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Why GetHashCode is in Object class?

Why GetHashCode is part of the Object class? Only small part of the objects of the classes are used as keys in hash tables. Wouldn't it be better to have a separate interface which must be implemented when we want objects of the class to serve as keys in hash table.

There must be a reason that MS team decided to include this method in Object class and thus make it available "everywhere".

Solution

  • It was a design mistake copied from Java, IMO.

    In my perfect world:

    • ToString would be renamed ToDebugString to set expectations appropriately
    • Equals and GetHashCode would be gone
    • There would be a ReferenceEqualityComparer implementation of IEqualityComparer<T>: the equals part of this is easy at the moment, but there's no way of getting an "original" hash code if it's overridden
    • Objects wouldn't have monitors associated with them: Monitor would have a constructor, and Enter/Exit etc would be instance methods.

    Equality (and thus hashing) cause problems in inheritance hierarchies in general - so long as you can always specify the kind of comparison you want to use (via IEqualityComparer<T>) and objects can implement IEquatable<T> themselves if they want to, I don't see why it should be on Object. EqualityComparer<T>.Default could use the reference implementation if T didn't implement IEquatable<T> and defer to the objects otherwise. Life would be pleasant.

    Ah well. While I'm at it, array covariance was another platform mistake. If you want language mistakes in C#, I can start another minor rant if you like ;) (It's still by far my favourite language, but there are things I wish had been done differently.)

    I've blogged about this elsewhere, btw.