Sort list by property/anonymous function?

Question

I've got a list defined like this...

var sets = new List<HashSet<int>>(numSets);

Why isn't there an overload so I can sort it like this?

sets.Sort(s => s.Count);

I want the largest set first. What's the easiest way to do that?

Answer 1

Because List<T> class was introduced in .NET 2.0 and the designers of this class decided so. You could use the OrderByDescending extension method:

sets = sets.OrderByDescending(s => s.Count).ToList();

Answer 2

Try this:

sets.Sort((setA, setB) => setB.Count.CompareTo(setA.Count));

This uses the Sort(Comparison<T> comparison) overload of List<T>.Sort. The fact that the expression compares B with A rather than A with B is what produces the descending-by-count order that you require.

The reason your code doesn't work is because List<T>.Sort, unlike Enumerable.OrderByDescending, does not have an overload that accepts a Func<TSource, TKey> key-selector.

@Darin Dimitrov's technique of using OrderByDescending is fine too, but note that this will create a sorted list out of place and reassign the reference you have to the original list to the new, sorted one.

Answer 3

Little confused...

var sw = new Stopwatch();

sw.Start();
for (int i = 0; i < 1000; ++i)
    sets.Sort((s1, s2) => s2.Count.CompareTo(s1.Count));
sw.Stop();
Console.WriteLine(sw.ElapsedMilliseconds);

sw.Start();
for (int i = 0; i < 1000; ++i)
    sets = sets.OrderByDescending(s => s.Count).ToList();
sw.Stop();
Console.WriteLine(sw.ElapsedMilliseconds);

Gives me "1499, 2107", but if I switch the order, I get "589, 2023". Why is there such a big difference?