Skip to main content

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.
Anonymous said…
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

Popular posts from this blog

sizeof vs Marshal.SizeOf !!!

There are two facilities in C# to determine the size of a type - sizeof operator and Marshal.SizeOf method. Let me discuss what they offer and how they differ. Pardon me if I happen to ramble a bit. Before we settle the difference between sizeof and Marshal.SizeOf , let us discuss why would we want to compute the size of a variable or type. Other than academic, one typical reason to know the size of a type (in a production code) would be allocate memory for an array of items; typically done while using malloc . Unlike in C++ (or unmanaged world), computing the size of a type definitely has no such use in C# (managed world). Within the managed application, size does not matter; since there are types provided by the CLR for creating\managing fixed size and variable size (typed) arrays. And as per MSDN, the size cannot be computed accurately. Does that mean we don't need to compute the size of a type at all when working in the CLR world? Obviously no, else I would

out, ref and InvokeMember !!!

When I was working on the .NET reflection extravaganza thing that I explained in my previous column, i learnt one another interesting thing, that is about the Type.InvokeMember. How will pass out or ref parameters for the method invoked using Type.InvokeMember ? If you are going to invoke a method with the prototype int DoSomething(string someString, int someInt); then you would use InvokeMember like this:- object obj = someType.InvokeMember("DoSomething", BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance, null, this, new object[] {"Largest Integer", 1}); or use some variables in the new object[] {...}. But what do you with the args if DoSomething takes out or ref parameters ? int DoSomething(out string someString, ref int someInt); Something like this will not work string someText = string.Empty; int someInt = 0; object obj = someType.InvokeMember("DoSomething", BindingFlags.Public | BindingFlag