Why are entries in addition order in a .Net Dictionary?

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If you use .NET Reflector on the 3.5 class libraries you can see that the implementation of Dictionary actually stores the items in an array (which is resized as needed), and hashes indexes into that array. When getting the keys, it completely ignores the hashtable and iterates over the array of items. For this reason, you will see the behavior you have described since new items are added at the end of the array. It looks like if you do the following:

add 1
add 2
add 3
add 4
remove 2
add 5

you will get back 1 5 3 4 because it reuses empty slots.

It is important to note, like many others have, you cannot count on this behavior in future (or past) releases. If you want your dictionary to be sorted then there is a SortedDictionary class for this purpose.

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