How Does .NET Seamlessly Interoperate with COM Types?

The secret of interoperability between .NET and COM types

The C# compiler exhibits extraordinary capabilities when interacting with COM (Component Object Model) types. From seamlessly instantiating interfaces to passing non-reference parameters to reference parameters, it does it all with ease.

Instancing interfaces: the magic of Type.GetTypeFromCLSID() and Activator.CreateInstance()

When you create an instance of a COM interface in C#, the compiler implicitly converts it into two separate method calls. It first calls Type.GetTypeFromCLSID() to retrieve the metadata associated with the COM interface. Subsequently, it calls Activator.CreateInstance() to instantiate an object that implements this interface. This hidden process allows you to call constructors on the interface, which seems impossible otherwise.

Passing non-reference parameters to reference parameters: Tips for local variables

In C# 4, you can easily pass non-reference parameters to reference parameters. In this case, the compiler's secret weapon is to create a local variable that stores the non-reference parameter value. This variable is then passed by reference, and the return value from the method call is effectively discarded. This is a neat trick that simplifies passing reference parameters.

Simulated interface instantiation: CoClass attribute appears

While the interface cannot be instantiated directly, there is a way to simulate it: the CoClass attribute. By applying the [CoClass(typeof(Test))] attribute to an interface, you specify a concrete class that implements the interface. When the compiler encounters this attribute, it generates code that simulates an interface instantiation by creating an instance of the specified class.

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