對象不能從DBNull轉換爲其他類型？ C＃

Question

我有一個數據庫查詢，當我在MySql中運行查詢時，它返回值。但我得到的對象不能從DBNULL轉換爲其他類型的異常，當我在代碼中運行它。我如何從代碼中檢索數據？

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

請使用下面的代碼和平。

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

解決此問題的一種方法是確保查詢從不返回空值。您可以通過更改這樣的查詢使用MySQL ifnull功能做到這一點：

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