新手问题!VB.NET中批量修改控件的问题.

在FORM1中有8个groupbox,我想批量将其背景色改为透明,这样的代码
Dim i As Integer
Dim a As Control
For i = 1 To 8
If TypeOf a Is GroupBox Then
Me.Controls(i).BackColor = Color.Transparent
End If
Next i
有错么,为什么编译后没效果啊?麻烦各位帮帮忙,在线等.