querying list within a list using linq

Question

I'm trying to learn linq, but struggling with some concepts. How would I transform this double foreach loop into a linq query please?

foreach (var l1 in list1)
{
    foreach (var l2 in list2)
    {
       if (l1 == l2)
       {
          list1.Remove(l1);
       }
    }
}

You wouldn't, because LINQ is about querying, not acting. Queries do not (should not) modify sequences. At best, you'd write a query that identifies the elements to be removed; then remove them in a second, separate step. Or derive a new sequence (with only a subset of the original elements) from the original sequences. — stakx - no longer contributing
– stakx - no longer contributing, Commented Feb 6, 2015 at 15:07
You shouldn't be mutating a list you are iterating over in the first place. — juharr
– juharr, Commented Feb 6, 2015 at 15:12
By the way: Note that your title is inaccurate. You do not have a list within a list; that would only be the case if you had a List<List<T>>. What you do have is a loop within a loop (both acting on a list and a list). — stakx - no longer contributing
– stakx - no longer contributing, Commented Feb 8, 2015 at 17:00

SLaks · Accepted Answer · 2015-02-06 15:06:48Z

7

var list3 = list1.Except(list2);

LINQ does not mutate lists.

answered Feb 6, 2015 at 15:06

SLaks

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3 Comments

Rawling Over a year ago

As long as nothing else holds a reference to list1 and expects it to be updated...

Jynn Over a year ago

I'm always amazed how simply some tasks can be done if only you know how. Anyway I'm getting there slowly. thanks!

Servy Over a year ago

@user3611219 If you spent some time looking through what all of the LINQ operators are and what each of them do (when trying to learn LINQ) you wouldn't be so surprised when one of them does exactly what you're trying to do. That should be step 1 of learning LINQ.

Community · Accepted Answer · 2017-05-23 12:20:37Z

3

If list1 is declared as a List<T>, then you can do this:

list1.RemoveAll(list2.Contains);

You might find that a little difficult to read. The above is essentially equivalent to:

list1.RemoveAll(item1 => list2.Contains(item1));

Note that this solution is not based on LINQ. However, if list2's type does not have a Contains method, then LINQ can help you out with its .Contains extension method; add a using System.Linq; directive to your code in that case.)

_{P.S.: Please make sure that you have read and understood my above comment: LINQ's purpose is querying for data, not modifying it.}

edited May 23, 2017 at 12:20

CommunityBot

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answered Feb 6, 2015 at 15:15

stakx - no longer contributing

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Comments

vittore · Accepted Answer · 2015-02-06 15:08:46Z

0

There is number of ways to do it, one example is to use Intersect:

var inBothLists = list1.Intersect(list2);

inBothLists.ToList().ForEach(i => list1.Remove(i));

answered Feb 6, 2015 at 15:08

vittore

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Comments

D Stanley · Accepted Answer · 2015-02-06 15:09:33Z

0

Linq is for querying, not updating, but you could query for the items that need to be removed and then remove them in a loop:

var itemsToRemove = list1.Where(l2.Contains(l1));
foreach(var item in itemsToRemove)
{
    list1.Remove(item)
}

answered Feb 6, 2015 at 15:09

D Stanley

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