Compare two List<T> objects for equality, ignoring order

Question

Yet another list-comparing question.

I have: List<MyType> list1; List<MyType> list2;

I need to check that they both have the same elements, regardless of their position within the list. Each MyType object may appear multiple times on a list. Is there a built-in function that checks this? What if I guarantee that each element appears only once in a list?

EDIT: Guys thanks for the answers but I forgot to add something, the number of occurrences of each element should be the same on both lists.

Answer 1

Thinking this should do what you want:

list1.All(item => list2.Contains(item)) &&
list2.All(item => list1.Contains(item));

if you want it to be distinct, you could change it to:

list1.All(item => list2.Contains(item)) &&
list1.Distinct().Count() == list1.Count &&
list1.Count == list2.Count

Answer 2

If you don't care about the number of occurrences, I would approach it like this. Using hash sets will give you better performance than simple iteration.

var set1 HashSet<MyType>(list1);
var set2 HashSet<MyType>(list2);
return set1.SetEquals(set2);

This will require that you have overridden .GetHashCode() on MyType if necessary.

Answer 3

This is a slightly difficult problem, which I think reduces to: "Test if two lists are permutations of each other."

I believe the solutions provided by others only indicate whether the 2 lists contain the same unique elements. This is a necessary but insufficient test, for example {1, 1, 2, 3} is not a permutation of {3, 3, 1, 2} although their counts are equal and they contain the same distinct elements.

I believe this should work though, although it's not the most efficient:

static bool ArePermutations<T>(IList<T> list1, IList<T> list2)
{
   if(list1.Count != list2.Count)
         return false;

   var l1 = list1.ToLookup(t => t);
   var l2 = list2.ToLookup(t => t);

   return l1.Count == l2.Count 
       && l1.All(group => l2.Contains(group.Key) && l2[group.Key].Count() == group.Count()); 
}

Answer 4

As written, this question is ambigous. The statement:

... they both have the same elements, regardless of their position within the list. Each MyType object may appear multiple times on a list.

does not indicate whether you want to ensure that the two lists have the same set of objects or the same distinct set.

If you want to ensure to collections have exactly the same set of members regardless of order, you can use:

// lists should have same count of items, and set difference must be empty
var areEquivalent = (list1.Count == list2.Count) && !list1.Except(list2).Any();

If you want to ensure two collections have the same distinct set of members (where duplicates in either are ignored), you can use:

// check that [(A-B) Union (B-A)] is empty
var areEquivalent = !list1.Except(list2).Union( list2.Except(list1) ).Any();

Using the set operations (Intersect, Union, Except) is more efficient than using methods like Contains. In my opinion, it also better expresses the expectations of your query.

EDIT: Now that you've clarified your question, I can say that you want to use the first form - since duplicates matter. Here's a simple example to demonstrate that you get the result you want:

var a = new[] {1, 2, 3, 4, 4, 3, 1, 1, 2};
var b = new[] { 4, 3, 2, 3, 1, 1, 1, 4, 2 };

// result below should be true, since the two sets are equivalent...
var areEquivalent = (a.Count() == b.Count()) && !a.Except(b).Any(); 

Answer 5

If you want them to be really equal (i.e. the same items and the same number of each item), I think that the simplest solution is to sort before comparing:

Enumerable.SequenceEquals(list1.OrderBy(t => t), list2.OrderBy(t => t))

Edit:

Here is a solution that performs a bit better (about ten times faster), and only requires IEquatable, not IComparable:

public static bool ScrambledEquals<T>(IEnumerable<T> list1, IEnumerable<T> list2) {
  var cnt = new Dictionary<T, int>();
  foreach (T s in list1) {
    if (cnt.ContainsKey(s)) {
      cnt[s]++;
    } else {
      cnt.Add(s, 1);
    }
  }
  foreach (T s in list2) {
    if (cnt.ContainsKey(s)) {
      cnt[s]--;
    } else {
      return false;
    }
  }
  return cnt.Values.All(c => c == 0);
}