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I have a List that has duplicates of objects. To solve that, I need to convert the List into a HashSet (in C#). Does anyone know how?

mathenthusiast8203
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3 Answers3

98

Make sure your object's class overrides Equals and GetHashCode and then you can pass the List<T> to HashSet<T> constructor. 

var hashSet = new HashSet<YourType>(yourList);

You may see: What is the best algorithm for an overridden System.Object.GetHashCode?

Community
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Habib
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10

An alternative way would be 

var yourlist = new List<SomeClass>();

// [...]

var uniqueObjs = yourlist.Distinct();  //Gives you a List with unique Objects of the List.

Note that this is only possible, if SomeClass overrides GetHashCode and Equals in some way. This is also true for

var uniqueObjs = new HashSet<SomeType>(yourOriginalList);

Otherwise you could implement you own IEqualityComnparer-class and pass it to distinct.

Note that with the Distinct() approach, you can also look for distinct property values of the object in the list:

var uniqueNames = yourlist.Select(obj => obj.Name).Distinct();

and some more...

nozzleman
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  • I much prefer this answer to the ones that answer the specific question. "I want a house with 2 doors, so I'm going to bulldoze this house with 1 door and build a new house from scratch" – Phylogenesis Jun 25 '15 at 14:22
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    you would need to implement IEquatable on 'SomeClass' for that to work – yohannist Jun 25 '15 at 14:22
  • @YTAM as far as i am concerned, this is also the case if you want to create a new Hashset based on the original list. – nozzleman Jun 25 '15 at 14:26
  • And the mere fact that they're considered duplicate means they must be mathematically/logically equatable in some way. – Phylogenesis Jun 25 '15 at 14:30
7

If your type implements IEquatable<T>, Equals() and GetHashCode() correctly, then you don't need to do the de-duplication yourself. You can use Linq's Distinct() to do so like this:

myList = myList.Distinct().ToList();
Matthew Watson
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  • Most of the time I need a set to remove duplicates from a list. This is a great way to achieve that. – Jabba Aug 06 '20 at 11:10