Inserting twice the same key in a HashTable, how is that possible?

Question

I am trying to understand how works the key sorting / insertion check in a hashtable. I've understood that when I'm adding an object to a hashtable, it checks at runtime that there isn't the same key already entered in there.

In my test, I've 2 hashtables which keys are filled in with: 1- Integers 2- An object which I've overriden the GetHashCode method to return always 1.

My issue here: while the first test is breaking when adding the same int key, the second test isn't! How come? The hashcodes that should be checked at the insertion are all returning 1.

Thank you in advance!

My code:

class Collections
{
    public Collections()
    {
        // Testing a hashtable with integer keys
        Dictionary<int, string> d1 = new Dictionary<int, string>();
        d1.Add(1, "one");
        d1.Add(2, "two");
        d1.Add(3, "three");
        // d1.Add(3, "three"); // Cannot add the same key, i.e. same hashcode
        foreach (int key in d1.Keys)
            Console.WriteLine(key);

        // Testing a hashtable with objects returning only 1 as hashcode for its keys
        Dictionary<Hashkey, string> d2 = new Dictionary<Hashkey, string>();
        d2.Add(new Hashkey(1), "one");
        d2.Add(new Hashkey(2), "two");
        d2.Add(new Hashkey(3), "three");
        d2.Add(new Hashkey(3), "three");
        for (int i = 0; i < d2.Count; i++)
            Console.WriteLine(d2.Keys.ElementAt(i).ToString());

    }


}

/// <summary>
/// Creating a class that is serving as a key of a hasf table, overring the GetHashcode() of System.Object
/// </summary>
class Hashkey
{
    public int Key { get; set; }

    public Hashkey(int key)
    {
        this.Key = key;
    }

    // Overriding the Hashcode to return always 1
    public override int GetHashCode()
    {
        return 1;
        // return base.GetHashCode();
    }

    // No override
    public override bool Equals(object obj)
    {
        return base.Equals(obj);
    }

    // returning the name of the object
    public override string ToString()
    {
        return this.Key.ToString();
    }        
}

}

score 4 · Answer 1 · answered Nov 15 '10 at 09:21

4

Dictionary will check for the hash code and object equality. The hash is just used to come up with a "first approximation" to find possibly equal keys very efficiently.

Your override of Equals just delegates to the base implementation, which uses reference equality. That means any two distinct instances of HashKey are unequal, even if they have the same value for the Key property.

What are you actually trying to achieve? Or are you just trying to understand how GetHashCode and Equals relate to each other?

answered Nov 15 '10 at 09:21

Jon Skeet

1,421,763
867
9,128
9,194

Thank you all for your answers. – Goul Nov 15 '10 at 13:17
Thank you all for your answers. I was just trying to find out how was managed the unique insertion in a hashtable. I thought this was only based on the hashcode and not the object equality. I updated my code by simply returning true all the time from Equal() for a quick test and got the error I was waiting for: "An item with the same key has already been added.". Cheers – Goul Nov 15 '10 at 13:23

spender · Answer 2 · 2010-11-15T09:33:33.517

The hash code is merely an heuristic to choose the right bucket to store the item in. Just because 2 items share the same hashcode doesn't mean that they are equal. In the case of collision (as will be occurring with your 1 hashcode), we revert to simple search within the bucket to find members that are equal to the searched key. As your equality test is checking if the references are the same, no 2 items will ever be equal.

score 1 · Answer 3 · answered Nov 15 '10 at 09:23

The Equals compares the reference of the HashKey.

Because that are different instances, they are not equal.

Your Equals should look like this:

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(this, obj))
            return true;

        var other = obj as Hashkey;

        return
            other != null &&
            Key.Equals(other.Key);
    }

Inserting twice the same key in a HashTable, how is that possible?

3 Answers3