Try like this
protected void billOption_SelectedIndexChanged(object sender, EventArgs e)
{
HideAllPanels();
string selection = billOption.SelectedValue;
switch (selection)
{
case "1":
Panel1.Visible = true;
break;
case "2":
Panel2.Visible = true;
break;
.
.
.
case "N":
PanelN.Visible = true;
break;
case default:
HideAllPanels();
break;
}
}
private void HideAllPanels()
{
Panel1.Visible = false;
Panel2.Visible = false;
Panel3.Visible = false;
.
.
.
PanelN.Visible = false;
}