I need to be able to pass streams across the Interop boundary. Using the
article http://support.microsoft.com/default.aspx?scid=kb;en-us;321695 I can
call into and ATL COM object that has a method

   STDMETHODIMP CSimpleObj::GetUnmanagedData(IStream** ppData)

from .NET using the following C#

   UnmanagedFuncs.IStream stream;
   UnmanagedFuncs.ISimpleObj obj = new UnmanagedFuncs.CSimpleObjClass();
   obj.GetUnmanagedData( out stream );
   UCOMIStream uStream = stream as UCOMIStream;
   ComStream ComStream = new ComStream( uStream );

and I now have my IStream data as a UCOMIStream in a Stream derived .NET
class.

However I'm having problems going the other way, ie passing stream data into
COM. If my method is

   STDMETHODIMP CSimpleObj::SetUnmanagedData(IStream* pData)

and my C# is
   System.IO.MemoryStream mStream2 = new System.IO.MemoryStream();
   System.IO.BinaryWriter bw = new System.IO.BinaryWriter( mStream2 );
   bw.Write( "Hello!" );
   ManagedIStream miStream2 = new ManagedIStream( mStream2 );
   UCOMIStream uStream2 = miStream2 as UCOMIStream;
   obj.SetUnmanagedData( uStream2 );

then it won't compile, with the error cannot convert from
'System.Runtime.InteropServices.UCOMIStream' to 'UnmanagedHelpers.IStream'.
How can I convince the compiler that my UnmanagedFuncs.IStream is really
just a bog standard IStream?

Cheers,

Chris