Question

I need to know if the List I am working with contains only some specific values.

var list = new List<string> { "First", "Second", "Third" };

If I want to know if the List contain at least one item with the value “First” I use the Any keyword:

var result = list.Any(l => l == "First");

But how I can write a Linq expression that will return true/false only if the List contains “First” and “Second” values?

Answer 1

I’m not entirely sure what you want, but if you want to ensure that “First” and “Second” are represented once, you can do:

var result = list.Where(l => l == "First" || l =="Second")
                 .Distinct()
                 .Count() == 2;

or:

var result = list.Contains("First") && list.Contains("Second");

If you’ve got a longer “whitelist”, you could do:

var result = !whiteList.Except(list).Any();

On the other hand, if you want to ensure that all items in the list are from the white-list and that each item in the white-list is represented at least once, I would do:

var set = new HashSet(list);
set.SymmetricExceptWith(whiteList);
var result = !set.Any();

EDIT: Actually, Jon Skeet’s SetEquals is a much better way of expressing the last bit.