c# - 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.