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[parvizwpf]

private static Random random = new Random();

        public static string RandomString()

        {

            const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

            const string numbers = "0123456789";

            var horoof = new string(Enumerable.Repeat(chars, 3)

              .Select(s => s[random.Next(s.Length)]).ToArray());



            var adad = new string(Enumerable.Repeat(numbers, 4)

              .Select(s => s[random.Next(s.Length)]).ToArray());



            return horoof + adad;

        }