[c#] How to use the DropDownList's SelectedIndexChanged event

I have two DropDownLists in my webform and when I select a value in the first dropdownlist, I would like a related value to be automatically selected in the second dropdownlist.

This is what I currently have:

    <table>   
        <tr>
            <td>
                <asp:Label ID="lbmanu" runat="server" Text="Furniture Manufacturer : 
                   "></asp:Label>
            </td>
            <td>
                <asp:DropDownList ID="ddmanu" runat="server" 
                    DataSourceID="Sql_fur_model_manu"    
                    DataTextField="manufacturer" DataValueField="manufacturer" 
                    onselectedindexchanged="ddmanu_SelectedIndexChanged">
                </asp:DropDownList>
                <asp:SqlDataSource ID="Sql_fur_model_manu" runat="server" 
                    ConnectionString="<%$ ConnectionStrings:conStr %>" 
                    SelectCommand="SELECT DISTINCT [manufacturer] FROM 
                     [furniture_manufacturer]">
                </asp:SqlDataSource>
            </td>
        </tr>
        <tr>
            <td>
                <asp:Label ID="lbtype" runat="server" Text="Furniture Type : 
                        "></asp:Label>
            </td>
            <td>
                <asp:DropDownList ID="ddtype" runat="server" AutoPostBack="True">
                   </asp:DropDownList>
            </td>
        </tr>
   </table>

Code Behind :

protected void ddmanu_SelectedIndexChanged(object sender, EventArgs e)
{
    string query = "select furniture from furniture_model where manufacturer='" + 
    ddmanu.SelectedValue.ToString() + "'";
    con.Open();
    cmd = new SqlCommand(query, con);
    DataTable dt = Select(query);
    cmd.ExecuteNonQuery();
    ddtype.DataSource = dt;
    ddtype.DataTextField = "manufacturer";
    ddtype.DataValueField = "furniture";
    ddtype.DataBind(); 
}

The most basic way you can do this in SelectedIndexChanged events of DropDownLists. Check this code..

    <asp:DropDownList ID="DropDownList1" runat="server" onselectedindexchanged="DropDownList1_SelectedIndexChanged" Width="224px"
        AutoPostBack="True" AppendDataBoundItems="true">
    <asp:DropDownList ID="DropDownList2" runat="server"
        onselectedindexchanged="DropDownList2_SelectedIndexChanged">
    </asp:DropDownList> 


protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    //Load DropDownList2

}
protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
{
    //Load DropDownList3
}

You should add AutoPostBack="true" to DropDownList1

                <asp:DropDownList ID="ddmanu" runat="server" AutoPostBack="true"
                    DataSourceID="Sql_fur_model_manu"    
                    DataTextField="manufacturer" DataValueField="manufacturer" 
                    onselectedindexchanged="ddmanu_SelectedIndexChanged">
                </asp:DropDownList>

I think this is the culprit:

cmd = new SqlCommand(query, con);

DataTable dt = Select(query);

cmd.ExecuteNonQuery();

ddtype.DataSource = dt;

I don't know what that code is supposed to do, but it looks like you want to create an SqlDataReader for that, as explained here and all over the web if you search for "SqlCommand DropDownList DataSource":

cmd = new SqlCommand(query, con);
ddtype.DataSource = cmd.ExecuteReader();

Or you can create a DataTable as explained here:

cmd = new SqlCommand(query, con);

SqlDataAdapter listQueryAdapter = new SqlDataAdapter(cmd);
DataTable listTable = new DataTable();
listQueryAdapter.Fill(listTable);

ddtype.DataSource = listTable;