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Asked: 2026-05-11T14:43:05+00:00

Is there a way of getting a unique identifier of an instance? GetHashCode() is

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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 }
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1 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.

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