How to generate continuous sequence number with a prefix in asp.net

public string serialId()
        {
            string Id = GenerateId("customer_details", "id", 6, "PB", true);
            return Id;
        }

        public string GenerateId(string TableName, string ColumnName, int ColumnLength, string Prefix, bool Padding)
        {
            string query, con, Id;
            con = "Data Source=.;Initial Catalog=login_;Integrated Security=True";
            SqlConnection cn = new SqlConnection(con);
            int preLength, padLength;
            preLength = Convert.ToInt32(Prefix.Length);
            padLength = ColumnLength - preLength;
            if (Padding == true)
            {
                query = "SELECT'" + Prefix + "'+REPLACE(STR(MAX(CAST(SUBSTRING(" + ColumnName + "," + Convert.ToString(preLength + 1) + "," + padLength + ")AS INTEGER))+1 ," + padLength + "),'',0)FROM " + TableName;
            }
            else
            {
                query = "SELECT'" + Prefix + "'+CAST(MAX(CAST(SUBSTRING(" + ColumnName + "," + Convert.ToString(preLength + 1) + "," + padLength + ")AS INTEGER))+1 AS VARCHAR) FROM " + TableName;
            }
            SqlCommand com = new SqlCommand(query, cn);
            cn.Open();
            if (com.ExecuteScalar().ToString() == "")
            {
                Id = Prefix;
                if (Padding == true)
                {
                    for (int i = 1; i <= padLength - 1; i++)
                    {
                        Id += "0";
                    }
                }
                Id += "1";
            }
            else
            {
                Id = Convert.ToString(com.ExecuteScalar());
            }
            cn.Close();
            return Id;
        }