this parameter modifier in C#?

Question

I'm curious about this code snippet:

public static class XNAExtensions {     /// <summary>     /// Write a Point     /// </summary>     public static void Write(this NetOutgoingMessage message, Point value)     {         message.Write(value.X);         message.Write(value.Y);     }     // ... }; 

What does the this keyword mean next to the parameter type? I can't seem to find any information about it anywhere, even in the C# specification.

Answer 1

That's an extension method.

The syntax means you can call the method as if it was a member of the NetOutgoingMessage class:

var msg = new NetOutgoingMessage(); msg.Write(somePoint); 

This is basically rewritten by the compiler to:

var msg = new NetOutgoingMessage(); XNAExtensions.Write(msg, somePoint); 

It's just nice syntactical sugar.

Answer 2

That is how an extension method is defined.

What this essentially means is that, even though this method is contained in an encapsulating static class, when using the type specified (in the extension method parameters labelled this) such a method will be automatically exposed such that:

var typeInstance = new TypeWithExtensionMethod(); typeInstance.ExtensionMethod(anyParametersRequiredButNotTypeInstance); 

Is possible, as opposed to:

var type = new TypeWithExtensionMethod(); ExtensionMethods.ExtensionMethod(typeInstance, anyOtherParametersRequired);