What motivates the design of the root Object type in Java and C#?

In Java and C#, all classes derive from the root class Object (java.lang.Object and System.Object respectively).

In both languages/libraries this class comes with a set of methods that are inherited by every class.

My question is: Why and when do designers add a method to the global root object class?

I’m asking because many of Object‘s current methods seem to me like design mistakes, primarily violations of the Separation-of-Concerns rule – but I trust the language and library designers to put a great deal of thought into their designs; do the designers now say it was a mistake to add those methods?

  • System.Object
    • Equals(Object)
      • Object reference-equality can be tested using Object.ReferenceEquals. Value-equality must be explicitly opted-into by overriding this method, but consumers won’t know if value-equality is actually defined. We also have IEquatable<T> – surely it makes more sense to not have any .Equals method and instead to always test for IEquatable<T>?
        • And in most cases consumers know the types they’re comparing so type-checking for IEquatable<T> is unnecessary and the compiler can make the IEquatable<T>.Equals(T) call directly (and as a bonus there’s no need to do a runtime type test of the Object argument value)
        • And why define bool Equals instead of the more general int Compare? While value-comparison operations are undefined for many types of objects, the same can be said for value-equality. It seems odd to single-out this particular method.
    • GetHashCode()
      • Not every type is used as a hash-table key. This method is essentially useless unless overridden, especially if the object is mutable. As with Equals, why not define IHashable?
        • Having an explicit IHashable means that hashtable types could use it as a generic type constraint on dictionary key types to help prevent the use of the default GetHashCode() implementation where it would introduce bugs.
      • And the BCL’s hash-table types allow the use of a custom hash-code generator through IEqualityComparer<T> (and all struct types can have a trivial compiler-generated method).
    • ToString()
      • While handy for debugging, we’re better served with [DebuggerDisplay] instead, and most classes don’t override it (the default implementation returns the type’s name). Additionally we already have IConvertable.ToString() for when it it’s meaningful.
    • MemberwiseClone()
      • This method strikes me as being dangerous, especially if the class’ author didn’t consider it being called: consider calling SqlConnection::MemberwiseClone() on an active connection. And again, the BCL has IClonable.Clone.

In Java, the situation is similar for its methods clone(), equals(Object), hashCode() and toString() – but Java also adds threading primitives directly to Object instead of in the threading library. This seems shortsighted given that threading libraries can (and do) change their designs more often than the design of the root Object should be changed.