数据读取器上已有一个已打开的连接

Question

我很难解决我的代码有什么问题，我仅用此错误就花了大约4个小时，我已经尝试关闭所有打开的数据读取器，但仍然收到错误消息“该连接已经有一个打开的DataReader，必须先关闭。”

  Try

        '///////////////////////
        Dim myconn As MySqlConnection = New MySqlConnection("Data Source=localhost;Database=csais;User ID=root;Password=;")
        myconn.Open()

        Dim query2 = "Select student_id FROM student_subject WHERE student_id = @student_id AND sub_id = @sub_id AND grade  = '5'  "

        cmd = New MySqlCommand(query2, myconn)
        cmd.Parameters.AddWithValue("@student_id", StudentID)
        cmd.Parameters.AddWithValue("@sub_id", SubjectID)
        cmd.Parameters.AddWithValue("@school_year", Sy)
        cmd.Parameters.AddWithValue("@semester", Sem)
        dr = cmd.ExecuteReader
        dr.Read()


        If dr.HasRows Then

            MessageBox.Show("Subject's failed!")
            query2 = "Update student_subject  SET school_year = concat(school_year, '" & lbl_As_SchoolYear.Text & "') , enrolled = '1'  WHERE student_id = '" & StudentID & "' AND sub_id = '" & SubjectID & "' "
            cmd = New MySqlCommand(query2, myconn)
            cmd.ExecuteNonQuery()
            dr.Close()
        End If

        Dim query3 = "Select student_id FROM student_subject WHERE student_id = @student_id AND sub_id = @sub_id  "

        cmd = New MySqlCommand(query3, myconn)
        cmd.Parameters.AddWithValue("@student_id", StudentID)
        cmd.Parameters.AddWithValue("@sub_id", SubjectID)
        cmd.Parameters.AddWithValue("@school_year", Sy)
        cmd.Parameters.AddWithValue("@semester", Sem)
        dr2 = cmd.ExecuteReader
        dr2.Read()


        If dr2.HasRows Then
            MessageBox.Show("Subject's already taken")
            dr2.Close()
            Exit Sub
        End If


        '///////////////////////////


        Dim query = "Insert Into student_subject (student_id, sub_Id, grade, enrolled,school_year,semester) Values (@student_id, @sub_id, 0, 1, @school_year, @semester);"
        Using cn As New MySqlConnection(connstring)
            Using cmd As New MySqlCommand(query, cn)
                cmd.Parameters.Add("@student_id", MySqlDbType.Int32).Value = StudentID
                cmd.Parameters.Add("@sub_id", MySqlDbType.Int32).Value = SubjectID
                cmd.Parameters.AddWithValue("@school_year", Sy)
                cmd.Parameters.AddWithValue("@semester", Sem)
                cn.Open()
                cmd.ExecuteNonQuery()
            End Using
        End Using
        MessageBox.Show("Successful enrollment")

        myconn.Close()


    Catch ex As Exception
        MessageBox.Show(ex.Message)
        MessageBox.Show("Insert Enrollement Function Error to!")
    End Try

非常感谢您。