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Implementing COM OutOfProc Servers in C# .NET !!!

Had to implement our COM OOP Server project in .NET, and I found this solution from the internet after a great deal of search, but unfortunately the whole idea was ruled out, and we wrapped it as a .NET assembly. This is worth knowing.

Step 1:

Implement IClassFactory in a class in .NET. Use the following definition for IClassFactory.

namespace COM
   static class Guids
      public const string IClassFactory = "00000001-0000-0000-C000-000000000046";
      public const string IUnknown = "00000000-0000-0000-C000-000000000046";
   /// IClassFactory declaration
   [ComImport(), InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid(COM.Guids.IClassFactory)]
   internal interface IClassFactory
      int CreateInstance(IntPtr pUnkOuter, ref Guid riid, out IntPtr ppvObject);
      int LockServer(bool fLock);

Step 2:

private static extern int CoRegisterClassObject(ref Guid rclsid,
[MarshalAs(UnmanagedType.Interface)]IClassFactory pUnkn,
int dwClsContext,
int flags,
out int lpdwRegister);

private static extern int CoRevokeClassObject(int dwRegister);

Step 3:

Use these functions to register your own IClassFactory

Step 4:

IClassFactory has a CreateInstance method. Implement this method to return a reference (IntPtr) to your own object. Use Marshal.GetIUnknownForObject to get IUnknown pointer to your object.

Step 5:

The COM client receives a pointer to this object, and can use it as a regular COM object. .NET does the reference counting for you, and the GC will collect these objects when the COM-reference-count decremetns to zero.

Walking through and closely examining the working of ClassFactories for COM will give a clear sight of the objects that you need to implement in .NET, and a solution for COM Server in managed world.


Anonymous said…
Can you give more details about how to implement the IClassFactory.
Updated the post with IClassFactory definition, which can be used to derive and implement the class factory.
Anonymous said…
Thanks a lot. Can you provide more about how to implement the IClassFactory. I tried to call Marshal.GetIUnknownForObject but Calling from native client always invokes to wrong method of .net object.
Marshal.GetComInterfaceForObject seems to work:
public void CreateInstance(object pUnkOuter, Guid riid, out IntPtr ppvObject)
ppvObject = Marshal.GetComInterfaceForObject(theObject, typeof(IDynRange)); // works
// ppvObject = Marshal.GetIUnknownForObject(theObject as IDynRange); // does not work

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