Create Unique ID

I am Trying To create unique IDs like PO0001, PO0002, so on and My code is given below which works fine for a single entity like PO0001:

 using (var context = new User_InfoEntities())             {                 int count = getData();                 count = count + 1;                                var con = new Project                 {                     ID = "PO0000" + count.ToString ...