Simple Usage

The yield keyword is used to define a function which returns an IEnumerable or IEnumerator (as well as their derived generic variants) whose values are generated lazily as a caller iterates over the returned collection. Read more about the purpose in the remarks section.

The following example has a yield return statement that’s inside a for loop.

public static IEnumerable<int> Count(int start, int count)
{
    for (int i = 0; i <= count; i++)
    {
        yield return start + i;
    }
}

Then you can call it:

foreach (int value in Count(start: 4, count: 10))
{
    Console.WriteLine(value);
}

Console Output

4
5
6
...
14

Live Demo on .NET Fiddle

Each iteration of the foreach statement body creates a call to the Count iterator function. Each call to the iterator function proceeds to the next execution of the yield return statement, which occurs during the next iteration of the for loop.