How Does the C# Compiler Handle COM Type Instantiation and Ref Parameter Passing?

Detecting COM types in C#

The way the C# compiler handles COM types is very unique. A notable example is the ability to instantiate interfaces, as shown in the following example:

Word.Application app = new Word.Application();

Although Application is an interface, it can be instantiated. This is achieved by converting calls to Type.GetTypeFromCLSID() and Activator.CreateInstance.

Another unique behavior in C# 4 is the ability to pass non-ref parameters to ref parameters. The compiler automatically adds a local variable to pass parameters by reference without affecting the actual value.

To understand how the compiler performs these transformations, we can create a custom example:

[ComImport, GuidAttribute("00012345-0000-0000-0000-000000000011")]
public interface Dummy
{
    void Foo(ref int x);
}

class Test
{
    static void Main()
    {
        Dummy dummy = null;
        dummy.Foo(10);
    }
}

This code will execute successfully and demonstrates how to pass the ref parameter by value.

However, let’s say we want to try instantiating the interface:

Dummy dummy = new Dummy();

While this code will not execute successfully, it highlights the need to investigate compiler "magic". To actually instantiate an interface we can use the CoClass attribute:

[System.Runtime.InteropServices.CoClass(typeof(Test))]
public interface Dummy { }

The CoClass attribute associates an implementation class with an interface, allowing it to be instantiated as a concrete object. This approach is typically used when a COM API requires instantiating an interface.

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