How do I obtain an ID that allows me to tell difference instances of a class apart?

Question

Imagine I have a single class, with two instances:

MyClass a = new MyClass();
MyClass b = new MyClass();

MyClass has a method PrintUniqueInstan

Answer 1

Revised answer

Based on the additional information we now have, I believe that you can solve your problem very easily using ConditionalWeakTable (which is only from .NET 4 onwards).

• it can be used to associate arbitrary data with managed object instances
• it does not keep objects "alive" just because they have been entered as keys into the table
• it uses reference equality to determine object identity; moveover, class authors cannot modify this behavior (handy since you are not the author of every class on earth)
• it can be populated on the fly

You can therefore create such a global table inside your "manager" class and associate each object with a long, a Guid or anything else you might want¹. Whenever your manager encounters an object it can get its associated id (if you have seen it before) or add it to the table and associate it with a new id created on the spot.

¹ Actually the values in the table must be of a reference type, so you cannot use e.g. long as the value type directly. However, a simple workaround is to use object instead and let the long values be boxed.

Original answer

Isn't this a basic usage example for static members?

class Foo
{
    private static int instanceCounter;
    private readonly int instanceId;

    Foo()
    {
        this.instanceId = ++instanceCounter;
    }

    public int UniqueId
    {
        get { return this.instanceId; }
    }
}

Of course you have to pay attention to the range of identifiers so that you don't start reusing them if billions of instances are created, but that's easy to solve.