ASP.net DropDownList - Visiblility

Question

I have the following DropDownList, car_make, when I select BMW I would like the dropdownlist bmw to show up, I have set it to Visible = false by default, I have the code in my CS file in the pageload, however that is making the whole page to refresh, how can I just make the dropdownlist appear without refreshing the whole page? Thank you.

ASP.Net code:

<asp:DropDownList ID="car_make" runat="server" Width="100%" ForeColor="Black">
    <asp:ListItem></asp:ListItem>
    <asp:ListItem>BMW</asp:ListItem>
    <asp:ListItem>Benz</asp:ListItem>
</asp:DropDownList>



<asp:DropDownList ID="bmw" runat="server" Width="100%" Visible = "false" ForeColor="Black">
    <asp:ListItem>Model 1</asp:ListItem>
    <asp:ListItem>Model 2</asp:ListItem>
    <asp:ListItem>Model 3</asp:ListItem>
</asp:DropDownList>

Page.cs

protected void Page_Load(object sender, EventArgs e)
    {
        if (car_make.Text == "BMW")
        {
            bmw.Visible = true;


        }
}

Answer 1

use Updatepanel. It won't refresh the whole page.

Here one example with your provided sample

Source :

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:DropDownList ID="car_make" AutoPostBack="True" runat="server" Width="100%" ForeColor="Black" OnSelectedIndexChanged="car_make_SelectedIndexChanged">
                    <asp:ListItem></asp:ListItem>
                    <asp:ListItem>BMW</asp:ListItem>
                    <asp:ListItem>Benz</asp:ListItem>
                </asp:DropDownList>



                <asp:DropDownList ID="bmw" runat="server" Width="100%" Visible="false" ForeColor="Black">
                    <asp:ListItem>Model 1</asp:ListItem>
                    <asp:ListItem>Model 2</asp:ListItem>
                    <asp:ListItem>Model 3</asp:ListItem>
                </asp:DropDownList>
            </ContentTemplate>
        </asp:UpdatePanel>

CODE :

protected void car_make_SelectedIndexChanged(object sender, EventArgs e)
{
  if (car_make.Text == "BMW")
     bmw.Visible = true; 
   else
     bmw.Visible = false;
}

What's new here in Source part is AutoPostBack which needed to get change. Another is OnSelectedIndexChanged