List<Control> list = new List<Control>();

            GetAllControl(this,list);

            foreach (Control control in list)
            {
                if (control.GetType() == typeof(Button))
                {
                    //all btn
                }
            }

        private void GetAllControl(Control c,List<Control> list)
        {
            foreach (Control control in c.Controls)
            {
                list.Add(control);

                if (control.GetType() == typeof(Panel))
                    GetAllControl(control,list);
            }
        }