Slow LINQ missing from list query

Question

I have a pretty standard block of code that compares two lists and returns the items from the first list that are not matched in the second list. (unmatched query)

But when both the lists are large-ish, the unmatched query takes 3 minutes to execute.

// names.Count ~ 91k 
var names = xxxxx.ToList();

// namePhonetics.Count ~ 91k
var namePhonetics = yyyyy.ToList();

// this line takes 3 minutes to run
var namesMissingPhonetics = names.Where(n => !namePhonetics.Any(np => np.NameId == n.Id)).ToList();

What can I do to increase the performance of this?

What is `xxxx` and `yyyy`. Are they by chance IQueryable? Then don't `ToList()` them. Working with 91k data is not trival. If not, use a hashset. — Christian Gollhardt, Oct 26 '17 at 16:06
@ChristianGollhardt No they are not IQueryable. I included the ToList to note that they are indeed Lists. — Rick the Scapegoat, Oct 26 '17 at 16:11
Then use [hashset](https://stackoverflow.com/questions/1247442/when-should-i-use-the-hashsett-type) as I said @rick — Christian Gollhardt, Oct 26 '17 at 16:41

score 2 · Accepted Answer · answered Oct 26 '17 at 16:04

2

Try this:

 var namePhoneticsDict = yyyyy.ToDictionary(x => x.NameId, x => x);
 var namesMissingPhonetics = names.Where(n => !namePhoneticsDict.ContainsKey(n.Id)).ToList();

answered Oct 26 '17 at 16:04

MistyK

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Phenomenal! Thank you! – Rick the Scapegoat Oct 26 '17 at 16:22

Slow LINQ missing from list query

1 Answers1