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?
If you use HashSet.Contains pass your custom comparer to 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.