Determine whether two or more objects in a list are equal according to some property?

First GroupBy MyObject. Order and then determine if Any of the groups have more than one member: bool be = myObjectList. GroupBy(x => x.

Order) . Any(g => g.Count() > 1); // be is true is there are at least two objects with the same Order // be is false otherwise.

Bool hasDuplicates = myObjectList. Count > new HashSet(myObjectList. Select(x => x.

Order)).Count.

Not a pure Linq-To-Objects-Solution, but how about: var ordersList = new List(myObjectList. Select(obj => obj. Order); bool allUnique = ordersList.

Count == new HashSet(ordersList). Count; One would have to test performance of all the approaches presented here. I'd be careful, otherwise you end up quickly with some slow O(n²) look-ups.

You could improve performance by avoiding the intermediate List. The HashSet constructor can take an IEnumerable directly. – LukeH Jan 29 '10 at 15:20 What has O(n^2) lookups?

– Jason Jan 29 '10 at 15:21 @Luke, yap thanks, I overcomplicating. Of course the ordersList will always have the same count as the original list. – herzmeister Jan 29 '10 at 15:37 1 @Jason, I think I originally saw some answers posted with some Count() statements nested.

And Count() in Linq is definately evil. – herzmeister Jan 29 '10 at 15:38.

Bool allUnique = ordersList. Count == ordersList. Select(o => o.

Order).Distinct().Count().

The Count() method takes a predicate, so you would do something like this: if(myObjectList. Count(x => x. Order == 1) >= 2) { // Do something }.

That only checks if there is a duplicate with MyObject. Order equal to 1. It ignores the possibility of there being a duplicate with MyObject.

Order equal to any other int. – Jason Jan 29 '10 at 14:44 This would only test whether there are multiple objects with Order==1, not if there are two objects with the same (unknown) value. – Hans Kesting Jan 29 '10 at 14:45.

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