Statement "yield".

    When we want to use operator foreach, but want to hide details of implementation we can use interface IEnumerable.

     Review next sample:

Let's display list of cities:

Cities cities = new Cities();
foreach (string city in cities)
{
    Trace.WriteLine("city: " + city);
}

where class Cities is implemented:

public class Cities
{
    readonly string[] cities = { "London", "Paris", "Rome", "Jerusalem" } ;
    public IEnumerator GetEnumerator()
    {
        return cities.GetEnumerator();
    }
}

Seems enough simple.

Now we need display cities in reverse order

Trace.WriteLine("Reverse order: ");
foreach (string city in cities.Reverese)
{
    Trace.WriteLine("city: " + city);
}

To receive reverse order we can add property Reverse:

public IEnumerable<string> Reverese
{
    get
    {
        for (int i = cities.Length - 1; i >= 0; i--)
        {
            yield return cities[i];
        }
    }
}

Reference:

yield (C# Reference)

Create Elegant Code with Anonymous Methods, Iterators and Partial Classes.