From C# how do I call an unmanaged OLE interface that requires a *VARIANT parameter?

Question

From C# I am trying to call an OLE interface written in C++. The signatures from the .ODL file of the C++ code are:

long GetData1( int* data );
long GetData2( double* data );
long GetData3( VARIANT* data );

Here is a sample of my C# code:

[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface IMyInterface
{
    [DispId(1)]
    long GetData1 ( [In, Out] ref int data );
    [DispId(1)]
    long GetData2 ( [In, Out] ref double data );
    [DispId(3)]
    long GetData3 ( [In, Out, MarshalAs(UnmanagedType.IUnknown)] ref IntPtr data);
}

GetData1() and GetData2() work fine, but GetData3() results in a "Type Mismatch" exception.

For GetData3() I have tried every combination of In, Out, Ref, MarshallAs(), C# type, etc. I can think of with no success.

Please note the C++ code can not change.

Any suggestions would be appreciated.

Answer 1

There are (at least) two ways to call a function that takes a VARIANT pointer.

The sane method is to declare the parameter as ref object data and .NET will marshal for you. This blog post has more details and a example.

The other method is to create and fill in a VARIANT struct in memory yourself and just pass the address as a IntPtr parameter. This is probably overkill and a VARIANT is big and complicated so the StructLayout for it is not going to be fun.