Create a method applicable to a String object

Question

I would like to create a method applicable to a String object, that will return a modified String:

String s = "blabla";
String result = s.MyNewMethod();

I have tried to create a new class String but keyword this seems unknown:

class String  {

    public String MyNewMethod() {
        String s = this.Replace(',', '.'); // Replace method is unknown here
        // ...
    }
}

Answer 1

You need to define an extension method:

public static class StringExtensions {
    public static string MyNewMethod(this string s) {
        // do something and return a string
    }
}

Note that extension methods are static and are defined in a static top-level class.