yield return works only for IEnumerable?

Question

Can I use yield return when the return type is an IGrouping<TKey, TElement> or an IDictionary<TKey, TValue>?

possible duplicate of [Some help understanding "yield"](http://stackoverflow.com/questions/317462/some-help-understanding-yield) — nawfal, Jul 09 '14 at 06:54

score 52 · Accepted Answer · answered Jul 01 '09 at 19:52

52

yield return works for exactly 4 cases:

IEnumerable
IEnumerable<T>
IEnumerator
IEnumerator<T>

This is because it has to build a state machine internally; a dictionary (etc) wouldn't be possible with this. You can of course just return a suitable type instead.

answered Jul 01 '09 at 19:52

Marc Gravell

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Handy article here http://csharpindepth.com/articles/chapter6/iteratorblockimplementation.aspx if you want to implement IEnumerable yourself. – Rory Sep 16 '12 at 23:01
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@Rory yes, implementing it yourself is a massive PITA, and I can't think of any "normal" reasons to ever do so, now that iterator blocks exist. I have a few such, but that's usually for compatibility on projects that need to build even on C# 1.2 compilers; not exactly the "normal" scenario. – Marc Gravell Sep 17 '12 at 06:48
Agreed, although IGrouping could be nice in some cases it seems easier to use an IEnumerable> instead so you get the magic of yield return. – Rory Sep 17 '12 at 10:25

score 16 · Answer 2 · answered Jul 01 '09 at 19:52

16

You could however return IEnumerable<KeyValuePair<K,V>> that would be similar to a dictionary. You would then yield return KeyValuePairs. You could even wrap this with another method that creates a dictionary out of the return. The only thing the first method would not guarantee is uniqueness in the keys.

answered Jul 01 '09 at 19:52

Timothy Carter

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Then you lose the O(1) access time, and have to cycle through every single KVP to find the correct key, taking at worst O(n) times. – Callum Rogers Aug 18 '09 at 19:30

score 5 · Answer 3 · answered Jul 01 '09 at 19:54

Answer: No. A yield return statement can be used only if the return type is IEnumerator, IEnumerator<T>, IEnumerable, or IEnumerable<T>.

From §8.14 of the C# 3.0 spec:

The yield statement is used in an iterator block (§8.2) to yield a value to the enumerator object (§10.14.4) or enumerable object (§10.14.5) of an iterator or to signal the end of the iteration.

From §10.14.4:

An enumerator object has the following characteristics:

It implements IEnumerator and IEnumerator<T>, where T is the yield type of the iterator.

[...]

From §10.14.5:

An enumerable object has the following characteristics:

It implements IEnumerable and IEnumerable<T>, where T is the yield type of the iterator.

[...]

score 2 · Answer 4 · answered Jul 01 '09 at 19:49

No, because an iterator block is simply a state machine built on your behalf by the compiler. This feature allows you to "yield" and item as a portion of a sequence.

If the return type was other than IEnumerable<T> (like IDictionary for example) the compiler would have to generate methods to implement that interface and at that point it wouldn't make much sense because you would be working with a collection rather than a sequence.

score 2 · Answer 5 · answered Jul 01 '09 at 19:57

2

Just call the iterator method and chain ToDictionary or GroupBy after it, and you have much the same thing. Put that in a one-line wrapper method if you need to call it like that from several places.

answered Jul 01 '09 at 19:57

Daniel Earwicker

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score 1 · Answer 6 · answered Jul 01 '09 at 19:49

1

I don't think so. While the documentation doesn't exactly spell it out, the way it is worded implies that it can only be used when the return type of the method is either IEnumerable or IEnumerable<T>. You might be able to write a class that implements IGrouping given an IEnumerable (that would be returned from your method using yield return), but that's about the only option really.

answered Jul 01 '09 at 19:49

Andy

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The language spec spells it out instead... see 8.18 in ECMA 334 – Marc Gravell Jul 01 '09 at 19:54

score 0 · Answer 7 · answered May 22 '22 at 22:24

you can always create a IAsyncEnumerable<KeyValuePair<T,V>> using a repository or list and yield return new KeyValuePair<T,V>(obj1,obj2); and a second function to call the method above that returns 'IAsyncEnumerable<KeyValuePair<T,V>>' add into a dictionary and add to the dictionary if (!dic.TryGetValue(output.Key,out object s)) assuming you want unique keys

score -1 · Answer 8 · answered Jul 01 '09 at 19:47

-1

nope, just ienumerable :-)

answered Jul 01 '09 at 19:47

Joel Martinez

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yield return works only for IEnumerable?

8 Answers8