Yield

When you use the yield keyword in a statement, you indicate that the method, operator, or get accessor in which it appears is an iterator. Using yield to define an iterator removes the need for an explicit extra class (the class that holds the state for an enumeration, see IEnumerator<T> for an example) when you implement the IEnumerable and IEnumerator pattern for a custom collection type.

public class PowersOf2
{
    static void Main()
    {
        // Display powers of 2 up to the exponent of 8: 
        foreach (int i in Power(2, 8))
        {
            Console.Write("{0} ", i);
        }
    }

    public static System.Collections.Generic.IEnumerable<int> Power(int number, int exponent)
    {
        int result = 1;

        for (int i = 0; i < exponent; i++)
        {
            result = result * number;
            yield return result;
        }
    }

    // Output: 2 4 8 16 32 64 128 256
}

You use a yield return statement to return each element one at a time.

You consume an iterator method by using a foreach statement or LINQ query. Each iteration of the foreach loop calls the iterator method. When a yield return statement is reached in the iterator method, expression is returned, and the current location in code is retained. Execution is restarted from that location the next time that the iterator function is called.

You can use a yield break statement to end the iteration.