Iterators are objects that traverse a container. They yield return each element one at a time.

  • Each time yield return is reached, the current location in code is remembered.
  • The next time the iterator is called, execution restarts from that location. Iterators return IEnumerable, IEnumberable or IEnumerator, IEnumerator. In asynchronous operations, they return IAsyncEnumerable or IAsyncEnumerator. Iterators can be a method or a get accessor.

Creating Iterator Methods

public IEnumerable<int> EvenSequence(int firstnum, int lastnum) 
    for (var number = firstnum, number <= lastnum, number++) 
        if (number % 2 == 0) 
            yield return number; // When this statement is reached, an expression is returned, and the
        }                    // current location in code is remembered.


public static IEnumerable SomeNumbers() 
    yield return 7;
    yield return 1;
    yield return 9;

Using Iterators

Iterators are called with a foreach statement:

foreach (int number in EvenSequence(5, 18)) 
     // Each iteration of this body creates a call to the iterator function which proceeds to the
}     // next yield return statement.

Using Iterators with a Generic List