Why isn't there a "set" interface in the .NET framework?
.net, collections, interface, set
Solution
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.
Problem
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.