ISerialized .Net, C#, Scrum and agile software development

A quick start guide to yield return and yield break

The yield statement was introduced in .Net 2.0, but I am a bit surprised that I meet many senior developers who has never used yield return (and yield break)! For some strange reason, yield has become some kind of hidden treasure in .Net. Through this post, I hope I can show some simple examples, and give some of the ideas, benefits and limitations with yield.

First up, what is yield? Yield is used to return an IEnumerable from eg. a loop within a method. For example, if I have a method that I want to return several values from, I could do it in the following way:

public static List<int> GetResultList()
{
    var l = new List<int>();
    for (int i = 0; i < 100; i++)
    {
        l.Add(i);
    }
    return l;
}

Here I create a generic list of integers, which I poplulate, before I return the full list, it works fine, and I guess thats why many developers never started using it! To create a similar functionality with yield, returning an IEnumerable, I can do the following:

public static IEnumerable<int> GetResult()
{
    for (int i = 0; i < 100; i++)
    {
        yield return i;
    }
}

I get rid of some unnecessary code, and its easier to see what I actually want to return! If I inside this loop want to abort the iteration and return, I do this through the yield break. The yield break, will do more than a normal break, as it will return from the method, and not only from the for-loop. Eg: If I have to consecutive for-loops in the same method, both using yield, and I do yield break in the first, the second for-loop will never be executed:

public static IEnumerable<string> GetMoreResults()
{
    for (int i = 0; i < 20; i++)
    {
        yield return "Value " + i;
        yield break;
    }

    //Do something else

    for (int i = 0; i < 20; i++)
    {
        yield return "Another value " + i;
    }
}

Here I will only get one value returned in my IEnumerable, as the yield break will end the method execution.

If you have written a couple of methods using yield return, you might  have noticed, is that the debugger will never step into a method containing a yield return, and you can not use yield return in combination with try/catch. This is due to the fact that the iterator blocks uses deferred execution, meaning that it is not actually executed until you start iterating (using the IEnumerable).

Happy coding!