Linq Except ignoring Custom comparer?

Question

In this toy code:

void Main()
{
    var x = new string[] {"abc", "DEF"};
    var y = new string[] {"ABC", "def"};
    var c = new CompareCI();
    var z = x.Except(y, c);
    foreach (var s in z) Console.WriteLine(s);
}

private class CompareCI : IEqualityComparer<string>
{
    public bool Equals(string x, string y)
    {
        return string.Equals(x, y, StringComparison.OrdinalIgnoreCase);
    }

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

It seems like the Except method is ignoring my customer comparer. I get these results:

abc
DEF

Which looks like the case is not being ignored. Also, when I ran it under debug and put a breakpoint at the call to string.Equals in the Customer Comparer, the breakpoint never hit, although the code ran and I got the result I posted. i expected no results, since the sequences are equal if case is ignored.

Guess I'm doing something wrong, but I need a second pair of eyes to spot it.

Answer 1

.NET Framework already provides a StringComparer Class, that uses specific case and culture-based or ordinal comparison rules - so in this case there is no need to create a custom comparer.

This will work:

var x = new string[] { "abc", "DEF" };
var y = new string[] { "ABC", "def" };
var z = x.Except(y, StringComparer.OrdinalIgnoreCase);