递归方法：

     private void FindSelecedControl(Control control)
        {
            if (control is TextBox)
            {
                TextBox txt = (TextBox)control;
                txt.Text = string.Empty;
            }
            else
            {
                for (int i = 0; i < control.Controls.Count; i++)
                {
                    FindSelecedControl(control.Controls[i]);
                }
            }
        }

主方法：

                foreach (Control control1 in this.Form.Controls)
                {
                    FindSelecedControl(control1);
                }