Dropdown list items based on value of another drop down list

Question

I am trying to create a second drop down list which based on the value of the first drop down list adjusts the list items as needed. I have written the following code, but even though there are no compiling errors the second drop down list remains always empty. Below are html and C# code.

  <table> <tr><td>System</td> <td> <asp:DropDownList ID="systemddl" runat="server" AutoPostBack="True" Height="24px" Width="142px"> <asp:ListItem>G1</asp:ListItem> <asp:ListItem>D2</asp:ListItem> <asp:ListItem> D3</asp:Listitem> <asp:ListItem> M4</asp:Listitem> <asp:ListItem> I5</asp:Listitem> </asp:DropDownList></td> </tr> <tr> <td>KPI</td> <td> <asp:DropDownList ID="kpiddl" runat="server" AutoPostBack="True" Height="24px" Width="142px"></asp:DropDownList></td> </tr> </table> 

  protected void systemddl_SelectedIndexChanged(object sender, EventArgs e)
{
    if (systemddl.SelectedValue == "G1")
    {
        var items1 = new List<ListItem>()
        {
            new ListItem("TEST1"),
            new ListItem("")

        };
        kpiddl.DataSource = items1;
        kpiddl.DataBind();
        kpiddl.SelectedValue = "";
        if (systemddl.SelectedValue == "D2")
        {
            var items2 = new List<ListItem>()
        {
            new ListItem("1"),
            new ListItem("2"),
            new ListItem("3"),
            new ListItem("4")
        };
            kpiddl.DataSource = items2;
            kpiddl.DataBind();
        }
        if (systemddl.SelectedValue == "I5")
        {
            var items3 = new List<ListItem>()
        {
            new ListItem("Total"),
            new ListItem("V Completed"),
            new ListItem("R found"),
            new ListItem("R sold"),
            new ListItem("A found"),
            new ListItem("Asold"),
            new ListItem("")

        };
            kpiddl.DataSource = items3;
            kpiddl.DataBind();
            kpiddl.SelectedValue = "";
        }
        if (systemddl.SelectedValue == "D3")
        {
            var items4 = new List<ListItem>()
        {
            new ListItem("FRFT"),
            new ListItem("")

        };
            kpiddl.DataSource = items4;
            kpiddl.DataBind();
        }
        if (systemddl.SelectedValue == "M4")
        {
            var items5 = new List<ListItem>()
        {
            new ListItem("A"),
            new ListItem("B"),
            new ListItem("C"),
            new ListItem("D"),
            new ListItem("")
        };
            kpiddl.DataSource = items5;
            kpiddl.DataBind();
            kpiddl.SelectedValue = "";
        }

Answer 1

Change markup like this

<asp:DropDownList 
    ID="ddl_1" 
    runat="server"
    AutoPostBack="true"
    OnSelectedIndexChanged="ddl_1_SelectedIndexChanged">
</asp:DropDownList>
<asp:DropDownList 
    ID="ddl_2" 
    runat="server">

and add handler for SelectedIndexChanged of first drop down like this

protected void ddl_1_SelectedIndexChanged(object sender, EventArgs e)
{
    switch (this.ddl_1.SelectedValue)
    {
        case "A":
            this.ddl_2.DataSource = source1;
            break;
        case "B":
            this.ddl_2.DataSource = source2;
            break;
        default:
            break;
    }

    this.ddl_2.DataBind();
}

Init second dropdown with source, which depends of current selected value of first drop down.
So, when user changing selected value of first drop down - postback occurs and second drop down filling with required values