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Editorial Team
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Asked: 2026-05-14T21:16:51+00:00

Dictionary<string, int> testdic = new Dictionary<string, int>(); testdic.Add(cat, 1); testdic.Add(dog, 2); testdic.Add(rat, 3); testdic.Remove(cat);

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Dictionary<string, int> testdic = new Dictionary<string, int>();
testdic.Add("cat", 1);
testdic.Add("dog", 2);
testdic.Add("rat", 3);
testdic.Remove("cat");
testdic.Add("bob", 4);

Fill the dictionary and then remove the first element. Then add a new element. Bob then appears at position 1 instead of at the end, therefore it seems to remember removed entries and re-uses that memory space?

Is this documented anywhere because I can’t see it on MSDN and has caused me a day of grief because I assumed it would just keep adding to the end.

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1 Answer

  1. Editorial Team

    It’s not documented because it’s just an implementation detail.

    The implementation could change at any time. (And unless you’ve closely examined the source code then you can’t even be sure that your assumptions about the current implementation are accurate.)

    The Dictionary<K,V> type does not guarantee to store its elements in any particular order, and you shouldn’t rely on any undocumented behaviour that you happen to observe. A dictionary is a map from keys to values, not an ordered list.

    From the MSDN documentation:

    For purposes of enumeration … [t]he order in which the items are returned is undefined.

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