Question

我是C＃的新手。我试图从名为'tblAddResult'的表中读取数据。在这个表中我有一些列，我想平均3列tblAddResult并将其保存到名为tblResult的新表中。

其他一些计算也需要像tblAddResult列那样采集50％的数据。

我这样做了（代码放在下面），但是我收到一个错误“读取器关闭时调用read的无效尝试”。我甚至不知道这是否是正确的方法，如果没有人可以帮助我写这种方式或任何有关这种方式的建议。我对这个问题真的很无奈。

     private void button1_Click(object sender, EventArgs e)
       {
        con.Open();
        cmd = new SqlCommand("SELECT   tblAddResult.* FROM  tblAddResult", con);
        SqlDataReader sdr = cmd.ExecuteReader();

        while (sdr.Read())
        {

            String subject = sdr.GetString(2);
            int january = sdr.GetInt32(3);
            int february = sdr.GetInt32(4);
            int march = sdr.GetInt32(5);
            int average = (january + february + march) / 3;
            int average40= average*40/(100);
            int marks = sdr.GetInt32(6);
            int marks50 = marks * 50 / 100;
            int WorkingDay = sdr.GetInt32(7);
            int Attandence = sdr.GetInt32(8);
            int Attendence10 = Attandence * 10 / 100;
            int totalMarks = average40 + marks50 + Attendence10;
            string grade = "" ;
            if (totalMarks < 51) { grade = "C"; }
            else if (totalMarks < 61) { grade = "B"; }
            else if (totalMarks < 71) { grade = "A-"; }
            else if (totalMarks < 81) { grade = "A"; }
            else if (totalMarks < 91) { grade = "A+"; }
            else if (totalMarks <= 100) { grade = "A++"; }
            con.Close();
            con.Open();
            SqlCommand comnd = new SqlCommand("INSERT INTO tblResult (Subject, [Full Marks], January, February, March, [Average Class Perfomance], [Earned Marks], [Working Day], Attendence, [Cls Attendence(40%)], [Exam Perfomance(50%)], [Attendence(10%)], [Marks(%)], Grade)VALUES  ('"+subject+"','"+"100"+"','"+january+"','"+february+"','"+march+"','"+average+"','"+marks+"','"+WorkingDay+"','"+Attandence+"','"+average40+"','"+marks50+"','"+Attendence10+"','"+totalMarks+"','"+grade+"')",con);
            comnd.ExecuteNonQuery();
            MessageBox.Show("Successfull!");

        }

Answer 1

您收到此错误是因为在返回/完成阅读器之前关闭了连接。删除两行con.Close();和con.Open();并在阅读器完成后（即，在while循环之外）连接连接。

Answer 2

编辑如下代码，以提高安全性。发生错误是因为DataReader操作仅在处于Opened Connection状态时才起作用。

        con.Open();
        cmd = new SqlCommand("SELECT tblAddResult.* FROM  tblAddResult", con);
        SqlDataReader sdr = cmd.ExecuteReader(System.Data.CommandBehavior.CloseConnection);//this will close the DataReader along with Connection close.

        while (sdr.Read())
        {

            String subject = sdr.GetString(2);
            int january = sdr.GetInt32(3);
            int february = sdr.GetInt32(4);
            int march = sdr.GetInt32(5);
            int average = (january + february + march) / 3;
            int average40= average*40/(100);
            int marks = sdr.GetInt32(6);
            int marks50 = marks * 50 / 100;
            int WorkingDay = sdr.GetInt32(7);
            int Attandence = sdr.GetInt32(8);
            int Attendence10 = Attandence * 10 / 100;
            int totalMarks = average40 + marks50 + Attendence10;
            string grade = "" ;
            if (totalMarks < 51) { grade = "C"; }
            else if (totalMarks < 61) { grade = "B"; }
            else if (totalMarks < 71) { grade = "A-"; }
            else if (totalMarks < 81) { grade = "A"; }
            else if (totalMarks < 91) { grade = "A+"; }
            else if (totalMarks <= 100) { grade = "A++"; }

            cmd = new SqlCommand("INSERT INTO tblResult (Subject, [Full Marks], January, February, March, [Average Class Perfomance], [Earned Marks], [Working Day], Attendence, [Cls Attendence(40%)], [Exam Perfomance(50%)], [Attendence(10%)], [Marks(%)], Grade)VALUES  ('"+subject+"','"+"100"+"','"+january+"','"+february+"','"+march+"','"+average+"','"+marks+"','"+WorkingDay+"','"+Attandence+"','"+average40+"','"+marks50+"','"+Attendence10+"','"+totalMarks+"','"+grade+"')",con);
            cmd.ExecuteNonQuery();           

        }
        con.Close();
        MessageBox.Show("Successfull!");

编辑：

使用DataTable代替使用DataReader方法，下面给出了一个示例。希望它会有所帮助。

cmd = new SqlCommand("SELECT * FROM  Test_Table1", con);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            da.Fill(dt);

            foreach (DataRow sdr in dt.Rows)
            {
                String subject = sdr[0].ToString();
                int january = Convert.ToInt32(sdr[1]);
                int february = Convert.ToInt32(sdr[2]);
                int march = 0;
                int average = (january + february + march) / 3;
                int average40 = average * 40 / (100);

                cmd = new SqlCommand("INSERT INTO Test_Table2 (Test_Col1, Test_Col2) VALUES  ('" + subject + "'," + average40 + ")", con);
                con.Open();
                cmd.ExecuteNonQuery();
                con.Close();
            }

在C＃中如何从表中读取数据并在一些计算后将这些数据保存在另一个表中？

2 个答案: