Question

I need a list from OldGuids that are not in the NewGuids, so I used the Contains method, the problem is that it is already running more then half hour, is there a faster way? or how much longer would it take about?

Dim OldGuids As New List(Of Guid) ' 18 million rows
Dim NewGuids As New List(Of Guid) ' 6 million rows
Dim Filtered = From n In OldGuids Where Not NewGuids.Contains(n)

Answer 1

Filtered = OldGuids.Except(NewGuids)

(Note that this will only return unique elements, if you want to preserve duplicates this is not what you want).

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var newGuidSet=new HashSet<Guid>(newGuids);
Filtered = OldGuids.Where(g=>!newGuidSet.Contains(g));