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;   
}

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;
    }
}

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.