Why isn't there a "set" interface in the .NET framework?

Question

I'm thinking specifically about the generic class HashSet<T>. It implements several interfaces, but none exposes the correct semantics of a set. Specifically, none supports an Add method returning bool. (ICollection<T> supports void Add, which can be used in a pinch.) Also unsupported by these interfaces are common set operations like unions and intersections. (Though it must be said that some of these operations are available via extensions to IEnumerable<T>.)

This means the class can only be used like a set with its direct implementation. I.e., you can't do something like this:

ISet<int> = new HashSet<int>;

Not as far as I know, anyway. So what motivated the choice to leave this out?

Maybe the most important thing is this: even if you can cast HashSet<T> to ICollection<T> et al, you lose semantic value in the API you're exposing. That is, consumers of your API have no indication they're working with a set. So while you could call ICollection<T>.Add and get by, people will be confused if they try to add an item twice and it doesn't work. A set interface would give people the right expectations.

Answer 1

While I am not sure why it hasn't made it into the base class library earlier, an ISet is available in .NET 4.0, which is only weeks from shipping. The interface has most of the methods you would expect on a set. These are some of the supported methods:

• Add, Clear, Contains, CopyTo, GetEnumerator, Remove (inherited)
• ExceptWith
• IntersectWith
• IsSubsetOf
• IsSupersetOf
• Overlaps
• UnionWith

The HashSet and SortedSet collections implement this new interface.