Why is GetHashCode not called during Contains?

Question

Until today my understanding was that a HashSet uses GetHashCode inside Contains. That is also said e.g. here.

I wrote a little IEqualityComparer:

public class MyComparer : IEqualityComparer<string>
{
    public bool Equals(string? a, string? b)
    {
        return a == b;
    }

    public int GetHashCode(string a)
    {
        throw new NotImplementedException();
    }
}

And used it like so:

public void TestMyComparer()
{
    var x = new HashSet<string>(new []{ "hello", "world" });
    bool helloInside = x.Contains("hello", new MyComparer());
}

But TestMyComparer does not throw a NotImplementedException, as I expected. Instead, it returns true.

Why?

Answer 1

If you want to use your custom comparer in HashSet.Contains, pass it to the the constructor.

var x = new HashSet<string>(new MyComparer());
x.Add("hello");
x.Add("world");
bool helloInside = x.Contains("hello");

Now GetHashCode is used since you use a set based collection and not Enumerable.Contains which just enumerates all items and compares them with Equals.