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

COM type detection in C# compiler

The C# compiler has special handling of COM types, such as allowing interfaces to be instantiated and non-ref parameters treated as ref parameters. This behavior results from the application of specific properties and techniques.

To simulate the behavior of creating an interface instance, consider using the CoClass attribute. Applying the [CoClass] attribute to an interface associates it with the concrete class that implements the interface:

<code>[CoClass(typeof(MyClass))]
public interface IMyInterface { }</code>

This makes it possible to instantiate IMyInterface as follows:

<code>IMyInterface instance = new MyClass();</code>

Alternatively, you can use the Type.GetTypeFromCLSID() and Activator.CreateInstance() methods to retrieve the type and create an instance respectively.

Regarding the handling of ref parameters, when a non-ref parameter is provided, the C# compiler adds a local variable to pass by reference. To illustrate this, consider the following code:

<code>// Filename 参数实际上是一个 ref 参数
app.ActiveDocument.SaveAs(Filename: "test.doc");</code>

In this case, the compiler-generated code creates a local variable to hold the Filename parameter and passes this variable as the ref parameter to the SaveAs method.

By leveraging the CoClass attribute and the compiler's ability to handle ref parameters, developers can efficiently interact with COM types in C# code.

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