Use of “this” keyword in formal parameters for static methods in C#

Question

I\'ve come across several instances of C# code like the following:

public static int Foo(this MyClass arg)

I haven\'t been able to find an

Answer 1

^{In addition to Preet Sangha's explanation:}
Intellisense displays the extension methods with a blue arrow (e.g. in front of "Aggregate<>"):

You need a

using the.namespace.of.the.static.class.with.the.extension.methods;

for the extension methods to appear and to be available, if they are in a different namespace than the code using them.