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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
   {
      [PreserveSig]
      int CreateInstance(IntPtr pUnkOuter, ref Guid riid, out IntPtr ppvObject);
      [PreserveSig]
      int LockServer(bool fLock);
   }
}

Step 2:

[DllImport("ole32.dll")]
private static extern int CoRegisterClassObject(ref Guid rclsid,
[MarshalAs(UnmanagedType.Interface)]IClassFactory pUnkn,
int dwClsContext,
int flags,
out int lpdwRegister);

[DllImport("ole32.dll")]
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.

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Comments

Anonymous said…
Can you give more details about how to implement the IClassFactory.
Thanks
ilatavor@gmail.com
Friday, February 27, 2009 9:51:00 PM
Anonymous said…
Updated the post with IClassFactory definition, which can be used to derive and implement the class factory.
Saturday, February 28, 2009 2:03:00 AM
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.
Calling
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
}

ilatavor@gmail.com
Saturday, February 28, 2009 10:44:00 PM
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