对象不能从 DBNull 转换为其他类型？ C# [重复]答案

【问题标题】：Object cannot be cast from DBNull to other types ? C# [duplicate]对象不能从 DBNull 转换为其他类型？ C# [重复]
【发布时间】：2018-01-02 14:09:32
【问题描述】：

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它时，我得到 Object cannot be cast from DBNull to other types 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;
    }

【问题讨论】：

stackoverflow.com/questions/870697/…

标签： c# mysql

【解决方案1】：

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;
    }

【讨论】：

【解决方案2】：

解决此问题的一种方法是确保查询永远不会返回 null。您可以使用 MySql ifnull 函数通过更改您的查询来做到这一点：

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

【讨论】：