How do you determine if two HashSets are equal (by value, not by reference)?

Question

I am trying to determine if two HashSet objects in .NET 3.5 (C#) are equal sets, i.e. contain the same values. This seems like something one would obviously want to do but none of the provided functions seem to give you this information.

The way I can think to do this is by checking if the count of the two sets are equal and one set is a subset (not proper) of the other. I think the only way that can happen is if they are equal sets. Example code:

HashSet<int> set1 = new HashSet<int>();
set1.Add(1);
set1.Add(2);
set1.Add(3);

HashSet<int> set2 = new HashSet<int>();
set2.Add(1);
set2.Add(2);
set2.Add(3);

if(set1.Count == set2.Count && set1.IsSubsetOf(set2))
{
    // do something
}

Would this always work? Is there a better way? Why doesn't HashSet have a public bool IsEqualSetWith() function?

Laminitis

It's called IEqualityComparer :) – Tarryn

There is nothing wrong with SetEquals. – Duplessismornay

Note that the order of addition has nothing to do with hashset equality. – Defence

For anyone wanting an interface for accessing SetEquals, there is ISet<T> in System.Collections.Generic – Venge

Answer 1

Look at the method SetEquals.

my_hashset.SetEquals(other);

Wein

Thanks Michael, I don't know how I completely missed that in the method list on MSDN... – Laminitis

Happens to everyone at one time or another. I just can't believe I beat out both Jon Skeet and Marc Gravell on a .NET question. I guess they do have to sleep sometime. – Wein

Easy to miss. With the prefix "Set", the method looks like it is a mutator if you're not paying close attention – Krak

@MerlynMorgan-Graham: I never thought that way in a set theory context but an interesting catch anyway. – Disharmony

Answer 2

IEqualityComparer<HashSet<int>> comp = HashSet<int>.CreateSetComparer();
Console.WriteLine("CreateSetComparer set1 == set2 : {0}", comp.Equals(set1, set2));
// or
bool areEqual = HashSet<int>.CreateSetComparer().Equals(set1, set2);

Torbart

This is useful when you for example need to create a Dictionary where HashSet is used as a key. – Delamare