arcscene 定制的一段代码

我想在arcscene的窗口中画一个矩形框，我使用了下面的代码，但是不行
执行到1处的时候,cpu的使用率迅速上升到100%,死机,不知道为什么

Private Sub UIToolControl1_MouseDown(ByVal button As Long, ByVal shift As Long, ByVal x As Long, ByVal y As Long)
Dim papp As IApplication
Set papp = Application
Dim psxdoc As ISxDocument
Set psxdoc = papp.Document
Dim psg As ISceneGraph
Set psg = psxdoc.Scene.SceneGraph
Dim pEnvelope As IEnvelope
Dim pScreen As IScreenDisplay
Dim pLineSymbol As ISimpleLineSymbol
Dim pRGBColor As IRgbColor
Dim pRubber As IRubberBand
Set pScreen = psg
Set pRubber = New RubberEnvelope
Set pEnvelope = pRubber.TrackNew(pScreen, Nothing) ' 1
Set pLineSymbol = New SimpleLineSymbol
Set pRGBColor = New RgbColor
With pRGBColor
.Red = 255
.Green = 128
.Blue = 128
End With
With pLineSymbol
.Width = 2
.Color = pRGBColor
.Style = esriSLSSolid
End With
With pScreen
.StartDrawing pScreen.hDC, esriNoScreenCache
.SetSymbol pLineSymbol
.DrawPolyline pEnvelope
.FinishDrawing
End With
End Sub

<P>哪位兄弟能帮忙看看，不胜感激</P>