C# SortedSet and equality

Question

I am a bit puzzled about the behaviour of SortedSet, see following example:

public class Blah
{
    public double Value { get; private set; }

    public Bla         


        

Answer 1

You can do this if you provide an alternate comparison when the Values are equal and the Compare method would otherwise return 0. In most cases this would probably just defer the problem instead of solving it. As others have noted, the SortedSet discards duplicates and when you provide a custom comparer it uses that to determine duplicity.

    static void Main(string[] args)
    {
        var blahs = new List
                        {
                            new Blah(1, 0), new Blah(2, 1),
                            new Blah(3, 2), new Blah(2, 3)
                        };

        blahs.Add(blahs[0]);

        //contains all 4 entries
        var set = new HashSet(blahs);

        //contains all 4 entries
        var sortedset = new SortedSet(blahs, new BlahComparer());

    }
}

public class Blah
{
    public double Value { get; private set; }

    public Blah(double value, int index)
    {
        Value = value;
        Index = index;
    }

    public int Index { get; private set; }

    public override string ToString()
    {
        return Value.ToString();
    }
}

public class BlahComparer : Comparer
{
    public override int Compare(Blah x, Blah y)
    {
        // needs null checks
        var referenceEquals = ReferenceEquals(x, y);
        if (referenceEquals)
        {
            return 0;
        }
        var compare = Comparer.Default.Compare(x.Value, y.Value);
        if (compare == 0)
        {
            compare = Comparer.Default.Compare(x.Index, y.Index);
        }
        return compare;
    }
}