How extension methods hook up

Question

I was just curious to know how Extension methods are hooked up to the Original class. I know in IL code it gives a call to Static Method, but how it does that and why dosen't it break encapsulation.

Answer 1

They don't "hook up".

The Visaul Studio IDE just makes it look like it does by showing them in the intellisense lists.

The compiler "knows" how to deal with the references in order to make the right method calls with the correct parameters.

This is simply syntactic sugar - the methods are simply static methods on a separate static class. Using the this modifier lets the compiler "know" to add the ExtensionAttribute to the class to mark it as an extension method.

Since extension methods do not in fact change the class and can only access public members on it, encapsulation is retained.

From MSDN:

Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type.

(emphasis mine)

Answer 2

Extension methods are specified by putting the this keyword in front of the first parameter of a static method:

public static void SomeExtension(this string s)
{
    ...
}

That is just syntactic sugar for decorating the method with System.Runtime.CompilerServices.ExtensionAttribute:

[Extension]
public static void SomeExtension(string s)
{
    ...
}

When the compiler sees that attribute, it knows to translate the extension method call to the appropriate static method call, passing the instance as the first parameter.

Since the calls are just normal static method calls, there is no chance to break encapsulation; the methods, like all static methods, only have access to the public interfaces of the extended types.

Answer 3

Extension methods are just syntactic sugar, they are just static methods. You are only able to access public fields or properties in them, just like normal static methods.