Why does the C# dictionary not call the hidden GetHashCode method

Question

I've a project where I am extensively using the generic C# dictionary. I require composite keys, so I've been using tuples as keys. At some point I was wondering whether it would be beneficial to use a custom class which caches the hash code:

public class CompositeKey<T1, T2> : Tuple<T1, T2>
{
    private readonly int _hashCode;

    public CompositeKey(T1 t1, T2 t2) : base(t1, t2)
    {
        _hashCode = base.GetHashCode();
    }

    public new int GetHashCode()
    {
        return _hashCode;
    }
}

I used new instead of override because I thought it would not make a difference for this test, since I was defining the dictionary using the concrete type:

var dict = new Dictionary<CompositeKey<string, int>, int>();

I noticed, however, that my custom GetHashCode method is not called at all. When I changed new to override it got called as expected.

Can somebody explain why the hidden GetHashCode method is not called? I would expect this behavior if I would define the dictionary like this

var dict = new Dictionary<Tuple<string, int>, int>();

but not if I specify the CompositeKey type explicitly as in my example.

P.S. I know that hiding the GetHashCode method is probably not a good idea.

Answer 1

Can somebody explain why the hidden GetHashCode method is not called? I would expect this behavior if I would define the dictionary like this

To be able to call CompositeKey.GetHashCode method, one must have the reference of the instance of CompositeKey typed as CompositeKey in compile time.

But codebase of Dictionary<TKey,TValue> isn't aware of your CompositeKey class(obviously). All it knows is TKey(generic type parameter) which is as equivalent as having System.Object without any constraints. Because you can't call any methods of T other than which is declared in System.Object without a constraint.

So, Dictionary ends up calling Object.GetHashCode which isn't overridden in your class --and thus it is not called.