CODEWITHSUNDEEP
c#linq
Don Thomas Boyle
Don Thomas Boyle

Reputation: 3045

Why do the 2 LINQ queries get evaluated differently

Query 1

       var resulty = db.QIS
            .Where(w=>w.QSA.Any(a => a.QSID != w.QSID))
            .Select(s => s.QSID).ToList();

Query 2

        var resultz = db.QIS
            .Where(w=>w.QSA.Where(h => h.QSID == w.QSID).Select(s => s.QSID).FirstOrDefault() != w.QSID)
            .Select(s => s.QSID).ToList();

Table QIS and QSA are related Many:Many. The idea here is to find how many QIS.ID's are not Found in QSA where QIS.QID == QSA.QID.

Query 1 returns 0

Query 2 on the other hand gives me what I expected to see ( the list off all non matching QIS records.)

Why will the any not work? - i found myself running into the same situation a couple of times now in seperate scenarios... thanks for any help / thoughts.

PS: Prefer method syntax.

Upvotes: 1

Views: 74

Answers (1)

Anders Abel
Anders Abel

Reputation: 69310

In the filtering in your second version, will only be true if the inner Where clause returns no elements, so that FirstOrDefault() returns null or 0 (depending on if the type is nullable or not).

w=>w.QSA.Where(h => h.QSID == w.QSID)
.Select(s => s.QSID).FirstOrDefault() != w.QSID

Which is equivalent to (now assuming QSID is a non nullable numeric type, if it is nullable, use null instead of zero):

w=>w.QSA.Where(h => h.QSID == w.QSID)
.Select(s => s.QSID).FirstOrDefault() == 0

which can be rewritten to:

w=>w.QSA.Where(h => h.QSID == w.QSID).FirstOrDefault() == null

which can be rewritten to:

w=>!w.QSA.Any(h => h.QSID == w.QSID)

which is nearly the same as your initial version, but not exactly. You still want to check for equivalence inside the Any() filter, but then negate the result.

Upvotes: 4

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