单击ASP.NET AJAX时DropDownList选择的值

I have this ddl:

我有这个ddl:

        <asp:DropDownList  ID="ddl" runat="server">
                <asp:ListItem Value="-1">Select</asp:ListItem>
                <asp:ListItem Value="2">My Images</asp:ListItem>
                <asp:ListItem Value="3">My Documents</asp:ListItem>
        </asp:DropDownList>
        <asp:Drop