Iterators

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.
    }
}

or:

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