Syntax issue IEnumerable<T> method using yield return

Question

Here is my method :

static IEnumerable<DateTime> GetMonths(DateTime from, DateTime to)
{
    // if logs is not uptodate
    TimeSpan logsMissingTimespan = to - from;

    if (logsMissingTimespan != new TimeSpan(0))
    {
        return GetMonthsBetweenTwoDates(from, to);
    }

    return null; // Why this line ?
}

private static IEnumerable<DateTime> GetMonthsBetweenTwoDates(DateTime from, DateTime to)
{

    DateTime date = from;
    DateTime lastDate = DateTime.MaxValue;

    while (date < to)
    {
        if (lastDate.Month != date.Month)
        {
            lastDate = date;
            yield return lastDate;
        }
        date = date.AddDays(1);
    }
}

it works fine but I think I can write something cleaner like this :

static IEnumerable<DateTime> GetMonths(DateTime from, DateTime to)
{
    TimeSpan logsMissingTimespan = to - from;

    if (logsMissingTimespan == new TimeSpan(0))
    {
        yield break;
    }

    return GetMonthsBetweenTwoDates(from, to);
}

But I have an error message :

Cannot return a value from an iterator. Use the yield return statement to return a value, or yield break to end the iteration.

Why should I have a return null and what is the correct syntax ?

EDIT :

So, the correct way is to use Enumerable.Empty :

static IEnumerable<DateTime> GetMonths(DateTime from, DateTime to)
{
    // if logs is not uptodate
    TimeSpan logsMissingTimespan = to - from;

    if (logsMissingTimespan != new TimeSpan(0))
    {
        return GetMonthsBetweenTwoDates(from, to);
    }

    return Enumerable.Empty<DateTime>();
}

Answer 1

Because you have used the word yield it now expects the method to yield one element at a time. It must only use yeild return or yield break to return one element per iteration.

You should use Enumerable.Empty<DateTime>(); instead of yield break.

Answer 2

The forms of your first two examples produce different sorts of output.

Your first example returns an IEnumerable<T> directly if the condition is satisfied, and a null reference if it is not. Your second example always returns an IEnumerable<T>, but the condition determines whether or not it has any elements.

The second example is done by using an iterator block. The yield syntax is used by the C# compiler to turn the function that you wrote into a custom (hidden) type that implements IEnumerable<T> and a type that implements IEnumerator<T>. These types implement the necessary state machines in order to achieve (hopefully) the logic that you've placed into the function. Because of this, you cannot mix paradigms; you must either return an instance of IEnumerable<T> from the function (and not use yield anywhere at all), or everything must be returned via yield.

If all you're concerned with is the fact that you're returning a null reference, you could make the methods semantically the same by returning Enumerable.Empty<DateTime> rather than null.