Incrementing a unique ID number in the constructor

Question

I'm working on an object in C# where I need each instance of the object to have a unique id. My solution to this was simply to place a member variable I call idCount in the class and within the constructor I would have:

objectID = idCount;
idCount++;

I thought that this would solve my problem but it seems that idCount never gets incremented even though the constructor gets called multiple times. For example if idCount = 1, the objectID for all the objects are still 1. Why doesn't idCount++ work?

Any help would be appreciated. Apologies if my explanation isn't adequate, I'm not sure how else to explain it.

Answer 1

You need a static property in your class, BUT, you need to assign it to an instance variable within the class if you want each object to contain the id it was created with.

Also, you'll want to use Interlocked.Increment on the counter in case you are updating multiple instances simultaneously:

    public class Foo
    {
        private static int m_Counter = 0;

        public int Id { get; set; }

        public Foo()
        {
            this.Id = System.Threading.Interlocked.Increment(ref m_Counter);
        }
    }

Answer 2

You could use a static variable in your class that gets updated when the object is initialized.

public class Foo
{
   private static int ID = 0;
   private int myId = 0;

   public int MyId
   {
      get { return myId; }
   }

   public Foo()
   {
       ID++;
       this.myId = ID;
   }
}

Answer 3

As everyone has pointed out, static variables are the specific answer to your question. But static variables only have scope within the process in which they were created and there is no relationship across processes (for example, a load balanced web environment).

If what you are looking for is a unique way to identify an object instance for the duration of its lifetime, I suggest something like:

byte[] bytes = new byte[8];

RNGCryptoServiceProvider crypto = new RNGCryptoServiceProvider();            
crypto .GetBytes( bytes );

long id = BitConverter.ToInt64( bytes, 0 );

This will give you a random number which has an extremely low (roughly 0-1 in 100,000,000) chance of collision and you don't need to worry about keeping track of it.