查找列表1中不存在且代码最少的行中的所有整数？

Question

Let's say I have two lists of integers:

List<int> list1 = new List<int> {1,2,3,4,5,6};
List<int> list2 = new List<int> {4,5,6,7,8,9};

What is the quickest way to find all of the integers that exist in list1 but not list2

The simplest solution I can think of is to create a union list from list1 and list2 and remove all of the members from this union that exist in list2

Union = {1,2,3,4,5,6,7,8,9}
Union - list2 = {1,2,3} <- This is my desired result

But maybe there is a simpler and faster one line of code way to get this job done?

Answer 1

list1.Except(list2) （如果使用.NET 3.5）

Answer 2

If the lists can potentially contain duplicate elements and you want to return any duplicates from list1 then you can do something like this:

var tempSet = new HashSet<int>(list2);
var results = list1.Where(x => !tempSet.Contains(x));

If list2 only contains a few elements then you can probably get away without using a HashSet<T> :

var results = list1.Where(x => !list2.Contains(x));

Though for larger collections you'll find that the HashSet<T> will easily outperform using the list directly: Contains is O(1) for HashSet<T> and O(n) for arbitrary IEnumerable<T> sequences.