COM Interop - Problems with marshalling .NET object to C++ VARIANT pointer

[sergmykhailov]

I have a COM interface declared in C++

// This interface has all required attributes to make it visible for .NET Interop so I skip them.
 
class IMyInterface
{
public:     
      HRESULT MyMethod(int param1, VARIANT param2, VARIANT* param3) = 0;
};
 

This interface is implemented in C++.

class MyInterfaceImpl : IMyInterface
{
public:     
      HRESULT MyMethod(int param1, VARIANT param2, VARIANT* param3)
{
      // Do some activities here…
      Print(param1);    // Let’s say that Print() is a function which is able to display the content of any type
      Print(param2);
      Print(*param3);
 
      *param3 = 4444;   // Assign value and pass it back to caller.
};
};
 

I also have this interface declaration in .NET assembly like shown below:

[ComImport]
[Guid("11111111-1111-1111-1111-111111111111")] // Some GUID
[TypeLibType(4160)]
[ComVisible(true)]
public interface IMyInterface
{
      [DispId(1)] void MyMethod(int param1, object param2, out object param3);
}
 

This interface is also implemented in C#. I’m facing the problem with passing ‘param3’ from .NET to C++. For example I have code:

// .NET code
IMyInterface interfacePtr;   
// Initialize ‘interfacePtr’. This part works fine so I’ll skip it.
 
int param1 = 1111;
object param2 = 2222;
object param3 = 3333;
interfacePtr.MyMethod(param1, param2, out param3);
Print(param3);
 

The output of this program looks like:

================================Output============ ===============
1111
2222
Empty
4444
================================Output============ ===============

If I put a breakpoint at C++ code (i.e. in MyInterfaceImpl::MyMethod() function) I see that ‘param1’ is equal to ‘1111’, param2 is equal to ‘2222’ but param3 is always empty. But when param3 is assigned ‘4444’ value on C++ side .NET side successfully accepts it. As you can the problem occurs when I pass ‘param3’ – somehow C++ side is not able to receive/read it but it’s able to assign the appropriate value.

Any ideas what I’m doing wrong and how I can resolve the situation to make output look like:

================================Output============ ===============
1111
2222
3333
4444
================================Output============ ===============

Thanks in advance

Best regards
Serg