C# how to calculate hashcode from an object reference.

Question

Folks, here's a thorny problem for you!

A part of the TickZoom system must collect instances of every type of object into a Dictionary<> type.

It is imperative that their equality and hash code be based on the instance of the object which means reference equality instead of value equality. The challenge is that some of the objects in the system have overridden Equals() and GetHashCode() for use as value equality and their internal values will change over time. That means that their Equals and GetHashCode are useless. How to solve this generically rather than intrusively?

So far, We created a struct to wrap each object called ObjectHandle for hashing into the Dictionary. As you see below we implemented Equals() but the problem of how to calculate a hash code remains.

public struct ObjectHandle : IEquatable<ObjectHandle>{
    public object Object;
    public bool Equals(ObjectHandle other) {
        return object.ReferenceEquals(this.Object,other.Object);
    }
}

See? There is the method object.ReferenceEquals() which will compare reference equality without regard for any overridden Equals() implementation in the object.

Now, how to calculate a matching GetHashCode() by only considering the reference without concern for any overridden GetHashCode() method?

Ahh, I hope this give you an interesting puzzle. We're stuck over here.

Sincerely, Wayne

Answer 1

The hash code doesn't have to be unique. (But uniqueness will improve performance).
So - one thing you can do is set the hash code by the type name. All object from the same type will have the same hash code.

Answer 2

You are breaking the pattern, this leads to questions that can't be solved. Method Equals should compare the contents of objects, instead of comparing references. That is what object.Equals does, why override with same behavior?

Now about GetHashCode. Again, hashcode is hash function applied to contents of object. You can't calculate it by reference only. You could use pointer to object and use it as hash, but in .net objects' addresses can be changed by GC.

Answer 3

RuntimeHelpers.GetHashCode() does exactly what is needed here.