不明白为要用两个循环！

一次性清空多个TextBox:
       foreach (System.Web.UI.Control control in Controls)
        {
            for (int i = 0; i < control.Controls.Count; i++)
            {
                if (control.Controls[i] is System.Web.UI.WebControls.TextBox)
                {
                    System.Web.UI.WebControls.TextBox tb = (System.Web.UI.WebControls.TextBox)control.Controls[i];
                    tb.Text = "";
                }
            }
        }

不理解为什么要两个循环！
foreach (System.Web.UI.Control control in Controls) //历遍容器中每一个控件。
for (int i = 0; i < control.Controls.Count; i++)
 //control.Controls.Count 控件的控件的集合？不理解！上面foreach都历遍了，这里干嘛还要历遍？