c# Random, How to check if number exist and if not save on Database using ASP.Net MVC Project

I have Randum method for number generating and then save it if the number not exist in database. Is this way the write way to do? I'am not sure If I will write while (OrderNumber != null) Or while (OrderNumber == null); This is my code:

private string loadOrderNr()
        {
            using (Db db = new Db())
            {
                Random rnd = new Random();
                long part1 = rnd.Next(100000, 999999);
                long part2 = rnd.Next(1000, 9999);

                string OrderNr = "CA-" + part1 + "-" + part2;

                return OrderNr;

            } 
        }

And for saving in Database

using (Db db = new Db())
{
  string OrderNumber = loadOrderNr();
  var CheckifOrderNrExist = db.CarAnn.Where(x => x.OrderNr ==  OrderNumber).FirstOrDefault();

   do
     {
       OrderNumber = loadOrderNr();
     }
     while (CheckifOrderNrExist != null);

     CarAnn ann = new CarAnn();                         
     ann.Description = model.Description;
     ann.Title = model.Title;
     ann.OrderNr = OrderNumber; 
     db.CarAnn.Add(ann);
     db.SaveChanges();

}

I'am not sure about this Do and While statment. Thank you in advance!