C# Set collection?

Question

Does anyone know if there is a good equivalent to Java's Set collection in C#? I know that you can somewhat mimic a set using a Dictionary or a HashTable by populating but ignoring the values, but that's not a very elegant way.

Answer 1

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.

Answer 2

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...

Answer 3

As others have mentioned, there doesn't appear to be a set implementation in the .NET standard with set semantics.

See this question for why HashSet isn't always desired as a set implementation.

If you're interested in rolling your own set class, here's one simple approach:

public sealed class MathSet<T> : HashSet<T>, IEquatable<MathSet<T>>
{
    public override int GetHashCode() => this.Select(elt => elt.GetHashCode()).Sum().GetHashCode();

    public bool Equals(MathSet<T> obj) => SetEquals(obj);

    public override bool Equals(object obj) => Equals(obj as MathSet<T>);

    public static bool operator ==(MathSet<T> a, MathSet<T> b) =>
        ReferenceEquals(a, null) ? ReferenceEquals(b, null) : a.Equals(b);

    public static bool operator !=(MathSet<T> a, MathSet<T> b) => !(a == b);
}

Example usage:

var a = new MathSet<int> { 1, 2, 3 };
var b = new MathSet<int> { 3, 2, 1 };

var c = a.Equals(b);                        // true

var d = new MathSet<MathSet<int>> { a, b }; // contains one element

var e = a == b;                             // true

Answer 4

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.