实现在Winform中递归控件来清空指定类型控件的内容（因为在Winform中，各个控件是有层次关系的，不能简单地依靠遍历this.controls）

private void ClearContent4Controls(Control.ControlCollection controls)
{
            foreach (Control c in controls)
            {
                if (c is TextBox)
                {
                    (c as TextBox).Text = string.Empty;
                }
                else if (c is ComboBox)
                {
                    (c as ComboBox).SelectedIndex = -1;
                }
                else if (c is CheckBox)
                {
                    (c as CheckBox).Checked = false;
                }

                ClearContent4Controls(c.Controls);
            }
}