TL;DR; Explains how to synchronise two .NET collections by applying differences, with thread safety using locks, ensuring target lists match source lists efficiently.

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.
1 1:
1 2: Module SyncExtensions
1 3:
1 4: <System.Runtime.CompilerServices.Extension()> _
1 5: Public Sub Sync(Of TItem)(ByVal targetItems As ICollection(Of TItem), ByVal sourceItems As IEnumerable(Of TItem))
1 6: Dim o As Object = DirectCast(targetItems, ICollection).SyncRoot
1 7: If Monitor.TryEnter(o, TimeSpan.FromSeconds(10)) Then
1 8: ' Find items in source that are not in target
1 9: Dim itemsToAdd As New Collection(Of TItem)
1 10: For Each Item In sourceItems
1 11: If Not targetItems.Contains(Item) Then
1 12: itemsToAdd.Add(Item)
1 13: End If
1 14: Next
1 15: ' Apply all adds
1 16: For Each Item In itemsToAdd
1 17: targetItems.Add(Item)
1 18: Next
1 19: ' Find tags in target that should not be in source
1 20: Dim itemsToRemove As New Collection(Of TItem)
1 21: For Each Item In targetItems
1 22: If Not sourceItems.Contains(Item) Then
1 23: itemsToRemove.Add(Item)
1 24: End If
1 25: Next
1 26: ' Apply all removes
1 27: For Each Item In itemsToRemove
1 28: targetItems.Remove(Item)
1 29: Next
1 30: ' Dispose Timer
1 31: Monitor.Exit(o)
1 32: End If
1 33: End Sub
1 34:
1 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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