.NET unique object identifier

Question

Is there a way of getting a unique identifier of an instance?

GetHashCode() is the same for the two references pointing to the same instance. However, two different instances can (quite easily) get the same hash code:

Hashtable hashCodesSeen = new Hashtable();
LinkedList<object> l = new LinkedList<object>();
int n = 0;
while (true)
{
    object o = new object();
    // Remember objects so that they don't get collected.
    // This does not make any difference though :(
    l.AddFirst(o);
    int hashCode = o.GetHashCode();
    n++;
    if (hashCodesSeen.ContainsKey(hashCode))
    {
        // Same hashCode seen twice for DIFFERENT objects (n is as low as 5322).
        Console.WriteLine("Hashcode seen twice: " + n + " (" + hashCode + ")");
        break;
    }
    hashCodesSeen.Add(hashCode, null);
}

I'm writing a debugging addin, and I need to get some kind of ID for a reference which is unique during the run of the program.

I already managed to get internal ADDRESS of the instance, which is unique until the garbage collector (GC) compacts the heap (= moves the objects = changes the addresses).

Stack Overflow question Default implementation for Object.GetHashCode() might be related.

The objects are not under my control as I am accessing objects in a program being debugged using the debugger API. If I was in control of the objects, adding my own unique identifiers would be trivial.

I wanted the unique ID for building a hashtable ID -> object, to be able to lookup already seen objects. For now I solved it like this:

Build a hashtable: 'hashCode' -> (list of objects with hash code == 'hashCode')
Find if object seen(o) {
    candidates = hashtable[o.GetHashCode()] // Objects with the same hashCode.
    If no candidates, the object is new
    If some candidates, compare their addresses to o.Address
        If no address is equal (the hash code was just a coincidence) -> o is new
        If some address equal, o already seen
}

Answer 1

The reference is the unique identifier for the object. I don't know of any way of converting this into anything like a string etc. The value of the reference will change during compaction (as you've seen), but every previous value A will be changed to value B, so as far as safe code is concerned it's still a unique ID.

If the objects involved are under your control, you could create a mapping using weak references (to avoid preventing garbage collection) from a reference to an ID of your choosing (GUID, integer, whatever). That would add a certain amount of overhead and complexity, however.