C# in a Nutshell, Second Edition

Chapter 18. Integrating with COM Components

The CLR provides support both for exposing C# objects as COM objects and for using COM objects from C#. Additionally, CLR components can make use of COM+ services and can be used as configured components by CLR and classic COM applications.

Binding COM and C# Objects

Interoperating between COM and C# works through either early or late binding. Early binding allows you to program with types known at compile time, while late binding forces you to program with types via dynamic discovery, using reflection on the C# side and IDispatch on the COM side.

When calling COM programs from C#, early binding works by providing metadata in the form of an assembly for the COM object and its interfaces. TlbImp.exe takes a COM type library and generates the equivalent metadata in an assembly. With the generated assembly, it’s possible to instantiate and call methods on a COM object just as you would on any other C# object.

When calling C# programs from COM, early binding works via a type library. Both TlbExp.exe and RegAsm.exe allow you to generate a COM type library from your assembly. You can then use this type library with tools that support early binding via type libraries such as Visual Basic 6.