This sample code generate 30,000 random 5-character-alphanumeric code in C#. The codes are stored in Hashset to be used for duplicate check.
Random random = new Random(); const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"; int length = 6; int total = 30000; HashSet<string> codes = new HashSet<string>(); Response.Write(DateTime.Now.ToString() + "<br>"); int d = 0; for(int i=0;i<total;i++) { string randomAlpha = new string(Enumerable.Repeat(chars, length) .Select(s => s[random.Next(s.Length)]).ToArray()); if(codes.Contains(randomAlpha)) { Response.Write("found dup " + d); d++; i--; continue; // skip Add and generate again } codes.Add(randomAlpha); }