Remove sublist from a list

Question

I have 2 lists: list1 and list2 (both of type int)

Now I want to remove content of list2 from list1 . How I can do this in C#?

PS: Don't use loop.

Answer 1

IMPORTANT CHANGE

As was pointed out in the comments, .Except() uses a set internally, so any duplicate members of list1 will be absent in the final result.

Produces the set difference of two sequences

http://msdn.microsoft.com/en-us/library/system.linq.enumerable.except(v=vs.110).aspx

However, there is a solution that is both O(N) and preserves duplicates in the original list: Modify the RemoveAll(i => list2.Contains(i)) approach to use a HashSet<int> to hold the exclusion set.

List<int> list1 = Enumerable.Range(1, 10000000).ToList();
HashSet<int> exclusionSet = Enumerable.Range(500000, 10).ToHashSet(); 

list1.Remove(i => exclusionSet.Contains(i));

The extension method ToHashSet() is available in MoreLinq .

ORIGINAL ANSWER

You can use Linq

list1 = list1.Except(list2).ToList();

UPDATE

Out of curiosity I did a simple benchmark of my solution vs. @HighCore's.

For list2 having just one element, his code is faster. As list2 gets larger and larger, his code gets extremely slow. It looks like his is O(N-squared) (or more specifically O(list1.length*list2.length) since each item in list1 is compared to each item in list2 ). Don't have enough data points to check the Big-O of my solution, but it is much faster when list2 has more than a handful of elements.

Code used to test:

        List<int> list1 = Enumerable.Range(1, 10000000).ToList();
        List<int> list2 = Enumerable.Range(500000, 10).ToList(); // Gets MUCH slower as 10 increases to 100 or 1000

        Stopwatch sw = Stopwatch.StartNew();

        //list1 = list1.Except(list2).ToList();
        list1.RemoveAll(i => list2.Contains(i));

        sw.Stop();

        var ms1 = sw.ElapsedMilliseconds;

UPDATE 2

This solution assigns a new list to the variable list1 . As @Толя points out, other references (if any) to the original list1 will not be updated. This solution drastically outperforms RemoveAll for all but the smallest sizes of list2 . If no other references must see the update, it is preferable for that reason.

Answer 2

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

Answer 3

你可以用这个：

List<T> result = list1.Except(list2).ToList();

Answer 4

这将在删除每个项目secondList从firstList ：

firstList.RemoveAll( item => { secondList.Contains(item); } );