How to assign an integer ID to each class instace in C#?

"The problem with dictionary is that if i change the order of the instances, then even the ID changes."

That statement does not make sense. While it is correct that you should never rely on the KeyValuePairs in a Dictionary being in the same "order" you put them in (or, any order), using a Dictionary guarantees ... by virtue of the fact it does not allow duplicate Keys ... that each key is unique.

I think (guess) what you want is a unique id (int) for each instance of the Class; one way you can do that is:

using System.Collections.Generic;

namespace YourNameSpace
{
    public class AB
    {
        private static int AB_ID = 0;

        public static Dictionary<AB, int> ABInstanceToID = new Dictionary<AB, int>();

        public int ID { set; get; }

        public AB()
        {
            this.ID = AB_ID;
            ABInstanceToID.Add(this, this.ID);
            AB_ID++;
        }
    }
}

So, given a run-time instance of AB, you can read its ID by doing a lookup in the static Dictionary:

if(AB.ABInstanceToID.ContainsKey(someAB))
{
       int index = AB.ABInstanceToID[someAB];

       switch(index)
       {
           case 0:
              // handle case O
              break;

           default:
              break;
       }
}
else
{
     // throw an error ?
     // put up a warning with a MessageBox ?>
}

Note:

1.in "theory" the 'ContainsKey test should not be necessary, but I think it's always a good idea to check and re-check, and plan for errors.

2. while some might favor using Dictionary.TryGetValue for the check, in this case, where we can assume frequent attempts at look-up of missing Keys is very improbable, I see no reason to use that, here.

try this

AB a = new AB();
AB b = new AB();
AB c = new AB();
AB d = new AB();
AB e = new AB();

AB[] array = new AB[5];
array[0] = a;
array[1] = b;
array[2] = c;
array[3] = d;
array[4] = e;

int id = 2;
var instance = array[id];  // gets instance for c

Why not use an array of your class then ID is the index to the class you want to access?

Following gives 4 classes.

AB[] a = new AB[4];

Then a[ID] access the class you want.