SortedSet<string> Linq .Except with Comparer strangeness

Question

I'm using .NET 4.6 and I experienced serious strangeness. I'm trying to get an IEnumerable of all values from a sorted set that are NOT the case-insensitive equivalent of the passed in IEnumerable. From what I understand, that is the job of Except.

I have a class, CaseInsensitiveComparer which is implemented like so:

public class CaseInsensitiveComparer : IComparer<string>, IEqualityComparer<string>
{
    public static CaseInsensitiveComparer Instance { get; private set; }
    static CaseInsensitiveComparer()
    {
        Instance = new CaseInsensitiveComparer();
    }

    public int Compare(string a, string b)
    {
        var ret = string.Compare(a, b, true);
        return ret;
    }

    public bool Equals(string a, string b)
    {
        return Compare(a, b) == 0;
    }

    public int GetHashCode(string a)
    {
        return a.GetHashCode();
    }
}

I use it like this:

    public void DotNetWeirdness()
    {
        var a = new SortedSet<string>();
        a.Add("A");
        var b = a.Except(new string[] { "a" }, CaseInsensitiveComparer.Instance);
    }

The value of b is an IEnumerable containing A. Since I'm asking for all values in SortedSet except for a/A, shouldn't I have an empty set as the result? I'm quite confused by this.

Thanks!

Answer 1

The issue is that you're not overriding GetHashCode . "A".GetHashCode() returns a different value from "a".GetHashCode() .

A quick way to fix this is to change the GetHashCode function to this:

public int GetHashCode(string a)
{
    return a.ToLower().GetHashCode();
}

Also, do you know about StringComparer ? You can change your code to this:

var b = a.Except(new string[] { "a" }, StringComparer.InvariantCultureIgnoreCase);

And not have to worry about implementing your own comparer