对象不能从DBNull转换到其他类型？C#
对象不能从DBNull转换到其他类型？C#
提问于 2017-07-27 03:16:45
我有一个数据库查询，当我在MySql中运行查询时，它会返回值。但是，当我在代码中运行对象时，无法将对象从DBNull转换为其他类型的异常。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;
}回答 2
Stack Overflow用户
回答已采纳
发布于 2017-07-27 05:11:15
使用以下代码：
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;
}Stack Overflow用户
发布于 2017-07-27 03:51:15
解决该问题的一种方法是确保查询永远不返回null。您可以使用MySql ifnull函数来这样做，方法是将查询更改如下：
Select ifnull(max(ar.ActualReading), 0) ...页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/45340738
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