Question

What I’ve read on the HashSet is it uses the default comparer for a class. I’m expecting the code below to fail when adding the second Spork to the hash set. I think my understanding of what is happening is incomplete. From MSDN of the HashSet constructor:

The IEqualityComparer implementation to use when comparing values in the set, or null to use the default EqualityComparer implementation for the set type.

So what is the default comparer, and how can I tell .Net to use my own comparer?

public class Spork : IEquatable<Spork>
{
    public int Id { get; set; }


    public bool Equals(Spork other)
    {
        return other != null && other.Id == this.Id;
    }

    public override bool Equals(object obj)
    {
        var other = obj as Spork;
        return other != null && other.Id == this.Id;
    }

    public override int GetHashCode()
    {
        return Id.GetHashCode();
    }
}

public class Bjork
{
    public static HashSet<Spork> Sporks { get; set; }
    public static void Main()
    {
        Sporks = new HashSet<Spork>();
        Sporks.Add(new Spork() { Id = 0 });
        Sporks.Add(new Spork() { Id = 0 });     // come on, please throw an exception
    }
}

Answer 1

It is using your equality methods – but HashSet<T>.Add doesn’t throw an exception when you try to add an equal value – it just returns false.

If you change the last two lines to print out the return value of Add, you’ll see it returns True the first time, then False.