18.4 COM Mapping in C#When you use a COM object from C#, the RCW makes a COM method look like a normal C# instance method. In COM, methods normally return an HRESULT to indicate success or failure and use an out parameter to return a value. In C#, however, methods normally return their result values and use exceptions to report errors. The RCW handles this by checking the HRESULT returned from the call to a COM method and throwing a C# exception when it finds a failure result. With a success result, the RCW returns the parameter marked as the return value in the COM method signature.
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