ExceptWith in HashSet for complex types

Question

I have HashSet of my custom class:

public class Vertex
{
    public string Name;

    public override bool Equals(object obj)
    {
        var vert = obj as Vertex;
        if (vert !=null)
        {
            return Name.Equals(vert.Name, StringComparison.InvariantCulture);
        }
        return false;
    }
}

And now I have tow hashsets

HashSet<Vertex> hashSet1 = new HashSet<Vertex>();

HashSet<Vertex> hashSet1 = new HashSet<Vertex>();

And now I'd like to have in hashSet1 only Vertexes that are not in hashSet2 So I use ExceptWith method

hashSet1.ExceptWith(hashSet2);

But this doesn't work. I suppose that this doesn't work because I have complex type. So the question is: is there some interface required to be implemented in Vertex class to make this thing work? I know that while creation of HashSet I can pass a EqualityComparer but it seems to me that it would be more elegant to implement some comparing interface method in Vertex class.

Is it possible or I just doesn't understand sth?

Thanks.

http://msdn.microsoft.com/en-us/library/ms173147%28v=vs.80%29.aspx — Soner Gönül, Mar 16 '13 at 15:01

score 4 · Accepted Answer · answered Mar 16 '13 at 15:01

When overriding Equals you should also override GetHashCode. HashSet (and other hashing structures like Dictionary) will first calculate a hash code for your objects to locate them in tne structure before comparing elements with Equals.

public override int GetHashCode()
{
    return StringComparer.InvariantCulture.GetHashCode(this.Name);
}

score 3 · Answer 2 · answered Mar 16 '13 at 15:06

You don't have to implement any interface (although IEquatable<T>) is encouraged. When you create a hash-set without specifying an equality-comparer, it defaults to using EqualityComparer<T>.Default, which asks the objects themselves to compare themselves to each other (special-casing null references).

However, in your case, your equality contract is broken since you haven't overriden GetHashCode. Here's how I would fix your type:

public class Vertex : IEquatable<Vertex>
{
    public string Name { get; private set; }

    public Vertex(string name)
    {
        Name = name;
    }

    public override int GetHashCode()
    {
        return StringComparer.InvariantCulture.GetHashCode(Name);
    }

    public override bool Equals(object obj)
    {
        return Equals(obj as Vertex);
    }

    public bool Equals(Vertex obj)
    {
        return obj != null && StringComparer.InvariantCulture.Equals(Name, obj.Name);
    }
}

score 1 · Answer 3 · edited May 23 '17 at 12:28

1

Would you mind overriding the .GetHashCode()too?

Here's the reference.

edited May 23 '17 at 12:28

Community

1
1

answered Mar 16 '13 at 15:00

Freddie Fabregas

1,162
1
7
17

score 0 · Answer 4 · answered Mar 16 '13 at 15:00

0

You have to override GetHashCode with Equals overriding.

Object.Equals Method:

Types that override Equals(Object) must also override GetHashCode; otherwise, hash tables might not work correctly.

answered Mar 16 '13 at 15:00

MarcinJuraszek

124,003
15
196
263

ExceptWith in HashSet for complex types

4 Answers4