PlaceHolder里textbox值该怎么获取.

PlaceHolder里textbox值该如何获取....
现在可以点一下button1就能多生成一个textbox了，我想在点了button2后，把这些textbox里的值显示出来，请问下面的BUTTON2里的代码该如何写，多谢了~~~
  static int n = 1;
  protected void Page_Load(object sender, EventArgs e)
  {

  }
  protected void Button1_Click(object sender, EventArgs e)
  {
  for (int i = 0; i < n; i++)
  {
  TextBox txtbx = new TextBox();
  txtbx.ID = "txtbx" + i.ToString();
  PlaceHolder1.Controls.Add(txtbx);
  }
  n++;
  }
  protected void Button2_Click(object sender, EventArgs e)
  {
  for (int i = 0; i < n; i++)
  {
  TextBox ttb = new TextBox();
  ttb.ID = "txtbx" + i.ToString();
  Response.Write(ttb.Text);
  }
  }

------解决方案--------------------------------------------------------

C# code

foreach(Control txt in this.PlaceHolder1.Contols{         if(txt is TextBox){      Response.Write(((TextBox)txt).Text);}}
------解决方案--------------------------------------------------------
Button1_Click(null,null);  
厄...请问这句是啥意思？为啥要加NULL，NULL...

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textbox must first be created, this is to invoke button1_click function.