Thursday, July 17, 2008

.Net Faq's

1. Introduction
1.1 What is .NET?
That's difficult to sum up in a sentence. According to Microsoft, .NET is a "revolutionary new platform, built on open Internet protocols and standards, with tools and services that meld computing and communications in new ways".

A more practical definition would be that .NET is a new environment for developing and running software applications, featuring ease of development of web-based services, rich standard run-time services available to components written in a variety of programming languages, and inter-language and inter-machine interoperability.

Note that when the term ".NET" is used in this FAQ it refers only to the new .NET runtime and associated technologies. This is sometimes called the ".NET Framework". This FAQ does NOT cover any of the various other existing and new products/technologies that Microsoft are attaching the .NET name to (e.g. SQL Server.NET).

1.2 Is .NET just a new name for Windows DNA?
No. In many ways, the term Windows DNA was just a marketing term for an approach (i.e. the 3-tier approach) to using existing technology. .NET is much more radical and includes a complete new software development and runtime framework.

1.3 Does .NET only apply to people building web-sites?
No. If you write any Windows software (using ATL/COM, MFC, VB, or even raw Win32), .NET may offer a viable alternative (or addition) to the way you do things currently. Of course, if you do develop web sites, then .NET has lots to interest you - not least ASP+.

1.4 When was .NET announced?
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 7.

1.5 When will .NET be released?
No firm date yet, but it is expected in the second half of 2001.

1.6 How do I develop .NET apps?
The .NET Framework SDK contains command-line compilers and utilities which can be used to build .NET apps. The next version of Visual Studio (called Visual Studio 7 or Visual Studio.NET) will have fully integrated support for .NET development.

1.7 Where can I download the .NET SDK & Visual Studio 7?
You can download Beta 1 of the SDK from If you are an MSDN Universal subscriber, you can also download Beta 1 of Visual Studio 7.

1.8 What are the key technologies within .NET?
ASP.NET, CLR (Common Language Runtime), C# (a new Java-like language), SOAP, XML, ADO.NET, multi-language support (Eiffel, COBOL etc)

1.9 What platforms will the .NET Framework run on?
Beta 1 supports Windows 2000, NT4 SP6a, Windows Me and Windows 98 for development. The runtime is supported on Windows 95.

Microsoft are due to release a new version of Windows in a similar timeframe to the .NET runtime. It is codenamed 'Whistler' and is largely an incremental update to Windows 2000, with an emphasis on GUI changes. Microsoft will market this new OS as '.NET-enabled', but apparently it is not tied to the release of the .NET runtime. If the .NET runtime is ready in time it will be included with Whistler. If the .NET runtime is not ready, Whistler will ship without it.

1.10 What languages will the .NET Framework support?
Initially MS will provide compilers for C#, C++, VB and JScript. Other vendors have announced that they intend to develop .NET compilers for languages such as COBOL, Eiffel, Perl, Smalltalk and Python.

1.11 Will the .NET Framework go through a standardisation process?
Proposed draft standards for C# and something called the 'Common Language Infrastructure' have been submitted to ECMA. See

2. Basic terminology
2.1 What is the CLR?
CLR = Common Language Runtime. The CLR is a set of standard resources that (in theory) any .NET program can take advantage of, regardless of programming language. Robert Schmidt (Microsoft) lists the following CLR resources in his MSDN PDC# article:

Object-oriented programming model (inheritance, polymorphism, exception handling, garbage collection)
Security model
Type system
All .NET base classes
Many .NET framework classes
Development, debugging, and profiling tools
Execution and code management
IL-to-native translators and optimizers

What this means is that in the .NET world, different programming languages will be more equal in capability than they have ever been before, although clearly not all languages will support all CLR services.

2.2 What is the CTS?
CTS = Common Type System. This is the range of types that the .NET runtime understands, and therefore that .NET applications can use. However note that not all .NET languages will support all the types in the CTS. The CTS is a superset of the CLS.

2.3 What is the CLS?
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.

In theory this allows very tight interop between different .NET languages - for example allowing a C# class to inherit from a VB class.

2.4 What is IL?
IL = Intermediate Language. Also known as MSIL. All .NET source code (of any language) is compiled to IL. The IL is then converted to machine code at the point where the software is installed, or at run-time by a Just-In-Time (JIT) compiler.

2.5 What is C#?
C# is a new language which will run inside 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++."

Substitute 'Java' for 'C#' in the quote above, and you'll see that the statement still works pretty well :-).

If you are a C++ programmer, you might like to check out my C# FAQ.

2.6 What does 'managed' mean in the .NET context?
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. All C#, Visual Basic.NET and JScript.NET code is managed by default. VS7 C++ code is not managed by default, but the compiler can produce managed code by specifying a command-line switch (/com+).

Managed data: This is data that is allocated and de-allocated by the .NET runtime's garbage collector. C#, VB.NET and JScript.NET data is always managed. VS7 C++ data is unmanaged by default, even when using the /com+ switch, but it can be marked as managed using the __gc keyword.

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.

2.7 What is reflection?
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 (see System.Type.InvokeMember), or even create types dynamically at run-time (see System.Reflection.Emit.TypeBuilder).

3. Assemblies
3.1 What is an assembly?
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.

Finally, assemblies are the unit of versioning in .NET - more on this below.

3.2 How can I produce an assembly?
The simplest way to produce an assembly is directly from a .NET compiler. For example, the following C# program:

public class CTest
public CTest()
System.Console.WriteLine( "Hello from CTest" );

can be compiled into a library assembly (dll) like this:

csc /t:library ctest.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.

3.3 What is the difference between a private assembly and a shared assembly?
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.

3.4 How do assemblies find each other?
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.

3.5 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 (e.g. 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.

4. Application Domains
4.1 What is an Application Domain?
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 an application from other applications.

Win32 processes provide isolation by having distinct memory address spaces. This is effective, but it is expensive and doesn't scale well. 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.

4.2 How does an AppDomain get created?
AppDomains are usually created by hosts. Examples of hosts are the Windows Shell, ASP+ 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. Here is a C# sample which creates an AppDomain, creates an instance of an object inside it, and then executes one of the object's methods:

using System;
using System.Runtime.Remoting;

public class CAppDomainInfo : MarshalByRefObject
public string GetAppDomainInfo()
return "AppDomain = " + AppDomain.CurrentDomain.FriendlyName;


public class App
public static int Main()
AppDomain ad = AppDomain.CreateDomain( "Andy's new domain", null, null );
ObjectHandle oh = ad.CreateInstance( "appdomaintest.exe", "CAppDomainInfo" );
CAppDomainInfo adInfo = (CAppDomainInfo)(oh.Unwrap());
string info = adInfo.GetAppDomainInfo();

Console.WriteLine( "AppDomain info: " + info );
return 0;

4.3 Can I write my own .NET host?
Yes. For an example of how to do this, take a look at the source for the moniker developed by Jason Whittington and Don Box ( There is also a code sample in the .NET SDK called CorHost.

5. Garbage Collection
5.1 What is garbage collection?
Garbage collection is a system whereby a run-time component takes responsibility for managing the lifetime of objects and the heap memory that they occupy. This concept is not new to .NET - Java and many other languages/runtimes have used garbage collection for some time.

5.2 Is it true that objects don't always get destroyed immediately when the last reference goes away?
Yes. The garbage collector offers no guarantees about the time when an object will be destroyed and its memory reclaimed.

There is an interesting thread in the archives, started by Chris Sells, about the implications of non-deterministic destruction of objects in C#:

In October 2000, Microsoft's Brian Harry posted a lengthy analysis of the problem:

Chris Sells' response to Brian's posting is here:

5.3 Why doesn't the .NET runtime offer deterministic destruction?
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.

Futhermore, 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.

5.4 Is the lack of deterministic destruction in .NET a problem?
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 for the client 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.

5.5 Does non-deterministic destruction affect the usage of COM objects from managed code?
Yes. When using a COM object from managed code, you are effectively relying on the garbage collector to call the final release on your object. If your COM object holds onto an expensive resource which is only cleaned-up after the final release, you may need to provide a new interface on your object which supports an explicit Dispose() method.

5.6 I've heard that Finalize methods should be avoided. Should I implement Finalize on my class?
An object with a Finalize method is more work for the garbage collector than an object without one. Also there are no guarantees about the order in which objects are Finalized, so there are issues surrounding access to other objects from the Finalize method. Finally, there is no guarantee that a Finalize method will get called on an object, so it should never be relied upon to do clean-up of an object's resources.

Microsoft recommend the following pattern:

public class CTest
public override void Dispose()
... // Cleanup activities

protected override void Finalize()

In the normal case the client calls Dispose(), the object's resources are freed, and the garbage collector is relieved of its Finalizing duties by the call to SuppressFinalize(). In the worst case, i.e. the client forgets to call Dispose(), there is a reasonable chance that the object's resources will eventually get freed by the garbage collector calling Finalize(). Given the limitations of the garbage collection algorithm this seems like a pretty reasonable approach.

5.7 Do I have any control over the garbage collection algorithm?
A little. The System.GC class exposes a couple of interesting methods. The first is the Collect method - this forces the garbage collector to collect all unreferenced objects immediately. The other is RequestFinalizeOnShutdown(), which tells the garbage collector to definitely run the Finalize() method of each object on shutdown of the application. Normally the garbage collector puts a speedy exit at a higher priority than calling Finalize during application shutdown, so this method could be handy to force the runtime to be a little more responsible.

If you want to verify that this is not just a theoretical issue, try the following test program:

using System;

class CTest
protected override void Finalize()
Console.WriteLine( "This is the Finalizer." );

class CApplication
public static void Main()
Console.WriteLine( "This is Main." );
CTest test = new CTest();

// GC.RequestFinalizeOnShutdown();

Run the program, then re-run with the GC.RequestFinalizeOnShutdown() line un-commented. You should notice a difference ...

5.8 How can I find out what the garbage collector is doing?
Lots of interesting statistics are exported from the .NET runtime via the 'COM+ Memory' performance object. Use Performance Monitor to view them.

6. Attributes
6.1 What are attributes?
There are at least two types of .NET attribute. The first type I will 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 CTest {}

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. If you've come across Keith Brown's universal delegator you'll be familiar with this idea.

6.2 Can I create my own metadata attributes?
Yes. Simply derive a class from System.Attribute and mark it with the AttributeUsage attribute. For example:

public class InspiredByAttribute : System.Attribute
public string InspiredBy;

public InspiredByAttribute( string inspiredBy )
InspiredBy = inspiredBy;

[InspiredBy("Andy Mc's brilliant .NET FAQ")]
class CTest

class CApp
public static void Main()
object[] atts = typeof(CTest).GetCustomAttributes();

foreach( object att in atts )
if( att is InspiredByAttribute )
Console.WriteLine( "Class CTest was inspired by {0}", ((InspiredByAttribute)att).InspiredBy );

6.3 Can I create my own context attributes?
Yes. Take a look at Don Box's sample (called CallThreshold) at, and also Peter Drayton's Tracehook.NET at

7. Code Access Security
7.1 What is Code Access Security (CAS)?
CAS is the part of the .NET security model that determines whether or not a piece of 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.

7.2 How does CAS work?
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.)

7.3 Who defines the CAS code groups?
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. On my system it looks like this:

Level = Machine

Code Groups:

1. All code: Nothing
1.1. Zone - MyComputer: FullTrust
1.1.1. Honor SkipVerification requests: SkipVerification
1.2. Zone - Intranet: LocalIntranet
1.3. Zone - Internet: Internet
1.4. Zone - Untrusted: Nothing
1.5. Zone - Trusted: Internet
1.6. StrongName - 0024000004800000940000000602000000240000525341310004000003
AC1DF1734633C602F8F2D5: Everything

Note the hierarchy of code groups - the top of the hierarchy is the most general ('All code'), which is then sub-divided into several groups, each of which in turn can be sub-divided. Also note that (somewhat counter-intuitively) a sub-group can be associated with a more permissive permission set than its parent.

7.4 How do I define my own code group?
Use caspol. For example, suppose you trust code from and you want it have full access to your system, but you want to keep the default restrictions for all other internet sites. To achieve this, you would add a new code group as a sub-group of the 'Zone - Internet' group, like this:

caspol -ag 1.3 -site FullTrust

Now if you run caspol -lg you will see that the new group has been added as group 1.3.1:

1.3. Zone - Internet: Internet
1.3.1. Site - FullTrust

Note that the numeric label (1.3.1) is just a caspol invention to make the code groups easy to manipulate from the command-line. The underlying runtime never sees it.

7.5 How do I change the permission set for a code group?
Use caspol. If you are the machine administrator, you can operate at the 'machine' level - which means not only that the changes you make become the default for the machine, but also that users cannot change the permissions to be more permissive. If you are a normal (non-admin) user you can still modify the permissions, but only to make them more restrictive. For example, to allow intranet code to do what it likes you might do this:

caspol -cg 1.2 FullTrust

Note that because this is more permissive than the default policy (on a standard system), you should only do this at the machine level - doing it at the user level will have no effect.

7.6 Can I create my own permission set?
Yes. Use caspol -ap, specifying an XML file containing the permissions in the permission set. To save you some time, here is a sample file corresponding to the 'Everything' permission set - just edit to suit your needs. When you have edited the sample, add it to the range of available permission sets like this:

caspol -ap samplepermset.xml

Then, to apply the permission set to a code group, do something like this:

caspol -cg 1.3 SamplePermSet

(By default, 1.3 is the 'Internet' code group)

7.7 I'm having some trouble with CAS. How can I diagnose my problem?
Caspol has a couple of options that might help. First, you can ask caspol to tell you what code group an assembly belongs to, using caspol -rsg. Similarly, you can ask what permissions are being applied to a particular assembly using caspol -rsp.

7.8 I can't be bothered with all this CAS stuff. Can I turn it off?
Yes, as long as you are an administrator. Just run:

caspol -s off

8. Intermediate Language (IL)
8.1 Can I look at the IL for an assembly?
Yes. MS supply a tool called Ildasm which can be used to view the metadata and IL for an assembly.

8.2 Can source code be reverse-engineered from IL?
Yes, it is often relatively straightforward to regenerate high-level source (e.g. C#) from IL.

8.3 How can I stop my code being reverse-engineered from IL?
The only method currently available is to run ilasm with the /owner option. The IL for the resulting assembly cannot be viewed with ildasm. However, a determined code-thief could hack ildasm or write their own version of ildasm, so this method is only useful for deterring the casual thief.

Unfortunately the current .NET compilers do not have a /owner option, so to protect your C# or VB.NET assembly you'll need to do something like this:

csc helloworld.cs
ildasm / helloworld.exe
ilasm /owner

(This suggestion was posted to the DOTNET list by Hany Ramadan.)

In the longer term it is likely that IL obfuscation tools will become available (either from MS or from third parties). 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.

8.4 Can I write IL programs directly?
Yes. Peter Drayton posted this simple example to the DOTNET mailing list:

.assembly MyAssembly {}
.class MyApp {
.method static void Main() {
ldstr "Hello, IL!"
call void System.Console::WriteLine(class System.Object)

Just put this into a file called, and then run ilasm An exe assembly will be generated.

8.5 Can I do things in IL that I can't do in C#?
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.

9. Implications for COM
9.1 Is COM dead?
This subject causes a lot of controversy, as you'll see if you read the mailing list archives. Take a look at the following two threads:

FWIW my view is as follows: COM is many things, and it's different things to different people. But to me, COM is fundamentally about how little blobs of code find other little blobs of code, and how they communicate with each other when they find each other. COM specifies precisely how this location and communication takes place. In a 'pure' .NET world, consisting entirely of .NET objects, little blobs of code still find each other and talk to each other, but they don't use COM to do so. They use a model which is similar to COM in some ways - for example, type information is stored in a tabular form packaged with the component, which is quite similar to packaging a type library with a COM component. But it's not COM.

So, does this matter? Well, I don't really care about most of the COM stuff going away - I don't care that finding components doesn't involve a trip to the registry, or that I don't use IDL to define my interfaces. But there is one thing that I wouldn't like to go away - I wouldn't like to lose the idea of interface-based development. COM's greatest strength, in my opinion, is its insistence on a cast-iron separation between interface and implementation. Unfortunately, the .NET framework seems to make no such insistence - it lets you do interface-based development, but it doesn't insist. Some people would argue that having a choice can never be a bad thing, and maybe they're right, but I can't help feeling that maybe it's a backward step.

9.2 Is DCOM dead?
Pretty much, for .NET developers. The .NET Framework has a new remoting model which is not based on DCOM. Of course DCOM will still be used in interop scenarios.

9.3 Is MTS/COM+ dead?
No. The approach for the first .NET release is 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 are provided to try to make this as painless as possible. The PDC release of the .NET SDK includes interop support for core services (JIT activation, transactions) but not some of the higher level services (e.g. COM+ Events, Queued components).

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.

For more on this topic, search for postings by Joe Long in the archives - Joe is the MS group manager for COM+. Start with this message:

9.4 Can I use COM components from .NET programs?
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.

Here's a simple example for those familiar with ATL. First, create an ATL component which implements the following IDL:

import "oaidl.idl";
import "ocidl.idl";

helpstring("ICppName Interface"),

interface ICppName : IUnknown
[helpstring("method SetName")] HRESULT SetName([in] BSTR name);
[helpstring("method GetName")] HRESULT GetName([out,retval] BSTR *pName );

helpstring("cppcomserver 1.0 Type Library")
helpstring("CppName Class")
coclass CppName
[default] interface ICppName;

When you've built the component, you should get a typelibrary. Run the TLBIMP utility on the typelibary, like this:

tlbimp cppcomserver.tlb

If successful, you will get a message like this:

Typelib imported successfully to CPPCOMSERVERLib.dll

You now need a .NET client - let's use C#. Create a .cs file containing the following code:

using System;

public class MainApp
static public void Main()
CppName cppname = new CppName();
cppname.SetName( "bob" );
Console.WriteLine( "Name is " + cppname.GetName() );

Note that we are using the type library name as a namespace, and the COM class name as the class. Alternatively we could have used CPPCOMSERVERLib.CppName for the class name and gone without the using CPPCOMSERVERLib statement.

Compile the C# code like this:

csc /r:cppcomserverlib.dll csharpcomclient.cs

Note that the compiler is being told to reference the DLL we previously generated from the typelibrary using TLBIMP.

You should now be able to run csharpcomclient.exe, and get the following output on the console:

Name is bob

9.5 Can I use .NET components from COM programs?
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.

Here's a simple example. Create a C# file called testcomserver.cs and put the following in it:

using System;

namespace AndyMc
public class CSharpCOMServer
public CSharpCOMServer() {}
public void SetName( string name ) { m_name = name; }
public string GetName() { return m_name; }
private string m_name;

Then compile the .cs file as follows:

csc /target:library testcomserver.cs

You should get a dll, which you register like this:

regasm testcomserver.dll /tlb:testcomserver.tlb

Now you need to create a client to test your .NET COM component. VBScript will do - put the following in a file called comclient.vbs:

Dim dotNetObj
Set dotNetObj = CreateObject("AndyMc.CSharpCOMServer")
dotNetObj.SetName ("bob")
MsgBox "Name is " & dotNetObj.GetName()

and run the script like this:

wscript comclient.vbs

And hey presto you should get a message box displayed with the text "Name is bob".

(Note that at the time of writing there seem to be some path issues with accessing .NET classes as COM components - to avoid problems, run comclient.vbs from the same directory as testcomserver.dll)

An alternative to the approach above it to use the moniker developed by Jason Whittington and Don Box. Go to to check it out.

9.6 Is ATL redundant in the .NET world?
Yes, if you are writing applications that live inside the .NET framework. Of course many developers may wish to continue using ATL to write C++ COM components that live outside the framework, but if you are inside you will almost certainly want to use C#. Raw C++ (and therefore ATL which is based on it) doesn't have much of a place in the .NET world - it's just too near the metal and provides too much flexibility for the runtime to be able to manage it.

10. Miscellaneous
10.1 How does .NET remoting work?
.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. This (essentially stateless) model can be made stateful in the ASP+ environment by using the ASP+ state service to store application or session state.

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.

10.2 How can I get at the Win32 API from a .NET program?
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 );

11. Class Library
11.1 File I/O
11.1.1 How do I read from a text file?
First, use a System.IO.FileStream object to open the file:

FileStream fs = new FileStream( @"c:\test.txt", FileMode.Open, FileAccess.Read );

FileStream inherits from Stream, so you can wrap the FileStream object with a StreamReader object. This provides a nice interface for processing the stream line by line:

StreamReader sr = new StreamReader( fs );
string curLine;
while( (curLine = sr.ReadLine()) != null )
Console.WriteLine( curLine );

Finally close the StreamReader object:


Note that this will automatically call Close() on the underlying Stream object, so an explicit fs.Close() is not required.

11.1.2 How do I write to a text file?
Similar to the read example, except use StreamWriter instead of StreamReader.

11.1.3 How do I read/write binary files?
Similar to text files, except wrap the FileStream object with a BinaryReader/Writer object instead of a StreamReader/Writer object.

11.1.4 How do I delete a file?
Use the static Delete() method on the System.IO.File object:

File.Delete( @"c:\test.txt" );

11.2 Text Processing
11.2.1 Are regular expressions supported?
Yes. Use the System.Text.RegularExpressions.Regex class. For example, the following code updates the title in an HTML file:

FileStream fs = new FileStream( "test.htm", FileMode.Open, FileAccess.Read );
StreamReader sr = new StreamReader( fs );

Regex r = new Regex( "(.*)" );
string s;
while( (s = sr.ReadLine()) != null )
if( r.IsMatch( s ) )
s = r.Replace( s, "New and improved ${1}" );
Console.WriteLine( s );

11.3 Internet
11.3.1 How do I download a web page?
First use the System.Net.WebRequestFactory class to acquire a WebRequest object:

WebRequest request = WebRequestFactory.Create( "http://localhost" );

Then ask for the response from the request:

WebResponse response = request.GetResponse();

The GetResponse method blocks until the download is complete. Then you can access the response stream like this:

Stream s = response.GetResponseStream();

// Output the downloaded stream to the console
StreamReader sr = new StreamReader( s );
string line;
while( (line = sr.ReadLine()) != null )
Console.WriteLine( line );

Note that WebRequest and WebReponse objects can be downcast to HttpWebRequest and HttpWebReponse objects respectively, to access http-specific functionality.

11.3.2 How do I use a proxy?
Two approaches - to affect all web requests do this:

System.Net.GlobalProxySelection.Select = new DefaultControlObject( "proxyname", 80 );

Alternatively, to set the proxy for a specific web request, do this:

ProxyData proxyData = new ProxyData();
proxyData.HostName = "proxyname";
proxyData.Port = 80;
proxyData.OverrideSelectProxy = true;

HttpWebRequest request = (HttpWebRequest)WebRequestFactory.Create( "http://localhost" );
request.Proxy = proxyData;

11.4 XML
11.4.1 Is DOM supported?
Yes. Take this example XML document:


This document can be parsed as follows:

XmlDocument doc = new XmlDocument();
doc.Load( "test.xml" );

XmlNode root = doc.DocumentElement;

foreach( XmlNode personElement in root.ChildNodes )
Console.WriteLine( personElement.FirstChild.Value.ToString() );

The output is:


11.4.2 Is SAX supported?
No. Instead, a new XmlReader/XmlWriter API is offered. Like SAX it is stream-based but it uses a 'pull' model rather than SAX's 'push' model. Here's an example:

XmlTextReader reader = new XmlTextReader( "test.xml" );

while( reader.Read() )
if( reader.NodeType == XmlNodeType.Element && reader.Name == "PERSON" )
reader.Read(); // Skip to the child text
Console.WriteLine( reader.Value );

11.4.3 Is XPath supported?
Yes, via the XmlNavigator class (DocumentNavigator derives from XmlNavigator):

XmlDocument doc = new XmlDocument();
doc.Load( "test.xml" );

DocumentNavigator nav = new DocumentNavigator(doc);

nav.Select( "descendant::PEOPLE/PERSON" );

while( nav.MoveToNextSelected() )
Console.WriteLine( "{0}", nav.Value );

11.5 Threading
11.5.1 Is multi-threading supported?
Yes, there is extensive support for multi-threading. New threads can be spawned, and there is a system-provided threadpool which applications can use.

11.5.2 How do

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