...
 For Each document As Document In oAssembly.AllReferencedDocuments
  'Process only sheetmetal parts
  If document.DocumentType = DocumentTypeEnum.kPartDocumentObject And
   document.DocumentSubType.DocumentSubTypeID = "{9C464203-9BAE-11D3-8BAD-0060B0CE6BB4}"  And IsDocumentForExport(document)  Then
   ExportDxf(document)
...

Sub Main()
 'Assembly document must be active
 Dim oAssembly As AssemblyDocument = ThisDoc.Document
 'Iteration thru all referenced douments of active assembly document
 For Each document As Document In oAssembly.AllReferencedDocuments
  'Process only sheetmetal parts
  If document.DocumentType = DocumentTypeEnum.kPartDocumentObject And
   document.DocumentSubType.DocumentSubTypeID = "{9C464203-9BAE-11D3-8BAD-0060B0CE6BB4}" Then
   ExportDxf(document)
  End If
 Next
 MsgBox("Hotovo")
End Sub
'Constant value for all exports
Dim fSett As String = "FLAT PATTERN DXF?AcadVersion=2004&RebaseGeometry=True&OuterProfileLayer=0&OuterProfileLayerColor=0;0;0&InteriorProfilesLayer=0&InteriorProfilesLayerColor=0;0;0&InvisibleLayers=IV_ARC_CENTERS;IV_TANGENT;IV_ROLL;IV_ROLL_TANGENT;IV_ALTREP_BACK;IV_ALTREP_FRONT;IV_FEATURE_PROFILES_DOWN;IV_FEATURE_PROFILES;IV_TOOL_CENTER_DOWN;DIGI_MARKER_TOOL_1;DIGI_MARKER_TOOL_2;IV_BEND;IV_BEND_DOWN"                  'Nastaveni exportu
Sub ExportDxf(sheetmetalPart As PartDocument)
 Try
  'Reference to part parameters
  Dim parameters = sheetmetalPart.ComponentDefinition.Parameters
  'Get parameter values
  Dim fileStorage As String = GetParameterValue("FilesStorage", parameters)
  Dim typeOfConnection As String = GetParameterValue("type_of_connection", parameters)
  Dim lengthB As String = GetParameterValue("length_B", parameters)
  'Build target file name
  Dim fSname As String = String.Format("{0}JSMMU-{1}-B-{2}.dxf", fileStorage, typeOfConnection, lengthB) 'Nastaveni ulozeni cesta+nazev

  'Export flatpattern to DXF
  sheetmetalPart.ComponentDefinition.DataIO.WriteDataToFile(fSett, fSname) 'Save s nastavenim fSett a do cesty fSname
  '    MessageBox.Show("DXF was save: " & fSname,"Flat Pattern", MessageBoxButtons.OK) 'Message report
 Catch ex As Exception
  MsgBox(String.Format("Export se nezdaril.{2}FileName: {0}{2}Exception{1}", sheetmetalPart.FullFileName, ex, vbCrLf))
 End Try
End Sub
Function GetParameterValue(paramName As String, parameters As Parameters) As String
 Try
  'Returns parameter value as string if exists
  Return parameters(paramName).Value.ToString()
 Catch
  'Parameter with given name can't be found
  Return ""
 End Try
End Function