Question

I am having a basic issue here with linq . Though this can be solved with repeated loops . I am trying to know how can this be done in linq .

I have two lists .

List<string> a  = new List<string>{"a","b","c","d","e","f"};
List<string> b = new List<string> { "a", "b", "c", "x", "y", "z" };

I want to compare with list a and whichever element in b is found in a . I want to remove that element from b . In other words I want to remove {“a”,”b”,”c”} from b based on comparison from list a and want to contain only {“x”,”y”,”z”} in list b . Is there a single statement linq to solve this ?

Answer 1

(I’m sure this is a duplicate of another post just a few days ago, but I can’t find it…)

If you need a modification in place, you can use List<T>.RemoveAll:

b.RemoveAll(x => a.Contains(x));

Or more efficiently (if the lists are large):

HashSet<string> set = new HashSet<string>(a);
b.RemoveAll(set.Contains);

Note that modifying a collection in place isn’t idiomatic in LINQ, which is why the above uses the .NET 2 list-specific method.

Or if you’re happy to change b to refer to a new list instead, then you can use LINQ:

b = b.Except(a).ToList();