LINQ Except() Method Does Not Work

Question

I have 2 IList<T> of the same type of object ItemsDTO. I want to exclude one list from another. However this does not seem to be working for me and I was wondering why?

IList<ItemsDTO> related = itemsbl.GetRelatedItems();
IList<ItemsDTO> relating = itemsbl.GetRelatingItems().Except(related).ToList();

I'm trying to remove items in related from the relating list.

score 11 · Accepted Answer · answered Feb 15 '12 at 17:18

11

Since class is a reference type, your ItemsDTO class must override Equals and GetHashCode for that to work.

answered Feb 15 '12 at 17:18

Magnus

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I think it would be enough to override Equals alone. Why do you think you need to override GetHashCode? – gprasant Feb 15 '12 at 17:21
2

@gprasant http://stackoverflow.com/questions/371328/why-is-it-important-to-override-gethashcode-when-equals-method-is-overriden-in-c – Yuck Feb 15 '12 at 17:22
2

`Except` uses `HashSets` internally. – Magnus Feb 15 '12 at 17:22
@Magnus: Gosh, how did you know that? Anyways, for performance reasons? – gprasant Feb 15 '12 at 17:27
For help on using a good algorithm for the hashcode see [this](http://stackoverflow.com/q/263400/468973) answer. – Magnus Feb 15 '12 at 17:28
1

@gprasant No, a `HashSet` wont work at all if two equal objects return different hashcodes. – Magnus Feb 15 '12 at 17:41

Yuck · Answer 2 · 2012-02-15T17:47:58.030

From MSDN:

Produces the set difference of two sequences by using the default equality comparer to compare values.

The default equality comparer is going to be a reference comparison. So if those lists are populated independently of each other, they may contain the same objects from your point of view but different references.

When you use LINQ against SQL Server you have the benefit of LINQ translating your LINQ statement to a SQL query that can perform logical equality for you based on primary keys or value comparitors. With LINQ to Objects you'll need to define what logical equality means to ItemsDTO. And that means overriding Equals() as well as GetHashCode().

score 0 · Answer 3 · answered Aug 07 '13 at 01:44

I just ran into the same problem. Apparently .NET thinks the items in one list are different from the same items in the other list (even though they are actually the same). This is what I did to fix it:

Have your class inherit IEqualityComparer<T>, eg.

public class ItemsDTO: IEqualityComparer<ItemsDTO>
{
  public bool Equals(ItemsDTO x, ItemsDTO y)
  {
    if (x == null || y == null) return false;

    return ReferenceEquals(x, y) || (x.Id == y.Id); // In this example, treat the items as equal if they have the same Id
  }

  public int GetHashCode(ItemsDTO obj)
  {
    return this.Id.GetHashCode();
  }
}

gprasant · Answer 4 · 2012-02-15T17:26:20.170

0

Except works well for value types. However, since you are using Ref types, you need to override Equals and GethashCode on your ItemsDTO in order to get this to work

edited Feb 15 '12 at 17:26

answered Feb 15 '12 at 17:20

gprasant

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LINQ Except() Method Does Not Work

4 Answers4

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