Linq - Checking for all but only if there is at least one

Question

I'm writing a query in Linq which needs to check if one or more items in a list pass predicate A(), and none pass predicate B(). It needs to be a single query to enable Linq to SQL. So for the following lists the results should be as follows where a passes predicate A() and b passes predicate B():

1. [ a, a ] => true
2. [ a ]    => true
3. [ a, b ] => false
4. [ b, b ] => false
5. [ b ]    => false
6. [ ]      => false

I've tried the following, but each have certain situations where they fail:

// Fails in case 6
MyList.All(x => A(x) && !B(x));

// Fails in case 3
MyList.Where(x => !B(x)).Count(x => A(x)) > 0;

// This works, but it's not a single query anymore
MyList.All(x => A(x) && !B(x)) && Mylist.Count() > 0;

I feel like what I need here is something equivalent to all, but which returns false for an empty list. For example this would pass in all cases:

MyList.AllButReturnFalseIfListIsEmpty(x => A(x) && !B(x));

How can I achieve this?

score 2 · Answer 1 · answered Sep 06 '16 at 09:14

2

You can do it like this:

var result = MyList
    .Select(x => B(x) ? -1 : (A(x) ? 1 : 0))
    .Aggregate(0, (p, v) => v == -1 || p == -1 ? -1 : p + v) > 0;

The query works as follows:

Select produces -1 when B is true; otherwise, it produces 1 if A is true and 0 when A is false
Aggregate checks this list of numbers from -1, 0, 1. If it sees -1, the result of the aggregation is -1. Otherwise, the result of the aggregation is sum of positive values.
Comparison at the end makes sure there's at least one A and no Bs.

answered Sep 06 '16 at 09:14

Sergey Kalinichenko

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"*It needs to be a single query to enable Linq to SQL*" – Ivan Stoev Sep 06 '16 at 09:15
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@IvanStoev It *is* a single query. – Sergey Kalinichenko Sep 06 '16 at 09:16
I'm going to try this once I get my head around it - looks promising! – Alfie Woodland Sep 06 '16 at 09:25
Well, technically it is, but in reality it's a select query that returns calculated value per each record, and aggregation occurs at client side (`Aggregate` is basically `foreach` - there is no `Queryable.Aggregate`). I was thinking for something similar, but translatable to SQL. – Ivan Stoev Sep 06 '16 at 09:26
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@IvanStoev [Are you sure there is no `Queryable.Aggregate`?](https://msdn.microsoft.com/en-us/library/bb549412%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396) – Sergey Kalinichenko Sep 06 '16 at 09:28
Apparently I'm not :) May be I'm too much in EF and what is/is not supported there. Still I doubt that will be translated to SQL though. – Ivan Stoev Sep 06 '16 at 09:32
I've changed my accepted answer to Ivan's, as he's correct that this isn't translated to SQL. – Alfie Woodland Sep 06 '16 at 11:09

score 1 · Accepted Answer · edited May 23 '17 at 11:51

1

I would suggest you something similar to my answer to Linq All on empty collection:

var result = MyList.Max(x => B(x) ? (int?)2 : A(X) ? (int?)1 : (int?)0) == 1;

It utilizes how Max<int?> method works

(A) If the sequence is empty, it returns null
(B) If there is at least one element matching B, the result is 2
(C) If there is at least one element not matching B and matching A, the result is 1
(D) Otherwise the result is 0

So we simple check for case (C).

edited May 23 '17 at 11:51

Community

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answered Sep 06 '16 at 09:52

Ivan Stoev

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Clever! Full marks for being terse, although I'm not sure this is as easy to follow as the accepted answer. Both are fairly out there, though. – Alfie Woodland Sep 06 '16 at 10:36
Thank you. Out of curiosity (since I've also learned something today from @dasblinkenlight), does `Aggregate` query get translated to SQL (I'm pretty sure `Max` does)? – Ivan Stoev Sep 06 '16 at 10:47
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Nevermind. As I suspected, EF does not support `Aggregate`. – Ivan Stoev Sep 06 '16 at 10:57
Okay, just tested this and you're right. Switching this to my accepted answer. – Alfie Woodland Sep 06 '16 at 11:07

score 0 · Answer 3 · answered Sep 06 '16 at 09:10

0

Try this:

MyList.All(x => MyList.Any() && A(x) && !B(x));

answered Sep 06 '16 at 09:10

toadflakz

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This is part of a Linq to SQL query within EntityFramework, so I don't have access to the list as a variable. It's all a single expression, e.g. - `dbContext.MyTable.Any(x => x.thing == true);` – Alfie Woodland Sep 06 '16 at 09:19

Mauro Sampietro · Answer 4 · 2016-09-06T09:17:18.180

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Why don't you just check if there's any item after the evaluation?

MyList.Where(x => A(x) && !B(x)).Any();

edited Sep 06 '16 at 09:17

answered Sep 06 '16 at 09:13

Mauro Sampietro

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This returns true if the list contains some values which pass predicate `B()` – Alfie Woodland Sep 06 '16 at 09:15
@AlfieWoodland oops, meant Where. – Mauro Sampietro Sep 06 '16 at 09:16

score 0 · Answer 5 · answered Sep 06 '16 at 09:17

0

You should not use All() if you want it to return false for empty collection. Take a look at the documentation:

true if every element of the source sequence passes the test in the specified predicate, or if the sequence is empty; otherwise, false.

Instead, I would try something like this:

MyList.Where(x => A(x) && !B(x)).Any();

answered Sep 06 '16 at 09:17

J. Swietek

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This will return true for lists containing values which pass predicate `B()`. – Alfie Woodland Sep 06 '16 at 09:22

Linq - Checking for all but only if there is at least one

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