Improve coding with List Intersect using Net 3.5

Question

I've a class that has a list of IDs with a function for adding new IDs from a list of objects.

I've got this resolved, but I'm sure this can be done with much less code.

Public Class Page

  Private _IdList As List(Of Integer)
  Private _HasNewItems As Boolean = False

  Public Sub AddItems(ByVal Items As List(Of Item))

    Dim itemsID = From itemX In Items _
                  Select itemX.ID

    Dim list As List(Of Integer) = itemsID.ToList

    If _IdList.Intersect(list).Count = list.Count Then
      _HasNewItems = False
    Else
      _HasNewItems = True
      _IdList.AddRange(list)
      _IdList = _IdList.Distinct
    End If

  End Sub

End Class

So how can this be done with less code and more from a NET 3.5 perspective...

Answer 1

Try the following

Public Sub AddItems(ByVal items as List(Of Item))
  Dim oldCount = _IdList.Count
  _IdList.AddRange(items.Select(Function(x) x.ID).Except(_IdList))
  _HasNewItems = oldCount < _IdList.Count
End Sub

The trick here is that Except will return all items Except when they are present in the enumerable passed as the argument (In this case _IdList). So items.Except(_IdList) is all new items.

Answer 2

If your code allows it you could store the ids in a HashSet instead of a List. Then you can just add all the 'new' ids to the HashSet without creating duplicates.

Edit - added code (sorry c#):

    HashSet<int> idList;
    void AddItems(List<Item> items)
    {
        idList.UnionWith(from item in items select item.Id);
    }

Answer 3

Just for completeness, heres a VB version of it depends suggestion of using a HashSet:

Public Class Page

  Private _IdList As HashSet(Of Integer)
  Private _HasNewItems As Boolean = False

  Public Sub AddItems(ByVal Items As List(Of Item))

    Dim itemsID = From itemX In Items _
                  Select itemX.ID

    Dim localResult as Boolean

    For Each id As Integer In itemsID
     localResult = _IdList.Add(id)         
     _HasNewItems = _HasNewItems OrElse localResult
    Next
  End Sub

End Class