How does .NET know where to look for extension methods?

Question

I believe I understand the concept of extension methods, except one detail. It seems you can create a random (static) class with a random static method using this on a parameter. I believe this parameter has to match the type of class you want to extend (or one it's inheriting from). An example:

public static class MyExtendedMethods
{
    public static int square(this int num)
    {
        int result = 0;
        result = num * num;
        return result;
    }
}

class Program
{
    static void Main(string[] args)
    {
        int myNum = 3;
        myNum = myNum.square();
        Console.WriteLine(myNum);
    }
}

How does .NET know where to find this class and method (MyExtendedMethods.square())? I don't see the classes related in any way. Is it looking through all classes in the project for a possible extension?

Answer 1

It looks for static classes inside the namespaces you declared at the top of your file, like 'using System.Linq;' (if it cannot find anything in the current namespace) Then it simply replaces the call with

MyExtendedMethods.square