Ciclo su controlli form

        foreach (Control c in Page.Controls)
        {
            foreach (Control childc in c.Controls)
            {
                if (childc is DropDownList)
                {
                    if (((DropDownList)childc).SelectedValue == "0")
                    {
                        lblMessage.Text = "Campi obbligatori non completati";
                    }
                }
            }
        }

     
foreach (Control c in Form.Controls)
   if (childc is DropDownList && (((DropDownList)childc).SelectedValue == "0")
         lblMessage.Text = "Campi obbligatori non completati";