What does .NET Dictionary<T,T> use to hash a reference?

Question

So I'm thinking of using a reference type as a key to a .NET Dictionary...

Example:

class MyObj
{
    private int mID;
    public MyObj(int id)
    {
        this.mID = id;
    }
}

// whatever code here
static void Main(string[] args)
{
    Dictionary<MyObj, string> dictionary = new Dictionary<MyObj, string>();
}

My question is, how is the hash generated for custom objects (ie not int, string, bool etc)? I ask because the objects I'm using as keys may change before I need to look up stuff in the Dictionary again. If the hash is generated from the object's address, then I'm probably fine... but if it is generated from some combination of the object's member variables then I'm in trouble.

EDIT:

I should've originally made it clear that I don't care about the equality of the objects in this case... I was merely looking for a fast lookup (I wanted to do a 1-1 association without changing the code of the classes involved).

Thanks

Answer 1

The default implementation of GetHashCode/Equals basically deals with identity. You'll always get the same hash back from the same object, and it'll probably be different to other objects (very high probability!).

In other words, if you just want reference identity, you're fine. If you want to use the dictionary treating the keys as values (i.e. using the data within the object, rather than just the object reference itself, to determine the notion of equality) then it's a bad idea to mutate any of the equality-sensitive data within the key after adding it to the dictionary.

The MSDN documentation for object.GetHashCode is a little bit overly scary - basically you shouldn't use it for persistent hashes (i.e. saved between process invocations) but it will be consistent for the same object which is all that's required for it to be a valid hash for a dictionary. While it's not guaranteed to be unique, I don't think you'll run into enough collections to cause a problem.

Answer 2

The hash used is the return value of the .GetHashcode method on the object. By default this essentially a value representing the reference. It is not guaranteed to be unique for an object, and in fact likely won't be in many situations. But the value for a particular reference will not change over the lifetime of the object even if you mutate it. So for this particular sample you will be OK.

In general though, it is a very bad idea to use objects which are not immutable as keys to a Dictionary. It's way too easy to fall into a trap where you override Equals and GetHashcode on an object and break code where the type was formerly used as a key in a Dictionary.