第一次GridVIew在asp.net的默认下拉列表中未显示数据

Question

我不知道为什么会这样，实际上我在默认情况下通过下拉列表显示了一个名称，并且在数据库中还显示了它的数据，我也将其应用于gridview databind，但第一次却未见成效，因为它没有显示相同的选定值下拉菜单，但如果我确实喜欢更改并在列表中选择其他项目，然后再次选择下拉菜单中显示的值，则说明工作正常，原因是什么。 我在这里发送代码。

aspx.cs

<td>
                <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" 
                    DataSourceID="SqlDataSource1" DataTextField="Class" DataValueField="Class">
                </asp:DropDownList>
            </td>



 <asp:GridView ID="GridView3" runat="server" AutoGenerateColumns="False" DataKeyNames="ID"
                    DataSourceID="SqlDataSource10" OnRowCommand="GridView1_RowCommand" EmptyDataText="There are no data records to display.">
                    <Columns>
                        <asp:BoundField DataField="ID" HeaderText="ID" ReadOnly="True" Visible="false" SortExpression="ID" />
                        <asp:BoundField  DataField="ClassID" HeaderText="ClassID" SortExpression="ClassID" />
                        <asp:BoundField  DataField="FilePath" HeaderText="File Name" SortExpression="FilePath" />
                        <asp:ButtonField CommandName="Select" HeaderText="Download TimeTable" 
                            ShowHeader="True" Text="Download" />

                    </Columns>
                </asp:GridView>
                <asp:SqlDataSource ID="SqlDataSource10" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>"
                    DeleteCommand="DELETE FROM [UploadTabel] WHERE [ID] = @ID"
                    ProviderName="<%$ ConnectionStrings:ConnectionString.ProviderName %>"
                    SelectCommand="SELECT [ID], [ClassID], [FilePath] FROM [UploadTabel] Where ClassID=@Class" >
                    <DeleteParameters>
                        <asp:Parameter Name="ID" Type="Int32" />
                    </DeleteParameters>
                   <SelectParameters>
                        <asp:ControlParameter ControlID="DropDownList1" Name="Class" 
                            PropertyName="SelectedValue" Type="String" />
                    </SelectParameters>
                </asp:SqlDataSource>

CS

protected void Page_Load(object sender, EventArgs e)
    {

        GridView3.DataBind();
      //  lblSelectedDate.Text = AJAXDatePickerControl1.selectedDate.ToShortDateString();
    }

    protected void Page_PreLoad(object sender, EventArgs e)
    {
        try
        {
          //  GridView1.DataBind();
           GridView3.DataBind();
        }
        catch { }

    }

Answer 1

尝试以下代码：

 protected void Page_Load(object sender, EventArgs e)
        {
            if(!IsPostBack)
            {
            GridView3.DataBind();
            }
          //  lblSelectedDate.Text = AJAXDatePickerControl1.selectedDate.ToShortDateString();
        }