无法将‘System.DBNull’类型的对象转换为‘System.String’

Question

无法将‘System.DBNull’类型的对象转换为‘System.String’我在我的应用程序中得到了上面的错误。这是原始代码public string GetCustomerNumber(Guid id){      string accountNumber =            (string)DBSqlHelperFactory.ExecuteScalar(connectionStringSplendidmyApp,                            CommandType.StoredProcedure,                            "GetCustomerNumber",                            new SqlParameter("@id", id));      return accountNumber.ToString();  }我用public string GetCustomerNumber(Guid id){    object accountNumber =             (object)DBSqlHelperFactory.ExecuteScalar(connectionStringSplendidCRM,                                  CommandType.StoredProcedure,                                  "spx_GetCustomerNumber",                                  new SqlParameter("@id", id));     if (accountNumber is System.DBNull)     {        return string.Empty;     }     else     {        return accountNumber.ToString();     }}有什么更好的办法吗？