VB如何操作已经打开的CAD

如果用VB操作未打开CAD，第一次运行是有误的，第二次运行就OK，代码如下：
Set AcadApp = CreateObject("Autocad.Application")
Set ADWG = AcadApp.Documents.Open("c:\drawing1.dwg")
Set AcadDoc = AcadApp.ActiveDocument
Set Mospace = AcadDoc.ModelSpace
Set AcadUtil = AcadDoc.Utility
AcadApp.Visible = True
Dim AAA As Variant
AA = AcadUtil.GetReal(vbCrLf & "面积：")


如果用VB操作一个已经打开的CAD，一直是出错的，代码如下：
Set AcadApp = CreateObject("Autocad.Application")Set AcadDoc = AcadApp.ActiveDocument
Set Mospace = AcadDoc.ModelSpace
Set AcadUtil = AcadDoc.Utility
AcadApp.Visible = True
Dim AAA As Variant
AA = AcadUtil.GetReal(vbCrLf & "面积：")

希望高手给予以上两个例子进行纠错指导。