C# Set collection?

If you're using .NET 3.5, you can use HashSet<T>. It's true that .NET doesn't cater for sets as well as Java does though.

The Wintellect PowerCollections may help too.

Try HashSet:

The HashSet(Of 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(Of T) object is the number of elements that the object can hold. A HashSet(Of T) object's capacity automatically increases as elements are added to the object. The HashSet(Of T) class is based on the model of mathematical sets and provides high-performance set operations similar to accessing the keys of the Dictionary(Of TKey, TValue) or Hashtable collections. In simple terms, the HashSet(Of T) class can be thought of as a Dictionary(Of TKey, TValue) collection without values. A HashSet(Of T) collection is not sorted and cannot contain duplicate elements...

If you're using .NET 4.0 or later:

In the case where you need sorting then use SortedSet<T>. Otherwise if you don't, then use HashSet<T> since it's O(1) for search and manipulate operations. Whereas SortedSet<T> is O(log n) for search and manipulate operations.

I use Iesi.Collections http://www.codeproject.com/KB/recipes/sets.aspx

It's used in lot of OSS projects, I first came across it in NHibernate

I use a wrapper around a Dictionary<T, object>, storing nulls in the values. This gives O(1) add, lookup and remove on the keys, and to all intents and purposes acts like a set.

Have a look at PowerCollections over at CodePlex. Apart from Set and OrderedSet it has a few other usefull collection types such as Deque, MultiDictionary, Bag, OrderedBag, OrderedDictionary and OrderedMultiDictionary.

For more collections, there is also the C5 Generic Collection Library.

I know this is an old thread, but I was running into the same problem and found HashSet to be very unreliable because given the same seed, GetHashCode() returned different codes. So, I thought, why not just use a List and hide the add method like this

public class UniqueList<T> : List<T>
{
    public new void Add(T obj)
    {
        if(!Contains(obj))
        {
            base.Add(obj);
        }
    }
}

Because List uses the Equals method solely to determine equality, you can define the Equals method on your T type to make sure you get the desired results.