Compare two List objects for equality, ignoring order [duplicate]

Answer 1

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(IList list1, IList 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()); 
}