Why we use "this" in Extension Methods?

Question

I want to ask why we use "this" keyword before the parameter in an extension method (C# Language)........... like this function :

    public static int ToInt(this string number)
    {
        return Int32.Parse(number);
    }

I know that we have to use it but I don't know why.

Answer 1

Because that's the way you tell the compiler that it's an extension method in the first place. Otherwise it's just a normal static method. I guess they chose this so they didn't have to come up with a new keyword and potentially break old code.

Answer 2

For info, the significance of this as a contextual-keyword here is largely that it avoids introducing a new keyword. Whenever you introduce a new keyword you risk breaking code that would have used it as a variable / type name. this has a few useful features:

• it is close enough to indicating that this relates to an instance method
• it is an existing keyword...
• ...that would have been illegal when used in that location

This means that no existing code will be broken.

Beyond the choice of this as the keyword, it is just a convenient syntax for the compiler, and more convenient than adding [Extension] manually. Without either, it would just be a static method, without any special behaviour.