Creating a delegate in .NET is extremely simple. However, in a rare case, you might have to create an object of the .NET
delegate type with an unmanaged function pointer (in unmanaged world). This article explains a simple way to do the same.
The problem I faced was:
In an ATL COM environment, I had to create an object of the .NET
AsyncCallback. In .NET, to create an object of this kind and assign a method to it is simple, by using a parameterized constructor. But in unmanaged COM world, parameterized constructors are not available, and it would be very difficult to do the same.
I solved this problem by creating a helper class in .NET which wraps an unmanaged function pointer (sent from the unmanaged world) as an
AsyncCallback delegate. This delegate is then returned to the unmanaged world.
Using the code
Step 1. In .NET, create an interface
IDelegateHelper. This interface provides a method
AsyncCallbackDelegateWrapper that takes in a function pointer to an unmanaged function whose delegate is to be created.
DelegateHelperCLass implements this interface. The method definition involves using
Marshal.GeDelegateForFunctionPointer available in .NET to create a delegate out of the function pointer. The interface is provided for ease of use in the COM unmanaged world.
public interface IDelegateHelper
public class DelegateHelperCLass : IDelegateHelper
Delegate aAsyncCallbackDelegate =
return aAsyncCallbackDelegate as AsyncCallback;
Step 2. Export this .NET DLL into a TLB for use in COM. Use the tlbexp.exe utility to do the same. Register the .NET DLL using regasm.
Step 3. To use this in the unmanaged world (COM C++, here), create an instance of
CoCreateInstance. Then call the method
AsynCallbackDelegateWrapper, passing the function pointer of the method whose delegate is to be created. The result obtained is a .NET
//Import the TLBs
named_guids, rename("_Module", "ReflectionModule"),
raw_interfaces_only named_guids, no_namespace
//Add this to the library block in IDL
//file for recognizing IAsyncResult*
//Create a function pointer FuncPtr that has
//a signature same as that of the delgate methods
//CClient code that calls the helper methods
//First create an instance of DelegateHelper class
HRESULT hr =
//AsyncCallback delegate instnace
//that is to be populated with
_AsyncCallback* aCallBack = NULL;
FuncPtr aPtr = TestMethod;
hr = aHelper->AsynCallbackDelegateWrapper((long)aPtr,
//Now aCallBack can be used like
//a regular AsyncCallback instance!!
//Some static method whose delegate is to be created
void CClient::TestMethod(IAsyncResult* asyncResult_in)
//Your Callback code