Remove items from one list in another

Question

I'm trying to figure out how to traverse a generic list of items that I want to remove from another list of items.

So let's say I have this as a hypothetical example

List<car> list1 = GetTheList();
List<car> list2 = GetSomeOtherList();

I want to traverse list1 with a foreach and remove each item in List1 which is also contained in List2.

I'm not quite sure how to go about that as foreach is not index based.

Answer 1

You can use Except :

List<car> list1 = GetTheList();
List<car> list2 = GetSomeOtherList();
List<car> result = list2.Except(list1).ToList();

You probably don't even need those temporary variables:

List<car> result = GetSomeOtherList().Except(GetTheList()).ToList();

Note that Except does not modify either list - it creates a new list with the result.

Answer 2

您不需要索引，因为List<T>类允许您使用Remove函数按值而不是索引删除项目。

foreach(car item in list1) list2.Remove(item);

Answer 3

I would recommend using the LINQ extension methods . You can easily do it with one line of code like so:

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

This is assuming of course the objects in list1 that you are removing from list2 are the same instance.

Answer 4

In my case I had two different lists, with a common identifier, kind of like a foreign key. The second solution cited by "nzrytmn" :

var result =  list1.Where(p => !list2.Any(x => x.ID == p.ID && x.property1 == p.property1)).ToList();

Was the one that best fit in my situation. I needed to load a DropDownList without the records that had already been registered.

Thank you !!!

This is my code:

t1 = new T1();
t2 = new T2();

List<T1> list1 = t1.getList();
List<T2> list2 = t2.getList();

ddlT3.DataSource= list2.Where(s => !list1.Any(p => p.Id == s.ID)).ToList();
ddlT3.DataTextField = "AnyThing";
ddlT3.DataValueField = "IdAnyThing";
ddlT3.DataBind();

Answer 5

You could use LINQ, but I would go with RemoveAll method. I think that is the one that better expresses your intent.

var integers = new List<int> { 1, 2, 3, 4, 5 };

var remove = new List<int> { 1, 3, 5 };

integers.RemoveAll(i => remove.Contains(i));

Answer 6

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

Answer 7

Solution 1 : You can do like this :

List<car> result = GetSomeOtherList().Except(GetTheList()).ToList();

But in some cases may this solution not work. if it is not work you can use my second solution .

Solution 2 :

List<car> list1 = GetTheList();
List<car> list2 = GetSomeOtherList();

we pretend that list1 is your main list and list2 is your secondry list and you want to get items of list1 without items of list2.

 var result =  list1.Where(p => !list2.Any(x => x.ID == p.ID && x.property1 == p.property1)).ToList();

Answer 8

As Except does not modify the list, you can use ForEach on List<T> :

list2.ForEach(item => list1.Remove(item));

It may not be the most efficient way, but it is simple, therefore readable, and it updates the original list (which is my requirement).

Answer 9

I think it would be quick to convert list A to a dictionary and then foreach the second list and call DictA.Remove(item) otherwise I think most solutions will cause many iterations through list A either directly or under the covers.

If the lists are small, it probably won't matter.

Answer 10

Here ya go..

    List<string> list = new List<string>() { "1", "2", "3" };
    List<string> remove = new List<string>() { "2" };

    list.ForEach(s =>
        {
            if (remove.Contains(s))
            {
                list.Remove(s);
            }
        });