Why use the yield keyword, when I could just use an ordinary IEnumerable?

Answer 1

You can use yield to return items that aren't in a list. Here's a little sample that could iterate infinitely through a list until canceled.

public IEnumerable GetNextNumber()
{
    while (true)
    {
        for (int i = 0; i < 10; i++)
        {
            yield return i;
        }
    }
}

public bool Canceled { get; set; }

public void StartCounting()
{
    foreach (var number in GetNextNumber())
    {
        if (this.Canceled) break;
        Console.WriteLine(number);
    }
}

This writes

0
1
2
3
4
5
6
7
8
9
0
1
2
3
4

...etc. to the console until canceled.