LINQ to remove certain elements from a IList based on a IList

Question

How to use LINQ to remove certain elements from a IList based on another IList. I need to remove records from list1 where ID is present in list2. Below is the code sample,

class DTO
{

    Prop int ID,
    Prop string Name
}

IList<DTO> list1;

IList<int> list2;



foreach(var i in list2)
{
    var matchingRecord = list1.Where(x.ID == i).First();
    list1.Remove(matchingRecord);
}

This is how I am doing it, is there a better way to do the same.

Don't use Linq for this. Linq is all about filtering and creating new sequences, not modifying collections. — Matthew Watson, May 17 '13 at 10:12

score 23 · Answer 1 · answered May 17 '13 at 10:21

You could write a "RemoveAll()" extension method for IList<T> which works exactly like List.RemoveAll(). (This is generally useful enough to keep in a common class library.)

For example (error checking removed for clarity; you'd need to check the parameters aren't null):

public static class IListExt
{
    public static int RemoveAll<T>(this IList<T> list, Predicate<T> match)
    {
        int count = 0;

        for (int i = list.Count - 1; i >= 0; i--)
        {
            if (match(list[i]))
            {
                ++count;
                list.RemoveAt(i);
            }
        }

        return count;
    }

Then to remove the items from list1 as required would indeed become as simple as:

list1.RemoveAll(item => list2.Contains(item.ID));

Daniel Hilgarth · Answer 2 · 2013-05-17T10:18:11.897

16

I would use this approach:

var itemsToRemove = list1.Where(x => list2.Contains(x.ID)).ToList();
foreach(var itemToRemove in itemsToRemove)
    list1.Remove(itemToRemove);

This approach removes the items in place. It is best used for cases where there are many items in list1 and few in list2.
If the amount of items in list1 and list2 is similar, the approach from Cuong Le is a good alternative.

edited May 17 '13 at 10:18

answered May 17 '13 at 10:13

Daniel Hilgarth

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Why do a `ToList()` after the `Where`? Don't think that's needed. – George Duckett May 17 '13 at 10:34
2

@GeorgeDuckett: Yeah, it is needed. Otherwise `itemsToRemove` iterates over `list1`. Together with the `Remove` this would yield an `InvalidOperationException`: *Collection was modified; enumeration operation may not execute* – Daniel Hilgarth May 17 '13 at 10:40
Ahh, good point, forgot about that. – George Duckett May 17 '13 at 10:54

cuongle · Answer 3 · 2013-05-17T10:15:41.440

9

You can use Where for simpler:

list1 = list1.Where(x => !list2.Contains(x.ID))
             .ToList();

But you really need to remove in place, prefer @DanielHilgarth's answer

edited May 17 '13 at 10:15

answered May 17 '13 at 09:59

cuongle

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RemoveAll does not return a list, it returns an int which represents the count of items deleted. – Robert J. May 17 '13 at 10:09
@RobertJ. you should use RemoveAll, see my answer – Michiel van Vaardegem May 17 '13 at 10:16
It actually was in his answer before, hence my comment. – Robert J. May 17 '13 at 10:17
Never mind, it is an IList – Michiel van Vaardegem May 17 '13 at 10:17
@Cuong le : This is what I was looking for. I think this would work. Thanks. – passionatedeveloper May 17 '13 at 10:37

LINQ to remove certain elements from a IList based on a IList

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