How Does the C# Compiler Handle the Seemingly Unusual Instantiation and Parameter Passing of COM Types?

Deeply explore the secrets of how the C# compiler handles COM types

Seamless integration of COM types in C# sparks developer curiosity. This article takes a deep dive into the compiler's mechanism for detecting COM types and illustrates how it can achieve seemingly anomalous operations.

One puzzling aspect of COM type handling is the way interfaces such as Application are constructed in C#. The C# compiler appears to allow calling constructors on interfaces, behavior that is typically prohibited in other languages. This illusion is achieved by implicitly calling Type.GetTypeFromCLSID() to retrieve the correct COM class, and subsequently calling Activator.CreateInstance to create an instance of that class.

Additionally, C# 4 introduced a feature that allows developers to provide non-reference parameters for reference parameters. The compiler automatically creates a local variable to pass by reference, effectively discarding the original value. The following code illustrates this behavior:

<code class="language-c#">app.ActiveDocument.SaveAs(FileName: "test.doc");</code>

The FileName parameter is technically a reference parameter, but the code is passing a constant value. The compiler handles this difference transparently.

Trying to imitate the first scenario (instantiating the interface directly) has proven to be unsuccessful. However, the second scenario can be replicated using the ref and in modifiers, as follows:

<code class="language-c#">Dummy dummy = null;
dummy.Foo(in 10);</code>

Contrary to the expected new Dummy() construct, this code utilizes the in modifier to pass the variable by reference without changing its value.

The key to the compiler being able to recognize COM types is the CoClass attribute. By annotating an interface with [CoClass(typeof(Test))], the compiler can infer the existence of the underlying implementation class and generate the necessary code to instantiate it.

This discovery opens up new possibilities for interacting with COM types in C#, enhancing the language’s interoperability capabilities.

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