I'm trying to call a .NET 3.5 C# dll from a VB 6 GUI. I added the .NET 3.5 dll as a reference to the VB6 Project. The .NET dll is compiled with COM Interop enabled. If I compile the .NET dll in Visual Studio I have no problem with the VB6 GUI using the .NET dll. However when I put the .NET csproj in a buld script that compiles all my .NET dlls with MSBUILD.EXE I get a "Runtime error '429': Active X component can't create object" when the VB6 GUI tries to access it. I'm not sure what's the difference in compiling the .NET dll within Visual Studio 2010 and using MSBUILD. Is Visual Studio 2010 more forgiving? I tried to manually register it using regsvr32 and regasm but no luck Could anyone point me in the right direction? I'm on Windows 7. Thanks.