can be pre- and/or post-processed.
Can we create my own metadata attributes?
Answer: Yes. Simply derive a class from System.Attribute and mark it with the AttributeUsage attribute.
What is Code Access Security (CAS)?
Answer: CAS is the part of the .NET security model that determines whether or not code is allowed to run, and what resources it can use when it is running. For example, it is CAS that will prevent a .NET web applet from formatting your hard disk.
How does CAS work?
Answer: The CAS security policy revolves around two key concepts - code groups and permissions. Each .NET assembly is a member of a particular code group, and each code group is granted the permissions specified in a named permission set.
For example, using the default security policy, a control downloaded from a web site belongs to the 'Zone - Internet' code group, which adheres to the permissions defined by the 'Internet' named permission set. (Naturally the 'Internet' named permission set represents a very restrictive range of permissions.)
Who defines the CAS code groups?
Answer: Microsoft defines some default ones, but you can modify these and even create your own. To see the code groups defined on your system, run 'caspol -lg' from the command-line.
Can we look at the IL for an assembly?
Answer: Yes. MS supply a tool called Ildasm that can be used to view the metadata and IL for an assembly.
Can source code be reverse-engineered from IL?
Answer: Yes, it is often relatively straightforward to regenerate high-level source from IL.
How can we stop our code being reverse-engineered from IL?
Answer: We can buy an IL obfuscation tool. These tools work by 'optimising' the IL in such a way that reverse-engineering becomes much more difficult.
Of course if you are writing web services then reverse-engineering is not a problem as clients do not have access to your IL.
Can we do things in IL that we can't do in C#?
Answer: Yes. A couple of simple examples are that you can throw exceptions that are not derived from System.Exception, and you can have non-zero-based arrays.
Does .NET replace COM?
Answer: This subject causes a lot of controversy , .NET has its own mechanisms for type interaction, and they don't use COM. No IUnknown, no IDL, no typelibs, no registry-based activation. This is mostly good, as a lot of COM was ugly. Generally speaking, .NET allows us to package and use components in a similar way to COM, but makes the whole thing a bit easier.
Is DCOM dead?
Answer: Pretty much, for .NET developers. The .NET Framework has a new remoting model which is not based on DCOM. DCOM was pretty much dead anyway, once firewalls became widespread and Microsoft got SOAP fever. Of course DCOM will still be used in interop scenarios.
Is COM+ dead?
Answer: Not immediately. The approach for .NET 1.0 was to provide access to the existing COM+ services (through an interop layer) rather than replace the services with native .NET ones. Various tools and attributes were provided to make this as painless as possible. Over time it is expected that interop will become more seamless - this may mean that some services become a core part of the CLR, and/or it may mean that some services will be rewritten as managed code which runs on top of the CLR.
Can we use COM components from .NET programs?
Answer: Yes. COM components are accessed from the .NET runtime via a Runtime Callable Wrapper (RCW). This wrapper turns the COM interfaces exposed by the COM component into .NET-compatible interfaces. For oleautomation interfaces, the RCW can be generated automatically from a type library. For non-oleautomation interfaces, it may be necessary to develop a custom RCW which manually maps the types exposed by the COM interface to .NET-compatible types.
Can we use .NET components from COM programs?
Answer: Yes. .NET components are accessed from COM via a COM Callable Wrapper (CCW). This is similar to a RCW (see previous question), but works in the opposite direction. Again, if the wrapper cannot be automatically generated by the .NET development tools, or if the automatic behaviour is not desirable, a custom CCW can be developed. Also, for COM to 'see' the .NET component, the .NET component must be registered in the registry.
Is ATL redundant in the .NET world?
Answer: Yes. ATL will continue to be valuable for writing COM components for some time, but it has no place in the .NET world.
How do I spawn a thread?
Answer: Create an instance of a System.Threading.Thread object, passing it an instance of a ThreadStart delegate that will be executed on the new thread. For example:
class MyThread
{
public MyThread( string initData )
{
m_data = initData;
m_thread = new Thread( new ThreadStart(ThreadMain) );
m_thread.Start();
}
// ThreadMain() is executed on the new thread.
private void ThreadMain()
{
Console.WriteLine( m_data );
}
public void WaitUntilFinished()
{
m_thread.Join();
}
private Thread m_thread;
private string m_data;
}
In this case creating an instance of the MyThread class is sufficient to spawn the thread and execute the MyThread.ThreadMain() method:
MyThread t = new MyThread( "Hello, world." );
t.WaitUntilFinished();
How do I stop a thread?
Answer: There are several options. First, we can use our own communication mechanism to tell the ThreadStart method to finish. Alternatively the Thread class has in-built support for instructing the thread to stop. The two principle methods are Thread.Interrupt() and Thread.Abort(). The former will cause a ThreadInterruptedException to be thrown on the thread when it next goes into a WaitJoinSleep state. In other words, Thread.Interrupt is a polite way of asking the thread to stop when it is no longer doing any useful work. In contrast, Thread.Abort() throws a ThreadAbortException regardless of what the thread is doing. Furthermore, the ThreadAbortException cannot normally be caught (though the ThreadStart's finally method will be executed). Thread.Abort() is a heavy-handed mechanism which should not normally be required.
How do we use the thread pool?
Answer: By passing an instance of a WaitCallback delegate to the ThreadPool.QueueUserWorkItem() method
class CApp
{
static void Main()
{
string s = "Hello, World";
ThreadPool.QueueUserWorkItem( new WaitCallback( DoWork ), s );
Thread.Sleep( 1000 ); // Give time for work item to be executed
}
// DoWork is executed on a thread from the thread pool.
static void DoWork( object state )
{
Console.WriteLine( state );
}
}
How do we know when our thread pool work item has completed?
Answer: There is no way to query the thread pool for this information. We must put code into the WaitCallback method to signal that it has completed. Events are useful for this.
How do we prevent concurrent access to our data?
Answer: Each object has a concurrency lock (critical section) associated with it. The System.Threading.Monitor.Enter/Exit methods are used to acquire and release this lock. For example, instances of the following class only allow one thread at a time to enter method f():
class C
{
public void f()
{
try
{
Monitor.Enter(this);
...
}
finally
{
Monitor.Exit(this);
}
}
}
C# has a 'lock' keyword which provides a convenient shorthand for the code above:
class C
{
public void f()
{
lock(this)
{
...
}
}
}
Note that calling Monitor.Enter(myObject) does NOT mean that all access to myObject is serialized. It means that the synchronisation lock associated with myObject has been acquired, and no other thread can acquire that lock until Monitor.Exit(o) is called. In other words, this class is functionally equivalent to the classes above:
class C
{
public void f()
{
lock( m_object )
{
...
}
}
private m_object = new object();
}
Actually, it could be argued that this version of the code is superior, as the lock is totally encapsulated within the class, and not accessible to the user of the object.
Should we use ReaderWriterLock instead of Monitor.Enter/Exit?
Answer: Maybe, but be careful. ReaderWriterLock is used to allow multiple threads to read from a data source, while still granting exclusive access to a single writer thread. This makes sense for data access that is mostly read-only, but there are some caveats. First, ReaderWriterLock is relatively poor performing compared to Monitor.Enter/Exit, which offsets some of the benefits. Second, we need to be very sure that the data structures you are accessing fully support multithreaded read access. Finally, there is apparently a bug in the v1.1 ReaderWriterLock that can cause starvation for writers when there are a large number of readers.
Is there built-in support for tracing/logging?
Answer: Yes, in the System.Diagnostics namespace. There are two main classes that deal with tracing - Debug and Trace. They both work in a similar way - the difference is that tracing from the Debug class only works in builds that have the DEBUG symbol defined, whereas tracing from the Trace class only works in builds that have the TRACE symbol defined. Typically this means that you should use System.Diagnostics.Trace.WriteLine for tracing that you want to work in debug and release builds, and System.Diagnostics.Debug.WriteLine for tracing that you want to work only in debug builds.
Can we redirect tracing to a file?
Answer: Yes. The Debug and Trace classes both have a Listeners property, which is a collection of sinks that receive the tracing that you send via Debug.WriteLine and Trace.WriteLine respectively. By default the Listeners collection contains a single sink, which is an instance of the DefaultTraceListener class. This sends output to the Win32 OutputDebugString() function and also the System.Diagnostics.Debugger.Log() method. This is useful when debugging, but if we're trying to trace a problem at a customer site, redirecting the output to a file is more appropriate. Fortunately, the TextWriterTraceListener class is provided for this purpose.
Note the use of Trace.Listeners.Clear() to remove the default listener. If we don't do this, the output will go to the file and OutputDebugString(). Typically this is not what we want, because OutputDebugString() imposes a big performance hit.
Can we customise the trace output?
Answer: Yes. We can write our own TraceListener-derived class, and direct all output through it.
Are there any third party logging components available?
Answer: Log4net is a port of the established log4j Java logging component.
How does .NET remoting work?
Answer: .NET remoting involves sending messages along channels. Two of the standard channels are HTTP and TCP. TCP is intended for LANs only - HTTP can be used for LANs or WANs (internet).
Support is provided for multiple message serializarion formats. Examples are SOAP (XML-based) and binary. By default, the HTTP channel uses SOAP (via the .NET runtime Serialization SOAP Formatter), and the TCP channel uses binary (via the .NET runtime Serialization Binary Formatter). But either channel can use either serialization format.
There are a number of styles of remote access:
· SingleCall. Each incoming request from a client is serviced by a new object. The object is thrown away when the request has finished.
· Singleton. All incoming requests from clients are processed by a single server object.
· Client-activated object. This is the old stateful (D)COM model whereby the client receives a reference to the remote object and holds that reference (thus keeping the remote object alive) until it is finished with it.
Distributed garbage collection of objects is managed by a system called 'leased based lifetime'. Each object has a lease time, and when that time expires the object is disconnected from the .NET runtime remoting infrastructure. Objects have a default renew time - the lease is renewed when a successful call is made from the client to the object. The client can also explicitly renew the lease.
How can we get at the Win32 API from a .NET program?
Answer: Use P/Invoke. This uses similar technology to COM Interop, but is used to access static DLL entry points instead of COM objects. Here is an example of C# calling the Win32 MessageBox function:
using System;
using System.Runtime.InteropServices;
class MainApp
{
[DllImport("user32.dll", EntryPoint="MessageBox", SetLastError=true, CharSet=CharSet.Auto)]
public static extern int MessageBox(int hWnd, String strMessage, String strCaption, uint uiType);
public static void Main()
{
MessageBox( 0, "Hello, this is PInvoke in operation!", ".NET", 0 );
}
}
What is the difference between an event and a delegate?
Answer: An event is just a wrapper for a multicast delegate. Adding a public event to a class is almost the same as adding a public multicast delegate field. In both cases, subscriber objects can register for notifications, and in both cases the publisher object can send notifications to the subscribers. However, a public multicast delegate has the undesirable property that external objects can invoke the delegate, something we'd normally want to restrict to the publisher. Hence events - an event adds public methods to the containing class to add and remove receivers, but does not make the invocation mechanism public.
What size is a .NET object?
Answer: Each instance of a reference type has two fields maintained by the runtime - a method table pointer and a sync block. These are 4 bytes each on a 32-bit system, making a total of 8 bytes per object overhead. Obviously the instance data for the type must be added to this to get the overall size of the object. So, for example, instances of the following class are 12 bytes each:
class MyInt
{
...
private int x;
}
However, note that with the current implementation of the CLR there seems to be a minimum object size of 12 bytes, even for classes with no data (e.g. System.Object).
Values types have no equivalent overhead.
Will .NET app run on 64-bit Windows?
Answer: 64-bit (x64) versions of Windows support both 32-bit and 64-bit processes, and corresponding 32-bit and 64-bit versions of .NET 2.0. (.NET 1.1 is 32-bit only).
.NET 1.x apps automatically run as 32-bit processes on 64-bit Windows.
.NET 2.0 apps can either run as 32-bit processes or as 64-bit processes. The OS decides which to use based on the PE header of the executable. The flags in the PE header are controlled via the compiler /platform switch, which allows the target of the app to be specified as 'x86', 'x64' or 'any cpu'. Normally we specify 'any cpu', and our app will run as 32-bit on 32-bit Windows and 64-bit on 64-bit Windows. However if we have some 32-bit native code in our app (loaded via COM interop, for example), we will need to specify 'x86', which will force 64-bit Windows to load our app in a 32-bit process. We can also tweak the 32-bit flag in the PE header using the SDK corflags utility.
What are the new features of .NET 2.0?
Answer: Generics, anonymous methods, partial classes, iterators, property visibility (separate visibility for get and set) and static classes.
What are the new 2.0 features useful for?
Answer: Generics are useful for writing efficient type-independent code, particularly where the types might include value types. The obvious application is container classes, and the .NET 2.0 class library includes a suite of generic container classes in the System.Collections.Generic namespace. Here's a simple example of a generic container class being used:
List
myList.Add( 10 );
Anonymous methods reduce the amount of code you have to write when using delegates, and are therefore especially useful for GUI programming. Here's an example
AppDomain.CurrentDomain.ProcessExit += delegate { Console.WriteLine("Process ending ..."); };
Partial classes is a useful feature for separating machine-generated code from hand-written code in the same class, and will therefore be heavily used by development tools such as Visual Studio.
Iterators reduce the amount of code you need to write to implement IEnumerable/IEnumerator. Here's some sample code:
static void Main()
{
RandomEnumerator re = new RandomEnumerator( 5 );
foreach( double r in re )
Console.WriteLine( r );
Console.Read();
}
class RandomEnumerator : IEnumerable
{
public RandomEnumerator(int size) { m_size = size; }
public IEnumerator
{
Random rand = new Random();
for( int i=0; i < m_size; i++ )
yield return rand.NextDouble();
}
int m_size = 0;
}
The use of 'yield return' is rather strange at first sight. It effectively synthethises an implementation of IEnumerator, something we had to do manually in .NET 1.x.
What's the problem with .NET generics?
Answer: .NET generics work great for container classes. But what about other uses? Well, it turns out that .NET generics have a major limitation - they require the type parameter to be constrained. For example, you cannot do this:
static class Disposer
{
public static void Dispose(T obj) { obj.Dispose(); }
}
The C# compiler will refuse to compile this code, as the type T has not been constrained, and therefore only supports the methods of System.Object. Dispose is not a method on System.Object, so the compilation fails. To fix this code, we need to add a where clause, to reassure the compiler that our type T does indeed have a Dispose method
static class Disposer
{
public static void Dispose(T obj) { obj.Dispose(); }
}
The problem is that the requirement for explicit contraints is very limiting. We can use constraints to say that T implements a particular interface, but we can't dilute that to simply say that T implements a particular method. Contrast this with C++ templates (for example), where no constraint at all is required - it is assumed (and verified at compile time) that if the code invokes the Dispose() method on a type, then the type will support the method.
In fact, after writing generic code with interface constraints, we quickly see that we haven't gained much over non-generic interface-based programming. For example, we can easily rewrite the Disposer class without generics:
static class Disposer
{
public static void Dispose( IDisposable obj ) { obj.Dispose(); }
}
What's new in the .NET 2.0 class library?
Answer: Here is a selection of new features in the .NET 2.0 class library:
· Generic collections in the System.Collections.Generic namespace.
· The System.Nullable
· The GZipStream and DeflateStream classes in the System.IO.Compression namespace.
· The Semaphore class in the System.Threading namespace.
· Wrappers for DPAPI in the form of the ProtectedData and ProtectedMemory classes in the System.Security.Cryptography namespace.
· The IPC remoting channel in the System.Runtime.Remoting.Channels.Ipc namespace, for optimised intra-machine communication.
What is C#?
Answer: C# is a programming language designed by Microsoft. It is loosely based on C/C++, and bears a striking similarity to Java. Microsoft describe C# as follows:
"C# is a simple, modern, object oriented, and type-safe programming language derived from C and C++. C# (pronounced 'C sharp') is firmly planted in the C and C++ family tree of languages, and will immediately be familiar to C and C++ programmers. C# aims to combine the high productivity of Visual Basic and the raw power of C++."
How do we develop C# apps?
Answer: The (free) .NET SDK contains the C# command-line compiler (csc.exe). Visual Studio has fully integrated support for C# development. On Linux we can use Mono.
Does C# replace C++?
Answer: There are three options open to the Windows developer from a C++ background:
· Stick with standard C++. Don't use .NET at all.
· Use C++ with .NET. Microsoft supply a .NET C++ compiler that produces IL rather than machine code. However to make full use of the .NET environment (e.g. garbage collection), a set of extensions are required to standard C++. In .NET 1.x this extended language is called Managed Extensions for C++. In .NET 2.0 ME C++ has been completely redesigned and renamed C++/CLI.
· Forget C++ and use C#.
Each of these options has merits, depending on the developer and the application. ME C++ (soon to be C++/CLI) is very useful for interop between new .NET code and old C++ code - simply write a managed wrapper class using ME C++, then use the managed class from C#. From experience, this works well.
Does C# have its own class library?
Answer: Not exactly. The .NET Framework has a comprehensive class library, which C# can make use of. C# does not have its own class library.
What standard types does C# use?
Answer: C# supports a very similar range of basic types to C++, including int, long, float, double, char, string, arrays, structs and classes. However, don't assume too much. The names may be familiar, but many of the details are different. For example, a long is 64 bits in C#, whereas in C++ the size of a long depends on the platform (typically 32 bits on a 32-bit platform, 64 bits on a 64-bit platform). Also classes and structs are almost the same in C++ - this is not true for C#. Finally, chars and strings in .NET are 16-bit (Unicode/UTF-16), not 8-bit like C++.
Is it true that all C# types derive from a common base class?
Answer: Yes and no. All types can be treated as if they derive from object (System.Object), but in order to treat an instance of a value type (e.g. int, float) as object-derived, the instance must be converted to a reference type using a process called 'boxing'. In theory a developer can forget about this and let the run-time worry about when the conversion is necessary, but in reality this implicit conversion can have side-effects that may trip up the unwary.
So I can pass an instance of a value type to a method that takes an object as a parameter?
Answer: Yes. For example:
class CApplication
{
public static void Main()
{
int x = 28;
string s = "SKP";
DisplayMe( x );
DisplayMe( s );
}
static void DisplayMe( object o )
{
System.Console.WriteLine( "You are {0}", o );
}
}
This would display:
You are 28
You are SKP
What are the fundamental differences between value types and reference types?
Answer: C# divides types into two categories - value types and reference types. Most of the intrinsic types (e.g. int, char) are value types. Structs are also value types. Reference types include classes, arrays and strings. The basic idea is straightforward - an instance of a value type represents the actual data, whereas an instance of a reference type represents a pointer or reference to the data.
The most confusing aspect of this for C++ developers is that C# has predetermined which types are represented as values, and which are represented as references. A C++ developer expects to take responsibility for this decision.
For example, in C++ we can do this:
int x1 = 3; // x1 is a value on the stack
int *x2 = new int(3) // x2 is a pointer to a value on the heap
but in C# there is no control:
int x1 = 3; // x1 is a value on the stack
int x2 = new int();
x2 = 3; // x2 is also a value on the stack!
So an int is a value type, and a class is a reference type. How can int be derived from object?
Answer: It isn't, really. When an int is being used as an int, it is a value. However, when it is being used as an object, it is a reference to an integer value (on the managed heap). In other words, when you treat an int as an object, the runtime automatically converts the int value to an object reference. This process is called boxing. The conversion involves copying the int to the heap, and creating an object instance which refers to it. Unboxing is the reverse process - the object is converted back to a value.
int x = 3; // new int value 3 on the stack
object objx = x; // new int on heap, set to value 3 - still have x=3 on stack
int y = (int)objx; // new value 3 on stack, still got x=3 on stack and objx=3 on heap
Are C# references the same as C++ references?
Answer: Not quite. The basic idea is the same, but one significant difference is that C# references can be null . So we cannot rely on a C# reference pointing to a valid object. In that respect a C# reference is more like a C++ pointer than a C++ reference. If we try to use a null reference, a NullReferenceException is thrown.
For example, look at the following method:
void displayStringLength( string s )
{
Console.WriteLine( "String is length {0}", s.Length );
}
The problem with this method is that it will throw a NullReferenceException if called like this:
string s = null;
displayStringLength( s );
Of course for some situations you may deem a NullReferenceException to be a perfectly acceptable outcome, but in this case it might be better to re-write the method like this:
void displayStringLength( string s )
{
if( s == null )
Console.WriteLine( "String is null" );
else
Console.WriteLine( "String is length {0}", s.Length );
}
Can we use typedefs in C#?
Answer: No, C# has no direct equivalent of the C++ typedef. C# does allow an alias to be specified via the using keyword:
using IntList = System.Collections.Generic.List
but the alias only applies in the file in which it is declared. A workaround in some cases is to use inheritance:
public class IntList : List
Structs are largely redundant in C++. Why does C# have them?
Answer: In C++, a struct and a class are pretty much the same thing. The only difference is the default visibility level (public for structs, private for classes). However, in C# structs and classes are very different. In C#, structs are value types (instances stored directly on the stack, or inline within heap-based objects), whereas classes are reference types (instances stored on the heap, accessed indirectly via a reference). Also structs cannot inherit from structs or classes, though they can implement interfaces. Structs cannot have destructors. A C# struct is much more like a C struct than a C++ struct.
Does C# support multiple inheritance (MI)?
Answer: No, though it does support implementation of multiple interfaces on a single class or struct.
Is a C# interface the same as a C++ abstract class?
Answer: No, not quite. An abstract class in C++ cannot be instantiated, but it can (and often does) contain implementation code and/or data members. A C# interface cannot contain any implementation code or data members - it is simply a group of method names & signatures. A C# interface is more like a COM interface than a C++ abstract class.
Are C# constructors the same as C++ constructors?
Answer: Very similar, but there are some significant differences. First, C# supports constructor chaining. This means one constructor
can call another:
class Person
{
public Person( string name, int age ) { ... }
public Person( string name ) : this( name, 0 ) {}
public Person() : this( "", 0 ) {}
}
Another difference is that virtual method calls within a constructor are routed to the most derived implementation
Error handling is also somewhat different. If an exception occurs during construction of a C# object, the destuctor (finalizer) will still be called. This is unlike C++ where the destructor is not called if construction is not completed.
Finally, C# has static constructors. The static constructor for a class runs before the first instance of the class is created.
Also note that (like C++) some C# developers prefer the factory method pattern over constructors.
Are C# destructors the same as C++ destructors?
Answer: No. They look the same but they are very different. The C# destructor syntax (with the familiar ~ character) is just syntactic sugar for an override of the System.Object Finalize method. This Finalize method is called by the garbage collector when it determines that an object is no longer referenced, before it frees the memory associated with the object. So far this sounds like a C++ destructor. The difference is that the garbage collector makes no guarantees about when this procedure happens. Indeed, the algorithm employed by the CLR garbage collector means that it may be a long time after the application has finished with the object. This lack of certainty is often termed 'non-deterministic finalization', and it means that C# destructors are not suitable for releasing scarce resources such as database connections, file handles etc.
To achieve deterministic destruction, a class must offer a method to be used for the purpose. The standard approach is for the class to implement the IDisposable interface. The user of the object must call the Dispose() method when it has finished with the object. C# offers the 'using' construct to make this easier.
If C# destructors are so different to C++ destructors, why did MS use the same syntax?
Answer: Presumably they wanted C++ programmers to feel at home.
Are all methods virtual in C#?
Answer: No. Like C++, methods are non-virtual by default, but can be marked as virtual.
How do we declare a pure virtual function in C#?
Answer: Use the abstract modifier on the method. The class must also be marked as abstract (naturally). Note that abstract methods cannot have an implementation (unlike pure virtual C++ methods).
Can we call a virtual method from a constructor/destructor?
Answer: Yes, but it's generally not a good idea. The mechanics of object construction in .NET are quite different from C++, and this affects virtual method calls in constructors.
C++ constructs objects from base to derived, so when the base constructor is executing the object is effectively a base object, and virtual method calls are routed to the base class implementation. By contrast, in .NET the derived constructor is executed first, which means the object is always a derived object and virtual method calls are always routed to the derived implementation. (Note that the C# compiler inserts a call to the base class constructor at the start of the derived constructor, thus preserving standard OO semantics by creating the illusion that the base constructor is executed first.)
The same issue arises when calling virtual methods from C# destructors. A virtual method call in a base destructor will be routed to the derived implementation.
Should we make my destructor virtual?
Answer: A C# destructor is really just an override of the System.Object Finalize method, and so is virtual by definition.
Can we use exceptions in C#?
Answer: Yes, in fact exceptions are the recommended error-handling mechanism in C# (and in .NET in general). Most of the .NET framework classes use exceptions to signal errors.
What types of object can we throw as exceptions?
Answer: Only instances of the System.Exception classes, or classes derived from System.Exception. This is in sharp contrast with C++ where instances of almost any type can be thrown.
Can we define our own exceptions?
Answer: Yes, just derive your exception class from System.Exception.
Does the System.Exception class have any cool features?
Answer: Yes - the feature which stands out is the StackTrace property. This provides a call stack which records where the exception was thrown from. For example, the following code:
using System;
class CApp
{
public static void Main()
{
try
{
f();
}
catch( Exception e )
{
Console.WriteLine( "System.Exception stack trace = \n{0}", e.StackTrace );
}
}
static void f()
{
throw new Exception( "f went pear-shaped" );
}
}
produces this output:
System.Exception stack trace =
at CApp.f()
at CApp.Main()
Note, however, that this stack trace was produced from a debug build. A release build may optimise away some of the method calls which could mean that the call stack isn't quite what you expect.
When should we throw an exception?
Answer: This is the subject of some debate, and is partly a matter of taste. However, it is accepted by many that exceptions should be thrown only when an 'unexpected' error occurs. How do you decide if an error is expected or unexpected? This is a judgement call, but a straightforward example of an expected error is failing to read from a file because the seek pointer is at the end of the file, whereas an example of an unexpected error is failing to allocate memory from the heap.
Does C# have a 'throws' clause?
Answer: No, unlike Java, C# does not require (or even allow) the developer to specify the exceptions that a method can throw.
How can we check the type of an object at runtime?
Answer: We can use the is keyword. For example:
using System;
class CApp
{
public static void Main()
{
string s = "SKP";
long i = 10;
Console.WriteLine( "{0} is {1}an integer", s, (IsInteger(s) ? "" : "not ") );
Console.WriteLine( "{0} is {1}an integer", i, (IsInteger(i) ? "" : "not ") );
}
static bool IsInteger( object obj )
{
if( obj is int || obj is long )
return true;
else
return false;
}
}
produces the output:
SKP is not an integer
10 is an integer
Can we get the name of a type at runtime?
Answer: Yes, use the GetType method of the object class (which all types inherit from). For example:
using System;
class CTest
{
class CApp
{
public static void Main()
{
long i = 10;
CTest ctest = new CTest();
DisplayTypeInfo( ctest );
DisplayTypeInfo( i );
}
static void DisplayTypeInfo( object obj )
{
Console.WriteLine( "Type name = {0}, full type name = {1}", obj.GetType(), obj.GetType().FullName );
}
}
}
produces the following output:
Type name = CTest, full type name = CTest
Type name = Int64, full type name = System.Int64
How do we do a case-insensitive string comparison?
Answer: Use the String.Compare function. Its third parameter is a boolean which specifies whether case should be ignored or not.
"skp" == "Skp" // false
System.String.Compare( "skp", "Skp", true ) // true
Does C# support a variable number of arguments?
Answer: Yes, using the params keyword. The arguments are specified as a list of arguments of a specific type, e.g. int. For ultimate flexibility, the type can be object. The standard example of a method which uses this approach is System.Console.WriteLine().
How can we process command-line arguments?
Answer: Like this:
using System;
class CApp
{
public static void Main( string[] args )
{
Console.WriteLine( "You passed the following arguments:" );
foreach( string arg in args )
Console.WriteLine( arg );
}
}
Does C# do array bounds checking?
Answer: Yes. An IndexOutOfRange exception is used to signal an error.
How can we make sure our C# classes will interoperate with other .NET languages?
Answer: Make sure our C# code conforms to the Common Language Subset (CLS). To help with this, add the [assembly:CLSCompliant(true)] global attribute to your C# source files. The compiler will emit an error if you use a C# feature which is not CLS-compliant.
How do we use the 'using' keyword with multiple objects?
Answer: We can nest using statements, like this:
using( obj1 )
{
using( obj2 )
{
...
}
}
However consider using this more aesthetically pleasing (but functionally identical) formatting:
using( obj1 )
using( obj2 )
{
...
}
What is the difference between == and object.Equals?
Answer: For value types, == and Equals() usually compare two objects by value. For example:
int x = 10;
int y = 10;
Console.WriteLine( x == y );
Console.WriteLine( x.Equals(y) );
will display:
True
True
However things are more complex for reference types. Generally speaking, for reference types == is expected to perform an identity comparison, i.e. it will only return true if both references point to the same object. By contrast, Equals() is expected to perform a value comparison, i.e. it will return true if the references point to objects that are equivalent. For example:
StringBuilder s1 = new StringBuilder("skp");
StringBuilder s2 = new StringBuilder("skp");
Console.WriteLine( s1 == s2 );
Console.WriteLine( s1.Equals(s2) );
will display:
False
True
s1 and s2 are different objects (hence == returns false), but they are equivalent (hence Equals() returns true).
Unfortunately there are exceptions to these rules. The implementation of Equals() in System.Object (the one you'll inherit by default if you write a class) compares identity, i.e. it's the same as operator==. So Equals() only tests for equivalence if the class author overrides the method (and implements it correctly). Another exception is the string class - its operator== compares value rather than identity.
Bottom line: If we want to perform an identity comparison use the ReferenceEquals() method. If we want to perform a value comparison, use Equals() but be aware that it will only work if the type has overridden the default implementation. Avoid operator== with reference types (except perhaps strings), as it's simply too ambiguous.
How do we enforce const correctness in C#?
Answer: We can't - at least not in the same way you do in C++. C# (actually, the CLI) has no real concept of const correctness, For example, there's no way to specify that a method should not modify an argument passed in to it. And there's no way to specify that a method does not modify the object on which it is acting.
What are the new features in C# 2.0?
Answer: Support for all of the new framework features such as generics, anonymous methods, partial classes, iterators and static classes.
Delegate inference is a new feature of the C# compiler which makes delegate usage a little simpler. It allows us to write this:
Thread t = new Thread(ThreadFunc);
instead of this:
Thread t = new Thread( new ThreadStart(ThreadFunc) );
Another minor but welcome addition is the explicit global namespace, which fixes a hole in namespace usage in C# 1.x. We can prefix a type name with global:: to indicate that the type belongs to the global namespace, thus avoiding problems where the compiler infers the namespace and gets it wrong.
C# 2.0 includes some syntactic sugar for the new System.Nullable type. We can use T? as a synonym for System.Nullable
As suggested by the name, this allows values of the type to be 'null', or 'undefined'.
Are C# generics the same as C++ templates?
Answer: No, not really. There are some similarities, but there are also fundamental differences.
Who is a protected class-level variable available to?
Answer: It is available to any sub-class (a class inheriting this class).
Are private class-level variables inherited?
Answer: Yes, but they are not accessible. Although they are not visible or accessible via the class interface, they are inherited. |
Describe the accessibility modifier “protected internal”.
Answer: It is available to classes that are within the same assembly and derived from the specified base class.
What’s the top .NET class that everything is derived from?
Answer: System.Object.
What does the term immutable mean?
Answer: The data value may not be changed. Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.
What’s the difference between System.String and System.Text.StringBuilder classes?
Answer: System.String is immutable. System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.
What’s the advantage of using System.Text.StringBuilder over System.String?
Answer: StringBuilder is more efficient in cases where there is a large amount of string manipulation. Strings are immutable, so each time a string is changed, a new instance in memory is created.
Can we store multiple data types in System.Array?
Answer: No.
What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?
Answer: The Clone() method returns a new array (a shallow copy) object containing all the elements in the original array. The CopyTo() method copies the elements into another existing array. Both perform a shallow copy. A shallow copy means the contents (each array element) contains references to the same object as the elements in the original array. A deep copy (which neither of these methods performs) would create a new instance of each element's object, resulting in a different, yet identacle object.
How can we sort the elements of the array in descending order?
Answer: By calling Sort() and then Reverse() methods.
What’s the .NET collection class that allows an element to be accessed using a unique key?
Answer: HashTable.
What class is underneath the SortedList class?
A sorted HashTable.
Will the finally block get executed if an exception has not occurred?
Answer: Yes.
What’s the C# syntax to catch any possible exception?
Answer: A catch block that catches the exception of type System.Exception. You can also omit the parameter data type in this case and just write catch {}.
Can multiple catch blocks be executed for a single try statement?
Answer: No. Once the proper catch block processed, control is transferred to the finally block (if there are any).
Explain the three services model commonly know as a three-tier application.
Answer: Presentation (UI), Business (logic and underlying code) and Data (from storage or other sources).
What is the syntax to inherit from a class in C#?
Answer: Place a colon and then the name of the base class.
Example: class MyNewClass : MyBaseClass
Can we prevent our class from being inherited by another class?
Answer: Yes. The keyword “sealed” will prevent the class from being inherited.
Can we allow a class to be inherited, but prevent the method from being over-ridden?
Answer: Yes. Just leave the class public and make the method sealed.
What’s an abstract class?
Answer: A class that cannot be instantiated. An abstract class is a class that must be inherited and have the methods overridden. An abstract class is essentially a blueprint for a class without any implementation.
When do we absolutely have to declare a class as abstract?
Answer:
§ When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.
§ When at least one of the methods in the class is abstract.
What is an interface class?
Answer: Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes.
Why can’t we specify the accessibility modifier for methods inside the interface?
Answer: They all must be public, and are therefore public by default.
Can you inherit multiple interfaces?
Answer: Yes. .NET does support multiple interfaces.
What happens if we inherit multiple interfaces and they have conflicting method names?
Answer: It’s up to us to implement the method inside your own class, so implementation is left entirely up to us. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares we’re okay.
What’s the difference between an interface and abstract class?
Answer: In an interface class, all methods are abstract - there is no implementation. In an abstract class some methods can be concrete. In an interface class, no accessibility modifiers are allowed. An abstract class may have accessibility modifiers.
What is the difference between a Struct and a Class?
Answer: Structs are value-type variables and are thus saved on the stack, additional overhead but faster retrieval. Another difference is that structs cannot inherit.
What’s the implicit name of the parameter that gets passed into the set method/property of a class?
Answer: Value. The data type of the value parameter is defined by whatever data type the property is declared as.
What does the keyword “virtual” declare for a method or property?
Answer: The method or property can be overridden.
How is method overriding different from method overloading?
Answer: When overriding a method, you change the behavior of the method for the derived class. Overloading a method simply involves having another method with the same name within the class.
Can we declare an override method to be static if the original method is not static?
Answer: No. The signature of the virtual method must remain the same. (Note: Only the keyword virtual is changed to keyword override)
What are the different ways a method can be overloaded?
Answer: Different parameter data types, different number of parameters, different order of parameters.
If a base class has a number of overloaded constructors, and an inheriting class has a number of overloaded constructors; can we enforce a call from an inherited constructor to a specific base constructor?
Answer: Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.
What’s a delegate?
Answer: A delegate object encapsulates a reference to a method.
What’s a multicast delegate?
Answer: A delegate that has multiple handlers assigned to it. Each assigned handler (method) is called.
What’s the difference between // comments, /* */ comments and /// comments?
Answer: Single-line comments, multi-line comments, and XML documentation comments.
How do we generate documentation from the C# file commented properly with a command-line compiler?
Answer: Compile it with the /doc switch.
What debugging tools come with the .NET SDK?
Answer:
1. CorDBG – command-line debugger. To use CorDbg, you must compile the original C# file using the /debug switch.
2. DbgCLR – graphic debugger. Visual Studio .NET uses the DbgCLR.
What does assert() method do?
Answer: In debug compilation, assert takes in a Boolean condition as a parameter, and shows the error dialog if the condition is false. The program proceeds without any interruption if the condition is true.
What’s the difference between the Debug class and Trace class?
Answer: Documentation looks the same. Use Debug class for debug builds, use Trace class for both debug and release builds.
Why are there five tracing levels in System.Diagnostics.TraceSwitcher?
Answer: The tracing dumps can be quite verbose. For applications that are constantly running you run the risk of overloading the machine and the hard drive. Five levels range from None to Verbose, allowing you to fine-tune the tracing activities.
Where is the output of TextWriterTraceListener redirected?
Answer: To the Console or a text file depending on the parameter passed to the constructor.
How do you debug an ASP.NET Web application?
Answer: Attach the aspnet_wp.exe process to the DbgClr debugger.
What are three test cases you should go through in unit testing?
Answer:
1. Positive test cases (correct data, correct output).
2. Negative test cases (broken or missing data, proper handling).
3. Exception test cases (exceptions are thrown and caught properly).
Can we change the value of a variable while debugging a C# application?
Answer: Yes. If we are debugging via Visual Studio.NET, just go to Immediate window.
What is the role of the DataReader class in ADO.NET connections?
Answer: It returns a read-only, forward-only rowset from the data source. A DataReader provides fast access when a forward-only sequential read is needed.
What are advantages and disadvantages of Microsoft-provided data provider classes in ADO.NET?
Answer: SQLServer.NET data provider is high-speed and robust, but requires SQL Server license purchased from Microsoft. OLE-DB.NET is universal for accessing other sources, like Oracle, DB2, Microsoft Access and Informix. OLE-DB.NET is a .NET layer on top of the OLE layer, so it’s not as fastest and efficient as SqlServer.NET.
Explain ACID rule of thumb for transactions.
Answer: A transaction must be:
1. Atomic - it is one unit of work and does not dependent on previous and following transactions.
2. Consistent - data is either committed or roll back, no “in-between” case where something has been updated and something hasn’t.
3. Isolated - no transaction sees the intermediate results of the current transaction).
4. Durable - the values persist if the data had been committed even if the system crashes right after.
What connections does Microsoft SQL Server support?
Answer: Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and password).
Between Windows Authentication and SQL Server Authentication, which one is trusted and which one is untrusted?
Answer: Windows Authentication is trusted because the username and password are checked with the Active Directory, the SQL Server authentication is untrusted, since SQL Server is the only verifier participating in the transaction.
What does the Initial Catalog parameter define in the connection string?
Answer: The database name to connect to.
What does the Dispose method do with the connection object?
Answer: Deletes it from the memory.
What is a pre-requisite for connection pooling?
Answer: Multiple processes must agree that they will share the same connection, where every parameter is the same, including the security settings. The connection string must be identical.
How is the DLL Hell problem solved in .NET?
Answer: Assembly versioning allows the application to specify not only the library it needs to run (which was available under Win32), but also the version of the assembly.
What are the ways to deploy an assembly?
Answer: An MSI installer, a CAB archive, and XCOPY command.
What is a satellite assembly?
Answer: When we write a multilingual or multi-cultural application in .NET, and want to distribute the core application separately from the localized modules, the localized assemblies that modify the core application are called satellite assemblies.
What namespaces are necessary to create a localized application?
Answer: System.Globalization and System.Resources.
What is the smallest unit of execution in .NET?
Answer: an Assembly.
When should we call the garbage collector in .NET?
Answer: As a good rule, we should not call the garbage collector. However, we could call the garbage collector when we are done using a large object (or set of objects) to force the garbage collector to dispose of those very large objects from memory. However, this is usually not a good practice.
How do you convert a value-type to a reference-type?
Answer: Use Boxing.
What happens in memory when we Box and Unbox a value-type?
Answer: Boxing converts a value-type to a reference-type, thus storing the object on the heap. Unboxing converts a reference-type to a value-type, thus storing the value on the stack.
Describe the role of inetinfo.exe, aspnet_isapi.dll andaspnet_wp.exe in the page loading process.
Answer: inetinfo.exe is theMicrosoft IIS server running, handling ASP.NET requests among other things.When an ASP.NET request is received (usually a file with .aspx extension), the ISAPI filter aspnet_isapi.dll takes care of it by passing the request tothe actual worker process aspnet_wp.exe.
What’s the difference between Response.Write() andResponse.Output.Write()?
Answer: Response.Output.Write() allows you to write formatted output.
What methods are fired during the page load?
Answer:
Init() - when the page is instantiated
Load() - when the page is loaded into server memory
PreRender() - the brief moment before the page is displayed to the user as HTML
Unload() - when page finishes loading.
When during the page processing cycle is ViewState available?
Answer: After the Init() and before the Page_Load(), or OnLoad() for a control.
What namespace does the Web page belong in the .NET Framework class hierarchy?
Answer: System.Web.UI.Page
Where do we store the information about the user’s locale?
Answer: System.Web.UI.Page.Culture
What’s the difference between Codebehind="MyCode.aspx.cs" andSrc="MyCode.aspx.cs"?
Answer: CodeBehind is relevant to Visual Studio.NET only.
What’s a bubbled event?
Answer: When we have a complex control, like DataGrid, writing an event processing routine for each object (cell, button, row, etc.) is quite tedious. The controls can bubble up their eventhandlers, allowing the main DataGrid event handler to take care of its constituents.
Suppose we want a certain ASP.NET function executed on MouseOver for a certain button. Where do we add an event handler?
Answer: Add an OnMouseOver attribute to the button. Example: btnSubmit.Attributes.Add("onmouseover","someClientCodeHere();");
What data types do the RangeValidator control support?
Answer: Integer, String, and Date.
Explain the differences between Server-side and Client-side code?
Answer: Server-side code executes on the server. Client-side code executes in the client's browser.
What type of code (server or client) is found in a Code-Behind class?
Answer: The answer is server-side code since code-behind is executed on the server. However, during the code-behind's execution on the server, it can render client-side code such as JavaScript to be processed in the clients browser. But just to be clear, code-behind executes on the server, thus making it server-side code.
Should user input data validation occur server-side or client-side? Why?
Answer: All user input data validation should occur on the server at a minimum. Additionally, client-side validation can be performed where deemed appropriate and feasable to provide a richer, more responsive experience for the user.
What is the difference between Server.Transfer and Response.Redirect? Why would we choose one over the other?
Answer: Server.Transfer transfers page processing from one page directly to the next page without making a round-trip back to the client's browser. This provides a faster response with a little less overhead on the server. Server.Transfer does not update the clients url history list or current url. Response.Redirect is used to redirect the user's browser to another page or site. This performas a trip back to the client where the client's browser is redirected to the new page. The user's browser history list is updated to reflect the new address.
Explain the difference between an ADO.NET Dataset and an
Answer: Valid answers are:
· A DataSet can represent an entire relational database in memory, complete with tables, relations, and views.
· A DataSet is designed to work without any continuing connection to the original data source.
· Data in a DataSet is bulk-loaded, rather than being loaded on demand.
· There's no concept of cursor types in a DataSet.
· DataSets have no current record pointer You can use For Each loops to move through the data.
· You can store many edits in a DataSet, and write them to the original data source in a single operation.
· Though the DataSet is universal, other objects in ADO.NET come in different versions for different data sources.
What is the Global.asax used for?
Answer:
The Global.asax (including the Global.asax.cs file) is used to implement application and session level events.
What are the Application_Start and Session_Start subroutines used for?
Answer: This is where you can set the specific variables for the Application and Session objects.
Describe the difference between inline and code behind.
Answer: Inline code written along side the html in a page. Code-behind is code written in a separate file and referenced by the .aspx page.
Explain what a diffgram is, and a good use for one?
Answer: The DiffGram is one of the two XML formats that you can use to render DataSet object contents to XML. A good use is reading database data to an XML file to be sent to a Web Service.
Whats MSIL, and why should my developers need an appreciation of it if at all?
Answer: MSIL is the Microsoft Intermediate Language. All .NET compatible languages will get converted to MSIL. MSIL also allows the .NET Framework to JIT compile the assembly on the installed computer.
Which method do we invoke on the DataAdapter control to load your generated dataset with data?
Answer: The Fill() method.
Can we edit data in the Repeater control?
Answer: No, it just reads the information from its data source.
Which template must you provide, in order to display data in a Repeater control?
Answer: ItemTemplate.
How can we provide an alternating color scheme in a Repeater control?
Answer: Use the AlternatingItemTemplate.
What property must we set, and what method must we call in our code, in order to bind the data from a data source to the Repeater control?
Answer: We must set the DataSource property and call the DataBind method.
What base class do all Web Forms inherit from?
Answer: The Page class.
Name two properties common in every validation control?
Answer: ControlToValidate property and Text property.
Which property on a Combo Box do we set with a column name, prior to setting the DataSource, to display data in the combo box?
Answer: DataTextField property.
Which control would you we if you needed to make sure the values in two different controls matched?
Answer: CompareValidator control.
How many classes can a single .NET DLL contain?
Answer: It can contain many classes.
What is the transport protocol we use to call a Web service?
Answer: SOAP (Simple Object Access Protocol) is the preferred protocol.
What does WSDL stand for?
Answer: Web Services Description Language.
What is ViewState?
Answer: ViewState allows the state of objects (serializable) to be stored in a hidden field on the page. ViewState is transported to the client and back to the server, and is not stored on the server or any other external source. ViewState is used the retain the state of server-side objects between postabacks.
What is the lifespan for items stored in ViewState?
Answer: Item stored in ViewState exist for the life of the current page. This includes postbacks (to the same page).
What does the "EnableViewState" property do? Why would we want it on or off?
Answer: It allows the page to save the users input on a form across postbacks. It saves the server-side values for a given control into ViewState, which is stored as a hidden value on the page before sending the page to the clients browser. When the page is posted back to the server the server control is recreated with the state stored in viewstate.
What are the different types of Session state management options available with ASP.NET?
Answer: ASP.NET provides In-Process and Out-of-Process state management. In-Process stores the session in memory on the web server. This requires the a "sticky-server" (or no load-balancing) so that the user is always reconnected to the same web server. Out-of-Process Session state management stores data in an external data source. The external data source may be either a SQL Server or a State Server service. Out-of-Process state management requires that all objects stored in session are serializable.
What Are the Goals of the Compact Framework?
Answer: The goals of the Compact Framework (and Smart Device Projects) are fourfold:
? To provide a portable (and small) subset of the Windows .NET Framework targeting multiple platforms. This includes the creation of a runtime engine, class libraries, and compilers for VB and C# that together are referred to as the .NET Compact Framework. This framework is not a simple port of the desktop version, but a complete rewrite designed to execute managed code on multiple CPU architectures and operating systems through an abstraction layer known as the Platform Adaption Layer (PAL). It is important to note that the Compact Framework was also designed to be a subset of the desktop version of the Framework. As a result, roughly 25% of the desktop types are represented in the Compact Framework.
?Leverage VS .NET. Since VS .NET already provides a high productivity development environment, it is only natural that this environment should be utilized in developing mobile applications as well. To that end, the Smart Device Projects for VS .NET 2003 provide the project templates, emulator, debugger, and device integration to use the same IDE for desktop as for mobile development.
? True Emulation. One of the requirements for developing robust (i.e., well-debugged) mobile applications is that they run as expected when installed on the device. To that end VS.NET includes emulators for Pocket PC and Windows CE that run exactly the same execution engine and class libraries as those installed on the device. In addition, the emulator supports localized packages for developing global applications. In this way, developers can be assured that the code they write and test with the emulator will execute correctly when deployed to the device.
?Enable Web Services on Devices. Since XML Web Services are by definition device independent, they can be used from a variety of devices. By building support for consuming web services directly into the Compact Framework, developers can easily call web services.
On Which Platforms Does It Run?
Answer: Since 1992 Microsoft has produced a series of platforms (9 if our count is correct) built on the Windows CE operating system. A platform includes a specific set of hardware coupled with a version of Windows CE and associated software that makes it easier for developers and device manufacturers to target their solutions. The Compact Framework supports the Pocket PC 2000, 2002, and embedded Windows CE .NET 4.1 platforms. Although not officially supported, Compact Framework code also runs on Pocket PC Phone Edition devices that are a superset of Pocket PC 2002. Also not supported are smartphones such as the SPV from Orange.
Because the Compact Framework is a managed environment, each device that executes applications written using the Compact Framework must have the framework installed. As of this writing, there were no devices shipping that included the Compact Framework as a standard component in RAM or in ROM. However, look for future devices running Windows CE .NET to include the runtime.
What About ASP.NET Mobile Controls (aka the Mobile Internet Toolkit)?
Answer: Many developers new to developing for mobility are at first confused by the differences between the Compact Framework and ASP.NET Mobile Controls. In a nutshell here are the primary differences.
? The ASP.NET Mobile Controls are a server side technology whereas the Compact Framework is a client side technology. This means that code written using mobile controls must execute on a server, and not directly on the device, by producing markup language that is interpreted by the device using a browser or parser. Code written for the Compact Framework executes directly on the device using just-in-time (JIT) compilation and native execution.
? The ASP.NET Mobile Controls are based on ASP.NET and use the ASP.NET HTTP runtime to handle and process requests via a set of ASP.NET server controls specially designed for small form factor devices. This means that mobile controls require an ASP.NET web server (IIS 5.0 or 6.0). The device issues requests that are processed on the web server by the controls producing markup language.
?Both ASP.NET Mobile Controls and the Compact Framework are based on the desktop Framework and VS .NET. In the case of mobile controls, developers use VS .NET to write ASP.NET code using the ASP.NET server controls. The server side code is JIT compiled and executed on the web server by the common language runtime. In the case of the Compact Framework the code is written using SDP and subsequently downloaded to the device for JITting and execution using a compact version of the common language runtime.
? The ASP.NET Mobile Controls are more flexible in that they can dynamically produce various markup languages depending on the requesting device they dynamically detect, including WML, cHTML, HTML, and XHTML (in Visual Studio .NET 2003 and the Framework v1.1). This means that mobile controls can support a broader range of devices (over 200 at last count) including wireless web phones, Pocket PCs, and virtually any device that supports HTTP and HTML. The ASP.NET Mobile Controls also include the ability to add device specific customizations so that as new devices come online their feature set can be identified.
? Since code written for the Compact Framework is downloaded to the device, the Compact Framework supports connected, occasionally connected, and disconnected scenarios and supports local data storage using XML or SQL Server 2000 Windows CE Edition. ASP.NET Mobile Controls only support the former since an HTTP connection is required to request an ASP.NET page that uses the mobile controls. Basically, this means that mobile controls are targeted for building web sites for a broad variety of connected devices and at the same time not having to rewrite the vast majority of code to deal with differences between those devices.
As a result we can (and in our estimation should) consider architecting and building your applications using the Compact Framework and SDP when the following are true.
? You are developing code that will execute on the device, as opposed to applications developed with the ASP.NET Mobile Controls
? Your applications must run in disconnected, connected, or occasionally connected modes
? You are targeting the Pocket PC 2000, 2002 or embedded Windows CE .NET platforms
What's Missing in the Compact Framework?
Answer: In order to accommodate the restricted resources on smart devices the Compact Framework was designed as a subset of the desktop libraries and in fact includes just over 1,700 types, or roughly 25% of the desktop framework. As a result, developers familiar with the desktop Framework should not expect all the functionality they are accustomed to. The most important omissions are listed here:
? ASP.NET. Because the Compact Framework is designed to support applications that execute on the device, it does not include any support for building web pages hosted on a web server running on the device.
? COM Interop. Since the Windows CE operating system and the embedded Visual C++ (eVC) tool support creating COM components and ActiveX controls, it would be nice if the Compact Framework supported the same COM Interop functionality (complete with COM callable wrappers and interop assemblies) as does the desktop Framework. Unfortunately, COM Interop did not make it into the initial release of the Compact Framework. However, it is possible to create a DLL wrapper for a COM component using eVC++ and then call the wrapper using the Platform Invoke (PInvoke) feature of the Compact Framework, which allows native APIs to be called.
? OleDb Access. The Compact Framework omits the System.Data.OleDb namespace and so does not support the ability to make calls directly to a database using the OleDb .NET Data Provider.
? Generic Serialization. The desktop Framework supports binary serialization of any object through the use of the Serializable attribute, the ISerializable interface, and the XmlSerializer class in the System.Xml.Serialization namespace. This functionality is not supported in the Compact Framework. However, the Compact Framework does support serializing objects to XML for use in XML Web Services and serializing DataSet objects to XML.
? Asynchronous Delegates. Delegates in both the desktop Framework and Compact Framework can be thought of as object-oriented function pointers. They are used to encapsulate the signature and address of a method to invoke at runtime. While delegates can be called synchronously, they cannot be invoked asynchronously and passed a callback method in the Compact Framework. However, it should be noted that asynchronous operations are supported for some of the networking functionality found in the System.Net namespace and when calling XML Web Services. In other cases, direct manipulation of threads or the use of a thread pool is required.
? .NET Remoting. In the desktop Framework, it is possible to create applications that communicate with each other across application domains using classes in the System.Runtime.Remoting namespace. This technique allows for data and objects serialized to SOAP or a binary format to be transmitted using TCP or HTTP. This functionality is not supported (in part because Generic Serialization is not supported) in the Compact Framework where instead XML Web Services and IrDA can be used.
? Reflection Emit. Although the Compact Framework does support runtime type inspection using the System.Reflection namespace, it does not support the ability to emit dynamically created MSIL into an assembly for execution.
? Printing. Although the Compact Framework does support graphics and drawing, it does not support printing through the System.Drawing.Printing namespace.
? XPath/XSLT. Support for XML is included in the Compact Framework and allows developers to read and write XML documents using the XmlDocument, XmlReader, and XmlWriter classes. However, it does not support executing XPath queries or performing XSL transformations.
? Server-side Programming Models. As you would expect, the Compact Framework also does not support the server-side programming models including System.EnterpriseServices (COM+), System.Management (WMI), and System.Messaging (MSMQ).
? Multi-module Assemblies. The desktop Framework supports the ability to deploy an assembly as a collection of files. This is useful for creating assemblies authored with multiple languages. This feature is not supported in the Compact Framework where a single file (.exe or .dll) represents the entire assembly.
However, the Compact Framework does support two additional namespaces to support features particular to smart devices (see Table 1).
In addition, support for the IrDA protocol has been included in the Compact Framework and exposed in the six classes found in the System.Net.Sockets and the System.Net namespaces.
What Do Developers Need to Learn to Use the Compact Framework Effectively?
Answer: Although VB 6.0 and embedded VB (eVB) developers will feel fairly comfortable with the VB syntax used in SDP, the object-oriented nature of the Compact Framework means that developers need to have a firm grounding in OO concepts in order to maximize their productivity.
For example, the class libraries in the Compact Framework make extensive use of both implementation and interface inheritance. Developers familiar with these concepts can not only more easily understand the framework, but can write polymorphic code using the framework that is more maintainable and flexible than the component based development typically done in eVB. In the long run, using OO in their own designs allows developers to write less code and make the code that they do write more reusable. Therefore, to take maximum advantage of OO, however, a development organization should also get up to speed on the use of design patterns.
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