Question

The Real World Functional Programming has this code in page 65.

The Tuple has two properties Item1 and Item2, and it has TupleExtensions class as follows.

static class TupleExtensions {
  public static Tuple<T1, T2> WithItems2<T1, T2> 
    (this Tuple<T1, T2> tuple, T2 newItem2) { // line3 ???
      return Tuple.Create(tuple.Item1, newItme2); //  line4 ???
  }
}

var pragueOld = Tuple.Create("Prague", 1188000);
var pragueNew = pragueOld.WithItem2(pragueOld.Item2 + 13195);

The pragueOld.WithItem2() has only one parameter, but the definition has two parameters.

I think line 3, 4 is the same as this code.

(T2 newItem2) {
return Tuple.Create(this.Item1, newItem2)

What’s the meaning of this as is prepended in the first parameter? What advantages does this provides compared to the simple this.Item1 usage?

Answer 1

the this keyword, in this context, means that you are declaring an extension method.

To declare an extension method you must be in the scope of a static class and the method must also be marked static.

This would be an example

namespace Utils
{
    public static class StringExtension
    {
        public static int NonWhitespaceLength(this string text)
        {
            return text.Replace(" ","").Length;
        }
    }
}

and you could call this by either

var a = "hello world ! !    !";

Console.WriteLine(Utils.StringExtension.NonWhitespaceLength(a));

or simply

using Utils.StringExtension;

var a = "hello world ! !    !";

Console.WriteLine(a.NonWhitespaceLength());

an additional note: the “this” parameter can actually be null. Which isn’t that intuitive and it took me a while to realize, so you can either expect the function to fail if null is passed, or add null reference checks, like so:

namespace Utils
{
    public static class StringExtension
    {
        public static int NonWhitespaceLength(this string text)
        {
            return text == null ? 0 : text.Replace(" ","").Length;
        }
    }
}