HRESULT Return Type?

you can't pass a CString as a parameter across COM, change it to a BSTR, using CComBSTR to wrap the CString at the calling end, and CString to wrap the BSTR on the callee end

so it becomes

[ id( 1 ), helpstring( "Chat" )] HRESULT DumpMessage([in]BSTR Message );

caller

CString msg(_T("hello world"));
object->DumpMessage(CComBSTR(msg));

callee

STDMETHOD(DumpMessage)(BSTR Message)
{
 CString msg(Message);
 OutputDebugString(msg);
}