GetHashCode Made Easy

Implementing GetHashCode is hard work and little understood. If you take a look on MSDN or StackOverflow for a few pointers, you’ll see a plethora of examples with all kinds of little used C# operators and magic numbers with little explanation for developers (Especially the newbies) what they are for and why we need them. This, for a method which exists on the Object class and is the root of all that is good and wholesome in C# is surprising.

Before I continue, I recommend reading Eric Lippert’s blog post about the subject. He does not show any code, just goes into when and where we need to implement the GetHashCode method. Eric does a much better job than I could do but in short, GetHashCode is implemented wherever you implement the Equals method and ideally your class should be immutable.

Now down to the nitty gritty. How do we make implementing GetHashCode easy. Well, suppose we have the following class:

In our example we have an immutable object with a variety of fields of different types, including a collection. One possible implementation of GetHashCode according to the highest rated StackOverflow post (If modified to fit our example and deal with nulls) may be:

I don’t know about you but that code looks awfully unwieldy to me. For a start we’ve got two different magic numbers 17 and 23. Why? As it happens these are prime numbers and reduces the chance of getting collisions between hashes (Two equal objects are supposed to have the same hash code but sometimes this is not the case due to some maths that I’m not going to go into).

We’ve also got the unchecked C# keyword which stops overflow checking to improve performance (That’s not something you see every day). Bear in mind that the whole point of the GetHashCode method is to allow things like the Dictionary to quickly retrieve objects.

I personally would not be able to remember how to do this each time I need to implement GetHashCode and it seems like you could very easily introduce bugs by making a typo. How about a helper class (Well actually a struct)?

The helper struct can be used in our SuperHero class like so:

Now isn’t that pretty? All the nasty magic numbers and unchecked code has been hidden away. It is a very lightweight and simple struct, so although we create new instances of it, it’s stored in the stack rather than the memory heap. What’s more, is that is code is just as fast (I’ve timed it)! We’re using generics so there is no boxing or unboxing going on. We’re still using the unchecked keyword, so overflow checking is still disabled.