Question

I am trying to use InteropServices.Marshal.QueryInterface to convert a COM object to a specific type in C#, but I am having trouble to pass in the first and last parameters.

I am assuming that IntPtr is a reference, but I cannot get IntPtr to reference my classes to use QueryInterface due to syntax errors.

Of course, the code that is shown below does not have the correct syntax, but it gives you an idea of what I am trying to do.

set
{
    ClassA a;
    Guid guid = Guid.Parse("XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX");
    System.Runtime.InteropServices.Marshal.QueryInterface(ref value, 
        ref guid, out a);
}

EDIT:

[ComVisible(true), Guid("XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX")]
public ClassA : COMInterface{ ... }

ClassA is the C# class, and COMInterface is the COM interface. It is the only class that implements COMInterface. value in the set scope is the reference to the COM Interface, COMInterface

Answer 1

You’re not going to be able to do this.

The IntPtr that you have is a pointer to the implementation of the COMInterface. It’s perfectly feasible that you could have a completely separate class implementing COMInterface and that’s what you’re holding a pointer to.

That said, the call to QueryInterface can only be used to get a reference to the .NET representation of COMInterface, not an instance of ClassA; it would require you to shape the contents of the class implementing COMInterface in unmanaged code into something that managed code can understand and then write it to ClassA which is a completely different thing altogether.