Sync extension for Lists/Collections or whatever



I recently found the need to Sync two lists. I have one list that is used for display, and I want to dynamically sync that list with a new one by applying a delta.

I thought that this would be difficult, but I was surprised at its ease.

   2: Module SyncExtensions
   4:     <System.Runtime.CompilerServices.Extension()> _
   5:     Public Sub Sync(Of TItem)(ByVal targetItems As ICollection(Of TItem), ByVal sourceItems As IEnumerable(Of TItem))
   6:         Dim o As Object = DirectCast(targetItems, ICollection).SyncRoot
   7:         If Monitor.TryEnter(o, TimeSpan.FromSeconds(10)) Then
   8:             ' Find items in source that are not in target
   9:             Dim itemsToAdd As New Collection(Of TItem)
  10:             For Each Item In sourceItems
  11:                 If Not targetItems.Contains(Item) Then
  12:                     itemsToAdd.Add(Item)
  13:                 End If
  14:             Next
  15:             ' Apply all adds
  16:             For Each Item In itemsToAdd
  17:                 targetItems.Add(Item)
  18:             Next
  19:             ' Find tags in target that should not be in source 
  20:             Dim itemsToRemove As New Collection(Of TItem)
  21:             For Each Item In targetItems
  22:                 If Not sourceItems.Contains(Item) Then
  23:                     itemsToRemove.Add(Item)
  24:                 End If
  25:             Next
  26:             ' Apply all removes
  27:             For Each Item In itemsToRemove
  28:                 targetItems.Remove(Item)
  29:             Next
  30:             ' Dispose Timer
  31:             Monitor.Exit(o)
  32:         End If
  33:     End Sub
  35: End Module

You need to remember to lock the object while you sync. This is to allow your threading to take place without incident. The nitty gritty is just a case of comparing the two lists and building a list of changes to make and then removing them 🙂


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