how to implement override of GetHashCode() with logic of overriden Equals()

Question

I have some classes as below, i have implemented the Equals(Object) method for almost all of them. But i don't know how to write GetHashCode() . As far I used these data types as value type in a Dictionary Collection, i think i should override GetHashCode().

1.I don't know how to implement GetHashCode() with logic of Equals(Object).

2.There are some derived classes, if i override GetHashCode() and Equals(Object) for base class ( Param ), is it still necessary to override it for childs?

class Param
{
    ...
    public Int16 id { get; set; }
    public String name { get; set; }
    ...
    public override bool  Equals(object obj)
    {
        if ( obj is Param){
            Param p = (Param)(obj);
            if (id > 0 && p.id > 0)
                return (id == p.id);
            else if (name != String.Empty && p.name != String.Empty)
                return (name.equals(p.name));
            else
                return object.ReferenceEquals(this, obj);
        }
        return false;
    }
}  
class Item
{
    public int it_code { get; set; }
    public Dictionary<String, Param> paramAr { get; set; }
    ...
    public override bool Equals(Object obj)
    {
        Item im = new Item();
        if (obj is Item)
            im = (Item)obj;
        else 
            return false;

        if (this.it_code != String.Empty && im.it_code != String.Empty)
            if (this.it_code.Equals(im.it_code)) 
                return true;

        bool reParams = true;
        foreach ( KeyValuePair<String,Param> kvp in paramAr ){
            if (kvp.Value != im.paramAr[kvp.Key]) {
                reParams = false;
                break;
            }
        }
        return reParams;
    }
}
class Order
{

    public String or_code { get; set; }
    public List <Item> items { get; set; }
    ...
    public override bool Equals( Object obj ){
        Order o = new Order();
        if (obj is Order)
            o = (Order)obj;
        else
            return false;

        if (this.or_code != String.Empty && o.or_code != String.Empty)
            if (this.or_code.Equals(o.or_code))
                return true;
        bool flag = true;
        foreach( Item i in  items){
            if (!o.items.Contains(i)) { 
                flag = false;
                break;
            }
        }
        return flag;
    }
}

EDIT: i get this warning:

Warning : 'Item' overrides Object.Equals(object o) but does not override Object.GetHashCode()

I believe the only consideration you have to concern yourself with when writing `GetHashCode`, is that when two objects are equal (by calling `Equals`) that their hash code must also be equal. Conversely, if both hash codes are equal, that doesn't imply that the objects themselves are equal (though they could be). Anytime you write a custom implementation of either `Equals` or `GetHashCode`, you'll have to write something for the other method to ensure the logic I explained above stays true. — Matthew, Mar 22 '12 at 18:11
Visual Studio helps with this now https://learn.microsoft.com/en-us/visualstudio/ide/reference/generate-equals-gethashcode-methods?view=vs-2017 — Nick, Nov 24 '18 at 19:47

score 17 · Accepted Answer · edited May 06 '14 at 16:25

17

Firstly, as I think you understand, wherever you implement Equals you MUST also implement GetHashCode. The implementation of GetHashCode must reflect the behaviour of the Equals implementation but it doesn't usually use it.

See http://msdn.microsoft.com/en-us/library/system.object.gethashcode.aspx - especially the "Notes to Implementers"

So if you take your example of the Item implementation of Equals, you're considering both the values of id and name to affect equality. So both of these must contribute to the GetHashCode implementation.

An example of how you could implement GetHashCode for Item would be along the lines of the following (note you may need to make it resilient to a nullable name field):

public override GetHashCode()
{
    return id.GetHashCode() ^ name.GetHashCode();
}

See Eric Lippert's blog post on guidelines for GetHashCode - http://ericlippert.com/2011/02/28/guidelines-and-rules-for-gethashcode/

As for whether you need to re-implement GetHashCode in subclasses - Yes if you also override Equals - as per the first (and main) point - the implementation of the two must be consistent - if two items are considered equal by Equals then they must return the same value from GetHashCode.

Side note: As a performance improvement on your code (avoid multiple casts):

if ( obj is Param){
    Param p = (Param)(obj);

Param p = obj as Param;
if (p != null) ...

edited May 06 '14 at 16:25

Eric Lippert

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answered Mar 22 '12 at 18:18

kaj

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what about `order` class,it has just an id and a list, i can't use all of values in its list to make a hashcode. – rene Mar 22 '12 at 18:25
A List object has a `GetHashCode` implementation which may be sufficient for your purposes. Because the list affects equality you HAVE to consider it in the `GetHashCode` implementation. However looking again at your `Equals` implementation, you're using them in a hierarchical sense - if ids match then the objects are equal regardless of their name values. So the `GetHashCode` implementation would need to be different from my initial suggestion; I haven't worked out how though... (yet) – kaj Mar 22 '12 at 18:46
@rene If that's what you're doing in Equals then yes, that's exactly what you need to do. – Servy Mar 22 '12 at 18:49
1

@KAJ The List's GetHashCode is going to be based on the reference of the list, not its contents. Two different lists with identical items won't necessarily have the same hash code. Now if you defined equality for Orders to be just based on the order code, and not on the items, then these problems go away. The Equals can just compare the order code and the getHashCode can just return the hash code of the order code. If the program needs to support having two different orders with the same order code and different items you then need to use each item in the list in both equals and GetHashCode. – Servy Mar 22 '12 at 18:53
@Servy: That's what I thought (was about to play with a proof of concept). The basic issue is, `GetHashCode` is supposed to be very quickly evaluated so iterating all items in a list doesn't sound like a good idea; however if its part of the test for equality, it *has* to be included. – kaj Mar 22 '12 at 18:56
@Servy "If the program needs to support having two different orders with the same order code and different items..." no it is not needed. i just want to check that the user don't add another order into databse with same items but different code, or reverse! – rene Mar 22 '12 at 18:59
I suppose the other option would be to have GetHashCode just return the hash of order code. As long as you can ensure that no orders that are the same will ever have different order codes it technically meets the requirements, and as long as it is rare/impossible for two different orders to have the same order code it's still reasonably effective at balancing. Technically if a group of different items all share a hashcode, as long as the Equals method really does check equality, you just have a higher collision rate but still have a working Dictionary. – Servy Mar 22 '12 at 19:03

score 9 · Answer 2 · edited May 23 '17 at 11:54

I prefer Josh Bloch's aproach.

Here's the example for the Param class.

override GetHashCode(object obj)
{
 unchecked
    {
        int hash = 17;

        hash = hash * 23 + id.GetHashCode();
        hash = hash * 23 + name.GetHashCode();
        return hash;
    }
}

Also, check this link out : .net - best algorithm for GetHashCode Properties used for the hashcode computation should be immutable as well.

how to implement override of GetHashCode() with logic of overriden Equals()

2 Answers2