gistfile1.txt

private static IEnumerable<string> Something()
{
    yield return "Something() One";
    yield return "Something() Two";
    yield return "Something() Three";
}


// When the compiler encounters an iterator block it emits a nested type for the state machine similar to this: 


private static IEnumerable<string> Something()
{
    SomethingEnumerator somethingEnumerator = new SomethingEnumerator(2);
    return somethingEnumerator;
}

private sealed class SomethingEnumerator : IEnumerable<string>, IEnumerator<string>
{
    private int state;
    private int initialThreadId;

    public string Current { get; private set; }

    object IEnumerator.Current
    {
        get { return Current; }
    }

    public SomethingEnumerator(int state)
    {
        this.state = state;
        initialThreadId = Environment.CurrentManagedThreadId;
    }

    public IEnumerator<string> GetEnumerator()
    {
        SomethingEnumerator somethingEnumerator;
        if (Environment.CurrentManagedThreadId == initialThreadId && state == 2)
        {
            state = 0;
            somethingEnumerator = this;
        }
        else
        {
            somethingEnumerator = new SomethingEnumerator(0);
        }

        return somethingEnumerator;
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public bool MoveNext()
    {
        switch (state)
        {
            case 0:
                state = -1;
                Current = "Something() One";
                state = 1;
                return true;
            case 1:
                state = -1;
                Current = "Something() Two";
                state = 2;
                return true;
            case 2:
                state = -1;
                Current = "Something() Three";
                state = 3;
                return true;
            case 3:
                state = -1;
                break;
        }

        return false;
    }

    public void Reset()
    {
        throw new NotSupportedException();
    }

    public void Dispose()
    {
    }
}