CODEWITHSUNDEEP
linqentity-frameworklambdalinq-expressions
Ben Ford
Ben Ford

Reputation: 1394

Linq All() / Any() but not empty

I have a Linq expression that is used in a few places. I went down the expression route as there wasn't a logical way to accomplish some searching logic without enumerating a very large table otherwise.

    private Expression<Func<Property, bool>> PropertyIsCompliant()
    {
        return (p) => p.CalculationSets.OfType<SingleDocumentCalculationSet>()
            .GroupBy(cs => cs.SourceDocument)
            .Select(g => g.OrderByDescending(d => d.DateTime).FirstOrDefault().CalculationResults)
            .SelectMany(cr => cr)
            .All(cr => cr.Outcome == CalculationOutcome.Success);
    }

My models are as such:

I'm trying to create an expression that will tell me if all the outcomes from the most recent calculationsets grouped by document ordered by most recent (ie the most recent distinct results) are Successful.

I can the SelectMany clause returns all the CalculationResults from the correct CalculationSets. I just cant figure out how to return true ONLY if the collection isn't empty AND they are all Outcome.Success.

I understand the All operator automatically returns true on an empty collection. I just can't think of a way around it!

Upvotes: 2

Views: 789

Answers (4)

Ben Ford
Ben Ford

Reputation: 1394

I didn't realise it was possible to use "&&" in expressions. So I've managed to combine 2 separate expressions that give the answer I need. The "&&" only returns true when both expressions evaluate "true"

            return (p) => 
            p.CalculationSets.OfType<SingleDocumentCalculationSet>()
            .GroupBy(cs => cs.SourceDocument)
            .Select(g => g.OrderByDescending(d => d.DateTime).FirstOrDefault().CalculationResults)
            .SelectMany(cr => cr).Any()
            &&
            p.CalculationSets.OfType<SingleDocumentCalculationSet>()
            .GroupBy(cs => cs.SourceDocument)
            .Select(g => g.OrderByDescending(d => d.DateTime).FirstOrDefault().CalculationResults)
            .SelectMany(cr => cr)
            .All(cr => cr.Outcome == CalculationOutcome.Success);

Upvotes: 0

D Stanley
D Stanley

Reputation: 152556

So your real condition is that there are not any unsuccessful outcomes. In that case use Any and reverse the condition:

                //V-- notice the ! inverse operator here
    return (p) => !(p.CalculationSets.OfType<SingleDocumentCalculationSet>()
        .GroupBy(cs => cs.SourceDocument)
        .Select(g => g.OrderByDescending(d => d.DateTime).FirstOrDefault().CalculationResults)
        .SelectMany(cr => cr)
        .Any(cr => cr.Outcome != CalculationOutcome.Success));

Upvotes: 3

usr
usr

Reputation: 171178

var countsBySuccess = 
 ...
 .GroupBy(cr => cr.Outcome == CalculationOutcome.Success) //group on success
 .Select(g => new { IsSuccessful = g.Key, Count = g.Count() });

You can now examine the two result rows to make sure that the unsuccessful count is zero and the successful count is non-zero.

Regarding performance, this will need to materialize the entire result set server-side and aggregate it. But it does so only once.

If you must use the calculation result as part of a bigger query, you must use another trick:

!countsBySuccess.Any(g =>
    g.IsSuccessful && Count == 0 ||
    !g.IsSuccessful && Count != 0)

This boolean expression determines whether the condition you are looking for holds with one scan of the data.

It is important to only scan the data once. Do not simply write:

myItems.All(cr => cr.Outcome == CalculationOutcome.Success) && myItems.Any()

Because that does two scans. SQL Server does not optimize this out.

Upvotes: 1

alergy
alergy

Reputation: 974

I think you're answering your question - if you know that All returns TRUE for empty then you have two checks to make. Excuse my C# (I'm not sure on the var query assignment, hopefully you get the idea) but you could do something like this:

private Expression<Func<Property, bool>> PropertyIsCompliant()
    {
        var query = (p) => p.CalculationSets.OfType<SingleDocumentCalculationSet>()
            .GroupBy(cs => cs.SourceDocument)
            .Select(g => g.OrderByDescending(d => d.DateTime).FirstOrDefault().CalculationResults)
            .SelectMany(cr => cr);

        return (query.Count > 0) & query.All(cr => cr.Outcome == CalculationOutcome.Success);
    }

Upvotes: 0

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