show modal popup from code behind

i have a dropdownlist

in codebehind,i have this function

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

now i want to show modal popup when a particular text is selected from the dropdownlist from th开发者_Go百科is function

if(DropDownList1.SelectedItem.Text.Equals("Some Text"))
{
     ModalPopupExtender1.Show();
}