DropDownList 上消失的项目

Question

我遇到了显示学生姓名的 DropDownList 的问题。出于某种原因，在我单击一个用于给它们评分的按钮后，它会丢失项目。我想找到一种方法来保存这些物品。该按钮不会以任何方式过滤显示的学生。生成的 DropDownLists 也应该将 autopostback 设置为 true。学生姓名不会在后面的代码中检索或更改，因此我不确定为什么这些姓名会从此 DropDownList 中消失。欢迎任何提示/解决方案。更新：我已经附加了来自前端的代码以及来自 .cs 文件的代码，用于为学生发送标记的按钮。输入分数后返回模块，出现项目消失问题。

<asp:SqlDataSource 
    ID="SQLStudentList" 
    runat="server" 
    ConnectionString="<%$ ConnectionStrings:UniString %>"
    SelectCommand="SELECT DISTINCT  students_profile.user_id, (first_name + ' ' + last_name ) AS studentDetails FROM students_profile  INNER JOIN classlist ON students_profile.user_id = classlist.user_id INNER JOIN class ON class.class_id = classlist.class_id INNER JOIN student_module_grade ON classlist.classlist_id = student_module_grade.classlist_id INNER JOIN student_module_repeat_grades ON student_module_grade.classlist_id = student_module_repeat_grades.classlist_id WHERE class.pathway_year_id = @idpathway AND  student_module_grade.module_on_pathway_id  =@modpwayid OR student_module_repeat_grades.module_on_pathway_id=@modpwayid">

  <SelectParameters>
    <asp:ControlParameter Name="idpathway" ControlID="degreeProgDropDown" Type="String"/>
    <asp:ControlParameter ControlID="modDropDown" Name="modpwayid" />
  </SelectParameters>
</asp:SqlDataSource>
<asp:DropDownList ID="StudentsList" 
  OnSelectedIndexChanged="StudentsList_SelectedIndexChanged" 
  runat="server" 
  width="420" 
  AutoPostBack="true" 
  EnableViewState="true"
  DataSourceID="SQLStudentList" 
  DataTextField="studentDetails" 
  DataValueField="user_id">
</asp:DropDownList>


    protected void Page_Load(object sender, EventArgs e)
{



    ////If there are no students the message below will be displayed
    ListItem selectedItem = StudentsList.SelectedItem;
    if (selectedItem != null && !String.IsNullOrEmpty(selectedItem.Text))
    {

    }
    else
    {
        changedFlag.Visible = true;
        changedFlag.Text = "There are currently no grades to change for any students for this module on this pathway";
        changedFlag.ForeColor = System.Drawing.Color.Red;
        EnterFinalMark.Visible = false;
        finalMarkAssignment.Visible = false;
        submitAssignmentMark.Visible = false;
        repeatSubmitAssignmentMark.Visible = false;
    }
    if (!IsPostBack)
    {

        StudentsList.DataSource = SQLStudentList;
        StudentsList.DataBind();
        String userName = Session["UserLoggedOn"].ToString();

        String conString = WebConfigurationManager.ConnectionStrings["UniString"].ConnectionString;
        SqlConnection myCon = new SqlConnection(conString);

        myCon.Open();


        String pathwaySelectionQuery = "SELECT pathway_years.id, pathway_years.pathway_year, pathway FROM pathways INNER JOIN pathway_years ON pathways.id = pathway_years.pathway_id";
        SqlCommand pathwaySelectionQuerycmd = new SqlCommand(pathwaySelectionQuery, myCon);
        SqlDataReader pwayReader = pathwaySelectionQuerycmd.ExecuteReader();
        while (pwayReader.Read())
        {
            //Put pathway year id in this table instead
            degreeProgDropDown.Items.Add(new ListItem(pwayReader["pathway_year"] + ": " + pwayReader["pathway"].ToString(), pwayReader["id"].ToString()));


        }


        myCon.Close();

    }
}

    protected void repeatSubmitAssignmentMark_Click(object sender, EventArgs e)
{
    String connectionString = WebConfigurationManager.ConnectionStrings["UniString"].ConnectionString;
    SqlConnection myConnection = new SqlConnection(connectionString);
    myConnection.Open();
    String repeatModgradeID = "SELECT repeat_module_grade_id from student_module_repeat_grades WHERE module_on_pathway_id = '" + modDropDown.SelectedValue + "'";
    SqlCommand repeatModuleGradeIDCommand = new SqlCommand(repeatModgradeID, myConnection);

    Int32 repeatModGradeIDResult = Convert.ToInt32(repeatModuleGradeIDCommand.ExecuteScalar().ToString());
    String repeatFindUserID = "SELECT classlist_id from classlist WHERE user_id = '" + StudentsList.SelectedValue + "'";
    SqlCommand repeatFindUserIDCommand = new SqlCommand(repeatFindUserID, myConnection);

    Int32 repeatClasslistval = Convert.ToInt32(repeatFindUserIDCommand.ExecuteScalar().ToString());
    String modOnPwayValue = modDropDown.SelectedValue;
    String repeatGrade = finalMarkAssignment.Text;
    Int32 repeatGradeval = Convert.ToInt32(repeatGrade);
    //Grade is a pass if it is equal to or greater than 40- otherwise it is a fail
    if (repeatGradeval >= 40)
    {
        //Pass assigned to the string which will be added to the table
        String passOrFail = "Pass";
        //Assigned to label
        pOrF.Text = passOrFail;

    }
    else
    {
        //Fail assigned to the string which will be added to the table
        String passOrFail = "Fail";
        //Assigned to label
        pOrF.Text = passOrFail;
    }
    if (repeatGradeval >= 0 && repeatGradeval <= 100)
    {
        changedVAL.Visible = false;
        SqlCommand addAssignmentGradeCommand = new SqlCommand("UPDATE student_module_repeat_grades SET classlist_id=@repeatClasslistid,module_on_pathway_id=@modOnPwayValue,grade=@grade,result_code=@PF,changed=1 WHERE module_on_pathway_id = '" + modDropDown.SelectedValue + "'", myConnection);
        addAssignmentGradeCommand.Parameters.AddWithValue(@"modOnPwayValue", modOnPwayValue);
        addAssignmentGradeCommand.Parameters.AddWithValue(@"repeatClasslistid", repeatClasslistval);
        addAssignmentGradeCommand.Parameters.AddWithValue(@"grade", repeatGradeval);
        addAssignmentGradeCommand.Parameters.AddWithValue(@"PF", pOrF.Text);
        addAssignmentGradeCommand.ExecuteNonQuery();
        myConnection.Close();
        success.Visible = true;
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "HideLabel();", true);
        success.ForeColor = System.Drawing.Color.Green;
        repeatSubmitAssignmentMark.Visible = false;

    }
    else
    {
        changedVAL.Visible = true;
        changedVAL.Text = "Please enter a grade between 0 and 100";
        changedVAL.ForeColor = System.Drawing.Color.Red;
    }
}

Answer 1

我最初的想法是，您目前可能没有检查页面的 Page_Load 事件中是否正在发生 PostBack，这将导致您的数据每次都反弹。

您通常可以通过在Page_Load 事件本身中执行检查来解决此问题：

protected void Page_Load(object sender, EventArgs e)
{
     // If it is an initial load
     if(!IsPostBack)
     {
         // Then perform your one-time data binding here
         StudentsList.DataSource = SQLStudentList;
         StudentsList.DataBind();
     }
     // Business as usual
}