What's the point of a protected member in System.Object?

Question

A protected member is accessible within its class and by derived class instances.

If every type in .NET derives from Object, is there any difference between a protected member and a public member in System.Object?

Answer 1

As you said, a protected member is accessible within its class and any inheriting classes. A public member is accessible from any other class.

I see two protected members of Object in the MSDN documentation: Finalize and MemberwiseClone. Those methods are callable within any inheriting classes (i.e. any class), but not publicly accessible. If we have the following:

class Foo
{
    object Test()
    {
        return this.MemberwiseClone();  
        // Works, because Foo can see protected MemberwiseClone
        // inherited from Object
    }
}

class Bar
{
    object Test()
    {
        var foo = new Foo();
        return foo.MemberwiseClone();
        // fails: Bar cannot see Foo's protected MemberwiseClone
        // because Bar does not inherit from Foo
    }
}

Answer 2

Yes

Just because you can access your "copy" of the protected member, it doesn't mean you can access another type's "copy". The following code would work with a "public" modifier.

The point in putting a protected member into Object (provided you were able to) would be to force all objects to to have that member, without exposing it to each other.

To prove it, I wrote this code (this won't compile):

class BaseClass
{
    protected bool sharedMember;
}

class DerivedClassA : BaseClass
{
   public DerivedClassA()
   {
      DerivedClassB otherObject = new DerivedClassB();
      otherObject.sharedMember = sharedMember; //Compiler error, cannot access protected member
   }
}

class DerivedClassB : BaseClass
{
}