在ASP .Net中的第一个PostBack之后，Update Panel中的下拉列表不起作用

更新时间：2023-10-15 22:15:16

不要让这个变得复杂。有一种非常简单的方法可以使用 Ajax CascadingDropDown 。

参考 - ASP.Net中的AJAX级联DropDown示例 [ ^ ]

Don't make this complicated. There is a very easy way of doing this using Ajax CascadingDropDown.

Refer - AJAX Cascading DropDown Example in ASP.Net[^]

而不是使用单个updatepanel使用updatepanel和每个下拉列表，请认为它可以正常工作。

Hi,
Instead of using a single updatepanel use updatepanel with every dropdown, think so it will work fine.

尝试使用单个更新面板....

设计师：

Try using Single Update Panel....

DESIGNER:

<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>Learn How Add ASP.NET 3.5 AJAX UpdatePanel Triggers</title></head>
<body>
    <form id="form1" runat="server">
<div>



    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <br />
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <asp:Label ID="Label1" runat="server" Text="Country"></asp:Label>
            &nbsp;&nbsp;&nbsp;&nbsp;
            <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True"

                onselectedindexchanged="DropDownList1_SelectedIndexChanged">
            </asp:DropDownList>
            <br />
            <asp:Label ID="Label2" runat="server" Text="State"></asp:Label>
            &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
            <asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="True"

                Enabled="False" onselectedindexchanged="DropDownList2_SelectedIndexChanged">
            </asp:DropDownList>
            <br />
            <asp:Label ID="Label3" runat="server" Text="City"></asp:Label>
            &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
            <asp:DropDownList ID="DropDownList3" runat="server" AutoPostBack="True"

                Enabled="False" onselectedindexchanged="DropDownList3_SelectedIndexChanged">
            </asp:DropDownList>

            <br />
            <asp:Label ID="Label4" runat="server"></asp:Label>
            <br />
            <br />
        </ContentTemplate>

    </asp:UpdatePanel>



</div>


    </form>


    </body>
</html>

背后的代码：

CODE BEHIND:

public partial class _Default : System.Web.UI.Page 
{
protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        List<string> l1 = new List<string>();
        l1.Add("    ");
        l1.Add("India");
        l1.Add("USA");


        foreach (string item in l1)
        {
            DropDownList1.Items.Add(item);
        }
    }
}


protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    DropDownList2.Enabled = true;
    DropDownList3.Enabled = false;
    Label4.Text = "";
    List<string> l2 = new List<string>();
 
    if (DropDownList1.SelectedItem.Text == "India")
    {
            DropDownList2.Items.Clear();
             l2.Add(" ");
             l2.Add("Maharashtra");
             l2.Add("Goa");
           
       
        foreach (string item in l2)
        {
            DropDownList2.Items.Add(item);
        }
    }

    else
    {
        DropDownList2.Items.Clear();
            l2.Add(" ");
            l2.Add("Indiana");
            l2.Add("Texas");
        

        foreach (string item in l2)
        {
            DropDownList2.Items.Add(item);
        }
    }


}


protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
{
    DropDownList3.Enabled = true;
    List<string> l3 = new List<string>();
   
    if (DropDownList2.SelectedItem.Text == "Maharashtra")
    {
        DropDownList3.Items.Clear();
        l3.Add(" ");
        l3.Add("Pune");
        l3.Add("Mumbai");


        foreach (string item in l3)
        {
            DropDownList3.Items.Add(item);
        }
    }

    else if (DropDownList2.SelectedItem.Text == "Goa")
    {

        DropDownList3.Items.Clear();
            l3.Add(" ");
            l3.Add("Panji");
            l3.Add("Vasco");
    

        foreach (string item   in l3)
        {
            DropDownList3.Items.Add(item);
        }
    }

    else if (DropDownList2.SelectedItem.Text == "Indiana")
    {

        DropDownList3.Items.Clear();
        l3.Add(" ");
        l3.Add("Wilkinson");
        l3.Add("Morocco");


        foreach (string item in l3)
        {
            DropDownList3.Items.Add(item);
        }
    }
    else if (DropDownList2.SelectedItem.Text == "Texas")
    {

        DropDownList3.Items.Clear();
        l3.Add(" ");
        l3.Add("Archer City");
        l3.Add("Bryan");


        foreach (string item in l3)
        {
            DropDownList3.Items.Add(item);
        }
    }

}
protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e)
{
    Label4.Text = DropDownList1.SelectedItem.Text.ToString() + " " + DropDownList2.SelectedItem.Text.ToString() + " " + DropDownList3.SelectedItem.Text.ToString();
}
}

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在ASP .Net中的第一个PostBack之后，Update Panel中的下拉列表不起作用

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