Question

What do you call this method, (is it available in .net?)

var list1 = new List<int>() { 1, 2, 2, 3, 4 };
var list2 = new List<int>() { 1, 2, 3};
var results = list1.diff(list2);

results:
{ 2, 4 }

Answer 1

This exactly return what you want, You can refactor it in a Extension Method:

var results = list1.GroupBy(p => p).Select(p => new { item = p.Key, count = p.Count() })
                .Concat(list2.GroupBy(p => p).Select(p => new { item = p.Key, count = -p.Count() }))
                .GroupBy(p => p.item).Select(p => new { item = p.Key, count = p.Sum(q => q.count) })
                .Where(p => p.count > 0)
                .SelectMany(p => Enumerable.Repeat(p.item, p.count));