Extension methods

It is sometimes useful to add functionality to a type without changing the implementation, creating a derived type, or recompiling code in general. We can do that by creating methods in helper classes. Let's say we want to have a function that reverses the content of a string because System.String does not have one. Such a function can be implemented as follows:

static class StringExtensions
{
    public static string Reverse(string s)
    {
        var charArray = s.ToCharArray();
        Array.Reverse(charArray);
        return new string(charArray);
    }
}

This can be invoked as follows:

var text = "demo";
var rev = StringExtensions.Reverse(text);

The C# language allows us to define this function in a way that enables us to call it as if it was an actual member...