Yield in C#

The yield keyword is used when the method or accessor is an enumerator. Using yield return returns one element at a time.

The thing to remember here is that IEnumerable/IEnumerable<T> is a stream, not a collection. This is where many developers misuse IEnumerable (myself included once upon a time). Consider the following code:

	public IEnumerable<Person> GetPeople()
	{
	var list = new List<Person>();
	// database connection/command logic removed for brevity
	while (reader.Read())
	{
	var person = MapPerson(reader);
	list.Add(person);
	}
	return list;
	}

view raw no-yield.cs hosted with ❤ by GitHub

This is probably what you are used to seeing, but is actually a misuse of IEnumerable. The code works just fine, but it is not leveraging the benefits of IEnumerable being a stream.

Let's refactor to use yield return:

	public IEnumerable<Person> GetPeople()
	{
	// database connection/command logic removed for brevity
	while (reader.Read())
	{
	var person = MapPerson(reader);
	yield return person;
	}
	}

view raw using-yield.cs hosted with ❤ by GitHub

What we are doing now is streaming the elements one at a time. Each iteration by the consumer of this method comes back to the point of the yield and executes the next statement, in this case the while. The benefit? We eliminated a loop and an object allocation. Rather than looping through all the records, building up a list, then looping through the list, we just loop through the records once.