c# - 在下拉列表的 selectedindexchanged 后没有页面刷新？

Question

在用户根据他们选择的内容选择第一个所需的下拉列表后，我正在使用两个下拉框，另一个下拉列表将加载最终列表。我有这个工作但是我不希望页面在选择第一个下拉选项后重新加载是否有解决方法下面是我的测试代码

Behind code

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                DropDownList1.Items.Add(new ListItem("3 Days", "3 Days"));
                DropDownList1.Items.Add(new ListItem("4 Days", "4 Days"));
                DropDownList1.Items.Add(new ListItem("5 Days", "5 Days"));
                DropDownList1.Items.Add(new ListItem("7 Days", "7 Days"));
            }
        }

        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            DropDownList2.Items.Clear();
            DropDownList3.Items.AddRange(GetListItems(DropDownList1.SelectedValue));
        }

        private ListItem[] GetListItems(string value)
        {
            var items = new List
                <ListItem>
                ();

            if (value == "3 Days")
            {
                DropDownList2.Items.Add(new ListItem("1", "1"));
            }

            if (value == "4 Days")
            {
                DropDownList2.Items.Add(new ListItem("2", "2"));
            }

            if (value == "5 Days")
            {
                DropDownList2.Items.Add(new ListItem("3", "3"));
            }

            if (value == "7 Days")
            {
                DropDownList2.Items.Add(new ListItem("4", "4"));
            }
            return items.ToArray();
        }

<div class="1">
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
  <asp:ListItem Text="Select..." Value="No selection made"></asp:ListItem>
</asp:DropDownList>
</div>
<div class="1">
  <asp:DropDownList ID="DropDownList2" runat="server">
    <asp:ListItem Text="Select..." Value="No selection made"></asp:ListItem>
  </asp:DropDownList>
</div>

Answer 1

( FROM MSDN )ScriptManager 控件和 UpdatePanel 控件。这些控件消除了每次回发时刷新整个页面的要求，从而改善了用户体验。默认情况下，UpdatePanel 控件内的回发控件(例如按钮)会导致部分页面更新。默认情况下，UpdatePanel 控件之外的按钮或其他控件会导致刷新整个页面，

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <fieldset>
            <div class="1">
                <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
                    <asp:ListItem Text="Select..." Value="No selection made"></asp:ListItem>
                </asp:DropDownList>
            </div>
            <div class="1">
                <asp:DropDownList ID="DropDownList2" runat="server">
                    <asp:ListItem Text="Select..." Value="No selection made"></asp:ListItem>
                </asp:DropDownList>
            </div>
        </fieldset>
    </ContentTemplate>
</asp:UpdatePanel>