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>();
}

It would be better if it returned Enumerable.Empty(), because currently the calling code must distinguish between empty and non-empty results — Damien_The_Unbeliever
– Damien_The_Unbeliever, Commented Nov 29, 2011 at 13:18

zero323 · Accepted Answer · 2013-11-03 09:07:29Z

7

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.

edited Nov 3, 2013 at 9:07

zero323

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answered Nov 29, 2011 at 13:18

DaveShaw

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Comments

Adam Robinson · Accepted Answer · 2011-11-29 13:19:47Z

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.

Jon Skeet · Accepted Answer · 2011-11-29 13:17:34Z

3

A method is either implemented with an iterator block or it's not - so either everything is in terms of yield return and yield break, or none of it is.

However, you don't need to do anything special. Your original GetMonthsBetweenTwoDates already works where to == from, because it'll never enter the while loop.

On the other hand, your use of lastDate looks suspicious to me - in particular, it looks like it'll do something different if from happens to be in the same month as DateTime.MaxValue.

answered Nov 29, 2011 at 13:17

Jon Skeet

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6 Comments

Florian Over a year ago

The example is stupid, it's just for illustrate my purpose :-)

Jon Skeet Over a year ago

@Florian: Then I suggest you pick a better example in future :) Seriously, it helps if it's representative of what you're actually trying to do, as there may well be a better approach.

Florian Over a year ago

Ok, I take your notice for the future.

Florian Over a year ago

Even if I think I'll never use stackoverflow to ask questions because I'll receive a very good book[1] very soon :-) [1]manning.com/skeet2

Adam Robinson Over a year ago

@Florian: What Jon doesn't mention is that his books are just the output of a sophisticated AI that scrapes answers from SO ;)

|

Turbot · Accepted Answer · 2011-11-29 13:27:36Z

0

you don't need yield break to end the iteration at your first check.

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

answered Nov 29, 2011 at 13:27

Turbot

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