a·gen·tic a·gil·i·ty

Sync extension for Lists/Collections or whatever

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

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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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