Integrating with COM Components In C#

Introduction

The CLR provides support both for exposing C# objects as COM objects and for using COM objects from C#. Additionally, CLR components can make use of COM+ services and can be used as configured components by CLR and classic COM applications.
Binding COM and C# Objects

Interoperating between COM and C# works through either early or late binding. Early binding allows you to program with types known at compile time, while late binding forces you to program with types via dynamic discovery, using reflection on the C# side and IDispatch on the COM side.

When calling COM programs from C#, early binding works by providing metadata in the form of an assembly for the COM object and its interfaces. TlbImp.exe takes a COM type library and generates the equivalent metadata in an assembly. With the generated assembly, it's possible to instantiate and call methods on a COM object just as you would on any other C# object.

When calling C# programs from COM, early binding works via a type library. Both TlbExp.exe and RegAsm.exe allow you to generate a COM type library from your assembly. You can then use this type library with tools that support early binding via type libraries such as Visual Basic 6.
Exposing COM Objects to C#

When you instantiate a COM object, you are actually working with a proxy known as the Runtime Callable Wrapper (RCW). The RCW is responsible for managing the lifetime requirements of the COM object and translating the methods called on it into the appropriate calls on the COM object. When the garbage collector finalizes the RCW, it releases all references to the object it was holding. For situations in which you need to release the COM object without waiting for the garbage collector to finalize the RCW, you can use the static ReleaseComObject method of the System.Runtime.InteropServices.Marshal type.

The following example demonstrates how to change your MSN Instant Messenger friendly name using C# via COM Interop:

// RenameMe.cs - compile with:
// csc RenameMe.cs /r:Messenger.dll
// Run RenameMe.exe "new name" to change your name
// as it is displayed to other users.
// Run TlbImp.exe "C:\Program Files\Messenger\msmsgs.exe"
// to create Messenger.dll
using System;
using Messenger;
class MSNFun {

static void Main(string[ ] args) {

MsgrObject mo = new MsgrObject( );
IMsgrService ims = mo.Services.PrimaryService;
ims.FriendlyName = args[0];

}

}

You can also work with COM objects using the reflection API. This is more cumbersome than using TlbImp.exe, but is handy in cases in which it's impossible or inconvenient to run TlbImp.exe. To use COM through reflection, you have to get a Type from Type.GetTypeFromProgID() for each COM type you want to work with. Then, use Activator.CreateInstance() to create an instance of the type. To invoke methods or set or get properties, use the reflection API:

using System;
using System.Reflection;
public class ComReflect {

public static void Main( ) {

object obj_msword; // Microsoft Word Application
Type wa = Type.GetTypeFromProgID("Word.Application", true);

// Create an instance of Microsoft Word
obj_msword = Activator.CreateInstance(wa);

// Use the reflection API from here on in...

}

}

Exposing C# Objects to COM

Just as an RCW proxy wraps a COM object when you access it from C#, code that accesses a C# object as a COM object must do so through a proxy as well. When your C# object is marshaled out to COM, the runtime creates a COM Callable Wrapper (CCW). The CCW follows the same lifetime rules as other COM objects, and as long as it is alive, a CCW maintains a traceable reference to the object it wraps. This keeps the object alive when the garbage collector is run.

The following example shows how you can export both a class and an interface from C# and control the Global Unique Identifiers (GUIDs) and Dispatch IDs (DISPIDs) assigned. After compiling IRunInfo and StackSnapshot, you can register both using RegAsm.exe.

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