请教老师，以下代码中，如果修改变量xx,yy的值，第一个方框的位置为何不随之变动呢？

代码如下 ：
   
   Dim doc As New PrintDoc

    Dim rt As  New prt.RenderText \'设置页眉的文本对象
    rt.Text="qwerwqreqwer"
    Doc.PageHeader=rt
   Dim g As New prt.RenderGraphics()

   Dim pn As New Pen(Color.Red,0.1) \'画笔
   Dim fnt As New Font("宋体",10)  \'文字字体
   Dim points_4cf(4) As Point  \'四边形
   Dim points_4lx(4) As Point  \' 菱形
   Dim xx,yy As Integer \'\'四方形左上点坐标
   Dim ll,hh As Integer \'四方形橫长及纵高
  
   xx=52 \'节点图左上点横坐标
   yy=152 \'节点图左上点纵坐标
   ll=80 \'四边形横长
   hh=30 \'四边形纵高

   \'箭头线条变量
   Dim linelength As Integer =30 \'线长
   Dim linelength1 As Integer =4 \'箭头线长
   Dim points1(1) As Point \'主线条
   Dim points11(1) As Point \'箭头左半部
   Dim points12(1) As Point \'箭头右半部

Dim s_name As String
   For i As Integer=1 To 10
       s_name="wwwwww"
      g.Graphics.DrawRectangle(pn,xx,yy,ll,hh) \'此处的xx,yy值在上面赋值无效???

\'文字
       g.Graphics.DrawString(s_name,fnt,Brushes.Red,xx-2,(yy+hh/2)-6)

\'竖箭头主线条
       points1(0) = New Point(xx+ll/2, yy+hh)
       points1(1) = New Point(xx+ll/2,yy+hh+linelength)
       g.Graphics.DrawLines(Pens.Green, points1)
\'箭头左半部
       points11(0) = New Point(xx+ll/2-linelength1,yy+hh+linelength-linelength1)
       points11(1) = New Point(xx+ll/2,yy+hh+linelength)
       g.Graphics.DrawLines(Pens.Green, points11)
\'箭头右半部
       points12(0) = New Point(xx+ll/2+linelength1,yy+hh+linelength-linelength1)
       points12(1) = New Point(xx+ll/2,yy+hh+linelength)
       g.Graphics.DrawLines(Pens.Green, points12)

       yy=yy+hh+linelength  \'下方下一个四边形左上点纵坐标

   Next

Doc.Body.Children.Add(g)
Doc.Preview()