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

bool b = list.Any(i => list.Any(j => j.ID == i.ID && j != i));

That's a bit of a brute-force approach but it works. There might be a smarter way to do it using the Except() extension method.

Edit: You didn't actually say that you needed to know which items are "duplicated", only that you needed to know whether any where. This'll do the same thing except give you a list you can iterate over:

list.Where(i => list.Any(j => j.ID == i.ID && j != i))

I like the grouping approach too (group by ID and find the groups with count > 1).