CODEWITHSUNDEEP
c#hashset
Ash
Ash

Reputation: 3379

C# Hashset Contains Non-Unique Objects

Using this class

public class Foo
{
    public string c1, c2;

    public Foo(string one, string two)
    {
        c1 = one;
        c2 = two;
    }

    public override int GetHashCode()
    {
        return (c1 + c2).GetHashCode();
    }
}

And this HashSet

HashSet<Foo> aFoos = new HashSet<Foo>();
Foo aFoo = new Foo("a", "b");

aFoos.Add(aFoo);
aFoos.Add(new Foo("a", "b"));

label1.Text = aFoos.Count().ToString();

I get the answer 2, when surely it should be 1. Is there a way to fix this so my HashSet contains only unique objects?

Thanks, Ash.

Upvotes: 19

Views: 18111

Answers (3)

Tomas Jansson
Tomas Jansson

Reputation: 23462

You need to override the equals method as well. The reason for this is that the hashcode is allowed to collide for two objects that is not equal. Otherwise it won't work.

public override bool Equals(Object obj)
{ 
   Foo otherObject = obj as Foo;
   return otherObject != null && otherObject.c1 == this.c1 && otherObject.c2 == this.c2;
}

Upvotes: 4

JaredPar
JaredPar

Reputation: 754525

The HashSet<T> type ultamitely uses equality to determine whether 2 objects are equal or not. In the type Foo you have only overridden GetHashCode and not equality. This means equality checks will default back to Object.Equals which uses reference equality. This explains why you see multiple items in the HashSet<Foo>.

To fix this you will need to override Equals in the Foo type.

public override bool Equals(object obj) { 
  var other = obj as Foo;
  if (other == null) {
    return false;
  }
  return c1 == other.c1 && c2 == other.c2;
}

Upvotes: 30

Victor Haydin
Victor Haydin

Reputation: 3548

You need to override Equals method. Only GetHashCode is not enough.

Upvotes: 7

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