Introduction

In the not-too-recent past I predicted that code generators would produce some of the most exciting and useful innovations. Because code that writes code—once perfected—is perfect every time. Humans cannot get something perfect every time, ever.

One innovation that is sure to grow on you, though not exciting, is very useful. It is the yield return idiom. The word yield when preceding the keyword return (or break) is actually a hint to the .NET framework to kick over the code generating engine.

In this article I will explain how yield return works and show you how to save a little work a lot of time.

Using Yield Return

The yield identifier itself is not a keyword. However, yield immediately before return is a key construct. To use yield return, simply define a function that returns IEnumerable<T>, write a foreach loop as you normally would and use yield return to return to the caller those elements that match some arbitrary criteria. That is, you can use yield return to copy a collection or a subset of that collection.

Listing 1 is the Main function of a console application. Supporting code implements a class called ScrabbleWord and a generic List<T> of ScrabbleWord objects, called Dictionary. The basic idea is that Dictionary represents a Scrabble^® dictionary. The main loop requests only those words that match the regular expression ^A\w+E$; that is, words beginning with A, ending with E, and having one letter in between.

Click Here for an introduction to Regular Expressions.

Listing 1: The Main function of a console application that is requesting a subset of a Scrabble dictionary.

static void Main(string[] args)
{
  Dictionary d = Dictionary.Create();

  //foreach(ScrabbleWord word in d)
  foreach(ScrabbleWord word in d.Get(@"^A\w+E$"))
    Console.WriteLine(word);

  Console.WriteLine("enter");
  Console.ReadLine();
}