How To Test If Linq Query Returned Results

[Kevin Marois]

I have this:

var query = from g in root.Groups
from r in g.Rules
from c in r.Conditions
from a in c.Actions
where g.Rules != null
where r.Conditions != null
where c.Actions != null
where a.ActionID == ActionId
select a;

List<RuleAction> items = query.ToList();

It fails on the last line with "Object reference not set to an instance of an object" because the query returned no results. How do I test for this before attempting to convert the results to a list? Thanks

Everything makes sense in someone's mind

[Not Active]

I believe you have other problems. Linq is smart enough to know to create an empty list when the query doesn't return any results.

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I know the language. I've read a book. - _Madmatt

[Chris Trelawny Ross]

The exception is thrown by one of the from clauses - because the where clauses are all executed after all the from clauses, so fail to excluded parent classes that have null collections. Try this instead:

var query = from g in root.Groups where g.Rules != null
	from r in g.Rules where r.Conditions != null
	from c in r.Conditions where c.Actions != null
	from a in c.Actions where a.ActionID == ActionId
	select a;

[User 2689127]

Just reading this and thought I would jump in. I typically use query.FirstorDefault to verify it. You could also use query.Count>0 However, I tried some sample code and got that message when I hadn't instantiated my datacontext properly. That may not be it but just something to verify.

[Gideon Engelberth]

If all you are doing is checking for any result, using query.Any() would be better than query.Count() since Any will stop after one item instead of always iterating the whole sequence.

[User 2689127]

Thanks Gideon - I hadn't used Any for that purpose. Awesome tip!