I am cross-posting this at...

http://www.codeproject.com/csharp/sets.asp?forumid=13500&exp=0&select=1296521&df=100&msg=1296521

...because I am not sure whose responsibility it should become.

I have a query about the SortedSet implementation. At present, it uses a SortedList as its backing store? However, I do not believe that, semantically, a SortedList is equivalent to a SortedSet?

Specially, as can be seen at http://www.dotnet247.com/247reference/System/Collections/SortedList/__rotor, SortedList uses IComparable and BinarySearch, not GetHashCode() and Equals(), to implement its Add, Contains, etc.

I have a class whose GetHashCode() and Equals() methods key off one property of the object, and whose CompareTo() keys off another. This is because my GetHashCode() and Equals() method use internal, unique identifiers useful to the code whereas my CompareTo() method uses human-readable, non-unique identifiers useful to people.

I believe this is a valid use case: I do not believe the contract of CompareTo() requires it use the same identifier as GetHashCode() and Equals()?

However, SortedSet breaks in this scenario because (I believe) it goes looking for an existing object using CompareTo() rather than by GetHashCode() and Equals(). This means it is possible to put multiple items into a SortedSet whose Equals() methods return true, which breaks the ISet contract (though of course is perfectly valid for a SortedList).

Have I gotten this completely wrong? Is there a better approach?

Regards,

Richard.

SortedSet in Java also uses IComparable (Comparable in Java), the documentation is a bit vague on what happens if elements are compared equal by compareTo but not by equals, but I think the behavior is the same as the .NET implementation currently has.

See http://java.sun.com/j2se/1.4.2/docs/api/java/util/SortedSet.html

I think you should just provide a comparator which will compare two elements equal only if they are really equal.