Today I ran into this problem for the second (and hopefully last) time. Let’s take a look at the words “sorted” and “list”. “sorted” suggests that the items of the list will somehow be sorted. So far, so good. “list” would suggest that you can put any item in the list. Specifically, it isn’t called “SortedSet”, so you could expect that you in fact can add the same item twice with the same key.

Well… that’s where you’d be wrong. The documentation says that the sorted list throws an ArgumentException if “An element with the same key already exists in the SortedList”. Unfortunately I don’t go around looking through the documentation to search for stuff like that. And in fact: you shouldn’t have to. A list is a list and not a set. And if you decide to implement a sorted list with a dictionary as the backing store, then you are – quite frankly – an idiot.

If you would like to save yourself a lot of grief, just don’t use the sorted list. Use a regular List and the Sort method. But be aware, that the Sort method uses the quicksort algorithm which is unstable (elements that are “equal” don’t keep their relative positions to one another).