Come filtrare i valori dell'elenco a discesa in base a un altro elenco a discesa in ASP.NET, c#

controlla i seguenti collegamenti per popolare l'elenco a discesa a cascata.

http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/CascadingDropDown/CascadingDropDown .aspx

http://www.codeproject.com/KB/aspnet/CascadingDropDown.aspx

http://www.aspsnippets.com/Articles /Creating-Cascading-DropDownLists-in-ASP.Net.aspx

Codice:

<table>
    <tr>
        <td>First</td>
        <td><asp:DropDownList ID="DDLFirst" runat="server"  AutoPostBack="true"
                onselectedindexchanged="DDLFirst_SelectedIndexChanged"></asp:DropDownList></td>
    </tr>
    <tr>
        <td>Secord</td>
        <td><asp:DropDownList ID="DDLSecond" runat="server" AutoPostBack="true"
                onselectedindexchanged="DDLSecond_SelectedIndexChanged">
            <asp:ListItem Text="Select" Value="Select"></asp:ListItem>    
            </asp:DropDownList></td>
    </tr>
    <tr>
        <td>Thrid</td>
        <td><asp:DropDownList ID="DDLThird" runat="server"><asp:ListItem Text="Select" Value="Select"></asp:ListItem>    </asp:DropDownList></td>
    </tr>
</table>

// Codice behindprotected void Page_Load(object sender, EventArgs e){if (!IsPostBack){// il tuo codice per associare il primo elenco a discesa

        }
    }

    protected void DDLFirst_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (DDLFirst.SelectedIndex > 0)
        {
            string FirstDDLValue = DDLFirst.SelectedItem.Value;
            // below your code to get the second drop down list value filtered on first selection


        }

    }

    protected void DDLSecond_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (DDLSecond.SelectedIndex > 0)
        {
            string SecondDDLValue = DDLSecond.SelectedItem.Value;
            // below your code to get the third drop down list value filtered on Second selection


        }
    }