Understand COM c# interfaces

c#, com, com-interop

Solution

It is a quirk, introduced in C# version 4. It is not exclusive to COM interop code, you can also get it in your own code. Try this:

using System;
using System.Runtime.InteropServices;

class Program {
    static void Example([Optional] object arg) { }
    static void Main(string[] args) {
        Example(   // <== Look at the IntelliSense popup here
    }
}

It is the [Optional] attribute that triggers this behavior. Been around forever but was never particularly useful in C# before. Unlike other languages like VB.NET and C++/CLI. Starting with C# v4, it interprets the attribute differently and the compiler will hard-code Type.Missing as the optional value for an argument type of object. Try changing the argument type to, say, string and note that the default becomes different. Null, as you'd expect.

This isn't very pretty of course, Type.Missing is a rather odd default value for object in normal C# code. Everybody would expect null instead. It is however very practical, writing Office interop code in C# in versions previous to 4 was a rather dreadful exercise. Companies can get into trouble when they do stuff like this btw, if Neelie Kroes gets wind of it she'd get Microsoft to pay a billion Euro fine for that :)

Problem

The Microsoft.Office.Interop.Word._Document interface has a method with the following signature: ``` void Close(ref object SaveChanges = Type.Missing, ref object OriginalFormat = Type.Missing, ref object RouteDocument = Type.Missing); ``` A few points I am having trouble understanding: - A ref parameter cannot have a default value. - A default value has to be a constant, and `Type.Missing` is not. - When calling this method, I can use `Close(false)` - normally a ref parameter requires an assignable variable? - When navigating to the definition of `Type` in Visual Studio, it takes me to the _Document.Type property, but this does not have a property named `Missing`. Is this a bug in VS? Thank you for any explanations.

Original source