The common language runtime exposes COM objects through a proxy called the runtime callable wrapper (RCW). Although the RCW appears to be an ordinary object to .NET clients its primary function is to marshal calls between a .NET client and a COM object. The runtime creates exactly one RCW for each COM object regardless of the number of references that exist on that object. The runtime maintains a single RCW per process for each object. If you create an RCW in one application domain or apartment and then pass a reference to another application domain or apartment a proxy to the first object will be used. As the following illustration shows any number of managed clients can hold a reference to the COM objects that expose INew and INewer interfaces. http://msdn.microsoft.com/en-us/library/8bwh56xe.aspx http://en.wikipedia.org/wiki/Runtime_Callable_Wrapper http://www.codeproject.com/Articles/5001/NET-COM-Interoperability