I was dealing with a support case recently where the developer was noticing an unexpected behavior: he was selecting multiple faces and his code was iterating through the SelectSet to create multiple MoveFaceFeatures one after the other. The first feature was created successfully, but the subsequent ones will fail.
There are two reasons why such an approach will fail:
1/ The content of the SelectSet will be cleared after each transacting operation, such as a feature creation, so you should store each selected entity in a collection or your custom data structure if the rest of your code is going to perform transaction, directly or as a result of invoking an API method.
2/ After any operation that is going to modify the BRep, you should assume that any pointer to BRep entities isn’t valid anymore, even if the modification was not involving precisely those entities (Faces, Edges and so on…). The correct approach for that scenario is to store ReferenceKeys of the selected entities prior to do any modification to the BRep and use those later when you need to process the entity.
Here is a VBA sample that illustrates the approach. It assumes the user has selected several faces from the UI and will create MoveFaceFeature for each of them:
Dim oPart As PartDocument
Set oPart = ThisApplication.ActiveDocument
Dim oDef As PartComponentDefinition
Set oDef = oPart.ComponentDefinition
Dim oFace As face, colFaces As FaceCollection
Dim oMFD As MoveFaceDefinition, oMF As MoveFaceFeature
' Get the reference keys for the selected faces.
Dim keyContext As Long
keyContext = oPart.ReferenceKeyManager.CreateKeyContext
Dim keyCollection As New collection
For Each oFace In oPart.SelectSet
Dim key() As Byte
Call oFace.GetReferenceKey(key, keyContext)
Dim tempKey As Variant
For Each tempKey In keyCollection
key = tempKey
Set oFace = oPart.ReferenceKeyManager.BindKeyToObject( _
Set colFaces = _
Set oMFD = oDef.Features.MoveFaceFeatures.CreateDefinition( _
Call oMFD.SetDirectionAndDistanceMoveType(0.1, oFace)
Set oMF = oDef.Features.MoveFaceFeatures.Add(oMFD)