LINQy way to check if any objects in a collection have the same property value

Question

I have a class Agent with a property Id

Given a collection of Agents I need to check if any of them have duplicate Ids.

I am currently doing this with a hash

Answer 1

For what it's worth, I just compared the two methods we've struck upon in this thread. First I defined a helper class:

public class Foo
{
    public int ID;
}

... and then made a big list of instances with a random ID:

var list = new List();

var r = new Random();

for (int i = 0; i < 10000; i++) list.Add(new Foo { ID = r.Next() });

... and lastly, timed the code:

var sw = new Stopwatch();
sw.Start();
bool b = list.Any(i => list.Where(j => i != j).Any(j => j.ID == i.ID));
Console.WriteLine(b);
Console.WriteLine(sw.ElapsedTicks);

sw.Reset();
sw.Start();
b = (list.GroupBy(i => i.ID).Count() != list.Count);
Console.WriteLine(b);
Console.WriteLine(sw.ElapsedTicks);

Here's one output:

False

59392129

False

168151

So I think it's safe to say that grouping and then comparing the count of groups to the count of items is way, way faster than doing a brute-force "nested Any" comparison.