[编程交流] at的读取字段表达式

<CommandMethod("foo")> _
       Public Sub 
GetFieldFromAttribute()
Dim doc As Document = 
Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument
Dim db As Database = 
doc.Database
Dim ed As Editor = doc.Editor
Try
Dim tab As String = 
Autodesk.AutoCAD.ApplicationServices.Application.GetSystemVariable("ctab").ToString
Dim blockname As String = ed.GetString(vbLf & "Enter blockname: 
").StringResult
Dim opts As PromptSelectionOptions = New 
PromptSelectionOptions
opts.SingleOnly = 
True
opts.MessageForRemoval = vbLf & "Select block 
only"
opts.MessageForAdding = vbLf & "Select single 
block"
Dim filt As SelectionFilter = New SelectionFilter({New TypedValue(0, "insert"), 
New TypedValue(2, blockname), New TypedValue(410, 
tab)})
Dim psr As PromptSelectionResult = ed.GetSelection(opts, 
filt)
If psr.Status <> PromptStatus.OK Then Exit Sub
If psr.Value.Count <> 1 Then Exit Sub
Dim id As ObjectId = psr.Value.GetObjectIds(0)
If id.IsNull Then Exit Sub
Using tr As Transaction = db.TransactionManager.StartTransaction()
Dim obj As DBObject = tr.GetObject(psr.Value.GetObjectIds(0), OpenMode.ForRead)
If TypeOf obj Is BlockReference Then
Dim bref As BlockReference = CType(tr.GetObject(id, OpenMode.ForRead), 
BlockReference)
Dim attcoll As AttributeCollection = 
bref.AttributeCollection
Dim att As 
AttributeReference
For Each aid As ObjectId In 
attcoll
Dim aobj As DBObject = tr.GetObject(aid, 
OpenMode.ForRead)
att = TryCast(DirectCast(aobj, AttributeReference), 
AttributeReference)
If att Is Nothing Then Exit 
Sub
If att.Tag.Equals("LENGTH", StringComparison.CurrentCultureIgnoreCase) 
Then
If att.HasFields 
Then
Dim fld As Field = tr.GetObject(att.GetField(), 
OpenMode.ForRead)
Dim code As String = 
fld.GetFieldCode
Dim pos1 As Int32 = 
code.IndexOf("_ObjId")
Dim pos2 As Integer = 
code.IndexOf(">%")
Dim leng1 As Integer = 
"_ObjId".Length
Dim leng2 As Integer = 
">%".Length
Dim idstr As String = code.Substring(pos1 + leng1 + 1, pos2 - (pos1 + leng1) - 
1)
MsgBox("ID from field code: " & idstr)
Dim objId As ObjectId = New ObjectId(New IntPtr(Convert.ToInt32(idstr)))
Dim fieldchild As DBObject = tr.GetObject(objId, 
OpenMode.ForRead)
If TypeOf fieldchild Is Polyline 
Then
Dim poly As Polyline = DirectCast(fieldchild, 
Polyline)
MsgBox("Compare:" & vbLf & "Return from attribute: " & 
att.TextString & vbLf & "Return from Polyline: " & 
poly.Area.ToString)
End 
If
Exit 
For
End 
If
End If
Next
End 
If
End Using
           Catch ex 
As 
Autodesk.AutoCAD.Runtime.Exception
ed.WriteMessage(ex.Message + vbLf + 
ex.StackTrace)
End Try
       End Sub