When should an iterator return an IEnumerator instead of an IEnumerable?

Question

Using yield return turns a method into an iterator. The return type of an iterator must be IEnumerable, IEnumerable<T>, IEnumerator, or IEnumerator<T>.

I have found that an iterator can only be used in a foreach loop if it returns one of the IEnumerable types. Given this limitation, when would it make sense to choose an IEnumerator return type instead of an IEnumerable?

The only place I've seen an iterator method returning an `IEnumerator` is for [Unity's coroutines](https://docs.unity3d.com/Manual/Coroutines.html). I assume this is because they're (ab)using the iterator machinery to do something different, and don't want people accidentally thinking they should be looping over the coroutine. — canton7, Apr 15 '20 at 12:36
I acutally thought that this code: https://dotnetfiddle.net/KOMtuL - would crash, since I am only returning the interface `IEnumerator` so who acutally implements the method `MoveNext()` ?!? Magic? — Rand Random, Apr 15 '20 at 12:42
@RandRandom [Demystified](https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA+ABATARgLABQGADAAQY4B0AwhADZ0xgAuAlhAHYDOA3ISeRwo+BAJDjx/AMzksggOyEA3oVHYsqlWNEA3AIZRSCUgF5SAMQgQAFAEoR4gO4ALVo2sJKAWQg6YAORgEZjtbVXEKAE4PGgBXKFgOZntVAF9VVQwZCiRSAEkAUQ5YgFsYKD1maAsrO01wihxyeVIAIgAJGAYIUgB1aDoAE1aHdIJUoA===) — canton7, Apr 15 '20 at 12:44
@RandRandom The block of code will be compiled into an IEnumerator state machine that will handle the MoveNext and Current. — juharr, Apr 15 '20 at 12:44

score 5 · Accepted Answer · answered Apr 15 '20 at 12:36

5

Pretty rarely, to be honest. The only times I've seen it used that way is when special-casing things for some reason in the GetEnumerator() method (before firing up the iterator block machinery), but still wanting an iterator block for the actual implementation, i.e.

public IEnumerator<T> GetEnumerator()
{
    if (someScenario) return SomethingSpecialPerhapsEmptyArrayEnumerator();
    if (anotherScenario) ThrowSomeException();
    return DoTheThing();
}
private IEnumerator<T> DoTheThing()
{
    // ... yield return
}

answered Apr 15 '20 at 12:36

Marc Gravell

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So `GetEnumerator` is automatically generated if an iterator returns `IEnumerable` but not if it returns `IEnumerator`? Are there any other differences between the two return types? – user200783 Apr 15 '20 at 15:55
1

@user200783 that's mostly it – Marc Gravell Apr 15 '20 at 19:09

When should an iterator return an IEnumerator instead of an IEnumerable?

1 Answers1