cant take backup of sql database

public void BackupDatabase(String databaseName, String userName,
           String password, String serverName, String destinationPath)
       {
           try
           {
               string drop_down_value = cmbDatabase.SelectedItem.ToString();

               string con = @"Data Source=.\SQLEXPRESS;AttachDbFilename='" + drop_down_value + "';Integrated Security=True;User Instance=True";

               Backup sqlBackup = new Backup();

               sqlBackup.Action = BackupActionType.Database;
               sqlBackup.BackupSetDescription = "ArchiveDataBase:" +
                                                DateTime.Now.ToShortDateString();
               sqlBackup.BackupSetName = "Archive";

               sqlBackup.Database = "event.mdf";

               BackupDeviceItem deviceItem = new BackupDeviceItem("D:\\test.bak", DeviceType.File);
               SqlConnection sqlCon = new SqlConnection(con);
               ServerConnection connection = new ServerConnection(sqlCon);
               //  ServerConnection connection = new ServerConnection(serverName, userName, password);
               Server sqlServer = new Server(connection);

               Database db = sqlServer.Databases[drop_down_value];

               sqlBackup.Initialize = true;
               sqlBackup.Checksum = true;
               sqlBackup.ContinueAfterError = true;

               sqlBackup.Devices.Add(deviceItem);
               sqlBackup.Incremental = false;

               sqlBackup.ExpirationDate = DateTime.Now.AddDays(3);
               sqlBackup.LogTruncation = BackupTruncateLogType.Truncate;

               sqlBackup.FormatMedia = false;

               sqlBackup.SqlBackup(sqlServer);
               MessageBox.Show("The support of the database was successfully performed", "Back", MessageBoxButtons.OK, MessageBoxIcon.Information);
           }
           catch (Exception ex)
           {
           }
       }