Why is it necessary to temporarily save keys to a list when iterating over a dictionary's items and modifying their values?

Question

foreach (var key in dict.Keys.ToList())
{
    dict[key] = false;
}

The call to ToList() makes this work, since it's pulling out and (temporarily) saving the list of keys, so the iteration works.

Why is the call to ToList() necessary here?

We're modifying values, not keys, and, to my best understanding, only modyfying the set of keys of a dictionary would break iteration over keys. In particular - to my, likely wrong understanding - the ordering of keys can only change if we are adding or removing items from a dictionary, which we are not doing.

score 5 · Answer 1 · answered May 24 '18 at 18:52

5

If you read the reference source for Dictionary<TKey, TValue>.Insert, you will see it is incrementing the version field which drives the guards while enumerating collections.

answered May 24 '18 at 18:52

Daniel A. White

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score 1 · Answer 2 · answered May 24 '18 at 18:49

1

Dictionaries have internal version numbers.

Setting a value in a dictionary updates its version number which will have the same effect as modifying an enumerated collection in a foreach loop.

answered May 24 '18 at 18:49

SchwiftyPython

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A reference to that info would be good – Eser May 24 '18 at 18:49
1

Daniel beat me to it. Also, possible duplicate - https://stackoverflow.com/questions/1070766/editing-dictionary-values-in-a-foreach-loop?rq=1 – SchwiftyPython May 24 '18 at 18:54

Why is it necessary to temporarily save keys to a list when iterating over a dictionary's items and modifying their values?

2 Answers2