Get all pairs in a list using LINQ

Question

How do I get all possible pairs of items in a list (order not relevant)?

E.g. if I have

var list = { 1, 2, 3, 4 };

I would like to get these tuples:

var pairs = {
   new Tuple(1, 2), new Tuple(1, 3), new Tuple(1, 4),
   new Tuple(2, 3), new Tuple(2, 4)
   new Tuple(3, 4)
}

Answer 1

Slight reformulation of cgeers answer to get you the tuples you want instead of arrays:

var combinations = from item1 in list
                   from item2 in list
                   where item1 < item2
                   select Tuple.Create(item1, item2);

(Use ToList or ToArray if you want.)

In non-query-expression form (reordered somewhat):

var combinations = list.SelectMany(x => list, (x, y) => Tuple.Create(x, y))
                       .Where(tuple => tuple.Item1 < tuple.Item2);

Both of these will actually consider n² values instead of n²/2 values, although they'll end up with the correct answer. An alternative would be:

var combinations = list.SelectMany((x, i) => list.Skip(i + 1), (x, y) => Tuple.Create(x, y));

... but this uses Skip which may also not be optimized. It probably doesn't matter, to be honest - I'd pick whichever one is most appropriate for your usage.