帮忙看下不能通过的这段代码 想实现前进和后退的

<P>看下不能通过的这段代码 想实现前进和后退的</P>
<P>Option Explicit<BR>Private m_pHookHelper As IHookHelper<BR>Private pActiveView As IActiveView</P>
<P><BR>Private Sub Command1_Click()<BR> <BR>  'Get the active view<BR>  </P>
<P>  Dim pActiveView As esriCarto.IActiveView<BR>   <BR> Set pActiveView = m_pHookHelper.FocusMap<BR>  <BR>  'Get the extent stack<BR>  Dim pMapExtent As esriCarto.IExtentStack<BR>  Set pMapExtent = pActiveView.ExtentStack<BR>  'Undo the extent<BR>  If pMapExtent.CanUndo Then<BR>    pMapExtent.Undo<BR>  End If</P>
<P>End Sub</P>
<P>Private Sub Command2_Click()<BR>    <BR>  'Get the active view<BR>  Dim pActiveView As esriCarto.IActiveView<BR> <BR>Set pActiveView = m_pHookHelper.FocusMap<BR>  'Get the extent stack<BR>  Dim pMapExtent As esriCarto.IExtentStack<BR>  Set pMapExtent = pActiveView.ExtentStack<BR>  'Redo the extent<BR>  If pMapExtent.CanRedo Then<BR>    pMapExtent.Redo<BR>  End If<BR>End Sub<BR>高手帮帮忙~~~~~``</P>
<P>谢谢了 </P>