LINQ expression to find if there are any matches between two arrays of string

Question

Suppose I have two lists of strings, List1 and list2, where List1 is a property of an object of type Foo in a list fooList.

I would like to remove a given Foo if no string in foo.List1 matches any string in list2 a la RemoveAll.

I can do this with nested for loops, but is there a way to do this with a single slick LINQ expression?

Long-winded code, building a new list rather than removing stuff from the existing list:

            var newFooList = new List<Foo>

            foreach (Foo f in fooList)
            {
                bool found = false;

                foreach (string s in newFooList)
                {
                    if (f.FooStringList.Contains(s))
                    {
                        found = true;
                        break;
                    }
                }

                if (found)
                    newFooList.Add(f);
            }

In my opinion you should include long-winded code as it would communicate your desired behaviour better than your description. — User 12345678
– User 12345678, Commented Oct 2, 2013 at 1:32

Ethan Brown · Accepted Answer · 2013-10-02 01:40:10Z

Yes:

var list2 = new List<string> { "one", "two", "four" };
var fooList = new List<Foo> {
    new Foo { List1 = new List<string> { "two", "three", "five" } },
    new Foo { List1 = new List<string> { "red", "blue" } }
};
fooList.RemoveAll( x => !x.List1.Intersect( list2 ).Any() );
Console.WriteLine( fooList );

Basically all the magic happens in RemoveAll: this only removes entries where the intersection of the entry's List1 property and list2 (i.e., the overlap) is empty.

I personally find the !....Any() construct kind of hard to read, so I like to have the following extension method on hand:

public static class Extensions {
    public static bool Empty<T>( this IEnumerable<T> l, 
            Func<T,bool> predicate=null ) {
        return predicate==null ? !l.Any() : !l.Any( predicate );
    }
}

Then I can re-write the magic line in a way that's a little clearer:

fooList.RemoveAll( x => x.List1.Intersect( list2 ).Empty() );

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