Question

It always seems to just “work” without ever having to do anything.

The only thing I can think of is that each class has a hidden sort of static identifier that Object.GetHashCode uses. (also, does anyone know how Object.GetHashCode is implemented? I couldn’t find it in the .NET Reflector)

I have never overridden GetHashCode but I was reading around and people say you only need to when overriding Equals and providing custom equality checking to your application so I guess I’m fine?

I’d still like to know how the magic works, though =P

Answer 1

It always seems to just “work” without ever having to do anything.

You didn’t tell us if you’re using value types or reference types for your keys.

If you’re using value types, the default implementation of Equals and GetHashCode are okay (Equals checks if the fields are equals, and GetHashCode is based on the fields (not necessarily all of them!)). If you’re using reference types, the default implementation of Equals and GetHashCode use reference equality, which may or may not be okay; it depends on what you’re doing.

The only thing I can think of is that each class has a hidden sort of static identifier that Object.GetHashCode uses.

No. The default is a hash code based on the fields for a value type, and the reference for a reference type.

(also, does anyone know how Object.GetHashCode is implemented? I couldn’t find it in the .NET Reflector)

It’s an implementation detail that you should never ever need to know, and never ever rely on it. It could change on you at any moment.

I have never overridden GetHashCode but I was reading around and people say you only need to when overriding Equals and providing custom equality checking to your application so I guess I’m fine?

Well, is default equality okay for you? If not, override Equals and GetHashCode or implmenet IEqualityComparer<T> for your T.

I’d still like to know how the magic works, though =P

Every object has Equals and GetHashCode. The default implementations are as follows:

1. For value types, Equals is value equality.
2. For reference types, Equals is reference equality.
3. For value types, GetHashCode is based on the fields (again, not necessarily all of them!).
4. For reference types, GetHashCode is based on the reference.

If you use a overload of Dictionary constructor that doesn’t take a IEqualityComparer<T> for your T, it will use EqualityComparer<T>.Default. This IEqualityComparer<T> just uses Equals and GetHashCode. So, if you haven’t overridden them, you get the implementations as defined above. If you override Equals and GetHashCode then this is what EqualityComparer<T>.Default will use.

Otherwise, pass a custom implementation of IEqualityComparer<T> to the constructor for Dictionary.