对象不能从DBNull转换为其他类型？ C＃

Question

我有一个数据库查询，当我在MySql中运行查询时，它返回值。但我得到的对象不能从DBNULL转换为其他类型的异常，当我在代码中运行它。我如何从代码中检索数据？

public int getLatestReading(string connstr, string gId, string mId, DateTime date) 
    { 

     int active = 0; 
     MySqlConnection myconn = new MySqlConnection(connstr); 
     String sqlstr = "Select max(ar.ActualReading) From actualmeterreading ar Inner Join meterreadingrecords mr On mr.ActualMeterReadingID = ar.ID Inner join machinemeterreadinglist mml on mml.ID = ar.MachineMeterReadingId Inner join grouping g on g.ID = mml.Grouping inner join machines m on m.ID = mml.MachineID where g.ID = '" + gId + "' and m.ID = '" + mId + "' and mr.rDate > '" + date + "'"; 
     MySqlCommand cmd = new MySqlCommand(sqlstr, myconn); 
     myconn.Open(); 
     active = Convert.ToInt32(cmd.ExecuteScalar()); 
     myconn.Close(); 
     myconn.Dispose(); 
     return active; 
    } 

Answer 1

请使用下面的代码和平。

public int getLatestReading(string connstr, string gId, string mId, DateTime date) 
    { 

     int active = 0; 
     MySqlConnection myconn = new MySqlConnection(connstr); 
     String sqlstr = "Select max(ar.ActualReading) From actualmeterreading ar Inner Join meterreadingrecords mr On mr.ActualMeterReadingID = ar.ID Inner join machinemeterreadinglist mml on mml.ID = ar.MachineMeterReadingId Inner join grouping g on g.ID = mml.Grouping inner join machines m on m.ID = mml.MachineID where g.ID = '" + gId + "' and m.ID = '" + mId + "' and mr.rDate > '" + date + "'"; 
     MySqlCommand cmd = new MySqlCommand(sqlstr, myconn); 
     myconn.Open(); 
     active = !Convert.IsDBNull(cmd.ExecuteScalar())?Convert.ToInt32(cmd.ExecuteScalar()):0; 
     myconn.Close(); 
     myconn.Dispose(); 
     return active; 
    } 

Answer 2

解决此问题的一种方法是确保查询从不返回空值。您可以通过更改这样的查询使用MySQL ifnull功能做到这一点：

Select ifnull(max(ar.ActualReading), 0) ...