What is the C# equivalent of the stl set?

You could use an HashSet

The HashSet<T> class provides high performance set operations. A set is a collection that contains no duplicate elements, and whose elements are in no particular order.

The capacity of a HashSet<T> object is the number of elements that the object can hold. A HashSet<T> object's capacity automatically increases as elements are added to the object.

1. If you require sorted set, use SortedDictionary<T,U>. This is implemented using a binary search tree. Admittedly, you will be using 64-bits per entry because you are storing a key-value pair underneath. You can write a wrapper around it like this:

   class Set<T> : SortedDictionary<T, bool>
   {
       public void Add(T item)
       {
           this.Add(item, true);
       }
   }
   

2. If you don't require a sorted set, use HashSet<T>.

3. Otherwise, check out C5 Generic Collection Library. In particular TreeSet<T>. It is a red-black tree and only stores the values.

Try RedBlackTree.NET. It's in VB but I think it can be easily converted to C#.

And I believe some of the collection type actually uses a red-black tree internally. So you might want to decompile the framework itself and look around for some clues.

I don't think a binary tree can be replaced by a HashSet. Their performance characteristics are different, roughly:

HashSet - O(1) lookup (n) search
Binary search tree - O(log n) lookup O(log n) search

If you want to store the values and later perform a search, you will want to be using a binary tree instead of a HashSet.