Implementing GetHashCode correctly

c# .net equals
Zivka picture Zivka · Jan 25, 2012 · Viewed 64.5k times · Source

I'd like to hear from the community on how I should go about implementing GetHashCode (or override it) for my object. I understand I need to do so if I override the equals method. I have implemented it a fair amount of times, sometimes just calling the base method. I understand that my object should equal another instance of the object if it contains the same details (members). What is the best way to get a hash code from the class's members?

Answer

jason picture jason · Jan 25, 2012

Let's say your class looks like this:

class Frob {
    public string Foo { get; set; }
    public int Bar { get; set; }
    public double FooBar { get; set; }
}

Let's say you define equals so that two instances of Frob are equal if their Foo and their Bar are equal, but FooBar doesn't matter.

Then you should define GetHashCode in terms of Foo and Bar. One way is like this:

return this.Foo.GetHashCode() * 17 + this.Bar.GetHashCode();

Basically, you just want to incorporate all the fields that go into defining the equality. One way is to just keep accumulating and multiplying by 17 like I've done. It's fast, it's simple, it's correct, and it usually gives a good distribution.

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