System.Object being the base class

Question

This question may be usual for many, i tried for an hour to understand the things but getting no proper explanation.

MSDN says, System.Object is the ultimate

Answer 1

Very easy. Ape inherits from animal, chimpanzee inherits from ape. Chimpanzee inherits from animal too, but not primarily, only through ape. In .NET, if class does not state its inheritance explicitly, the compiler adds IL code to inherit it from System.Object. If it does, it inherits System.Object through parent types.

Answer 2

Seems like you have a slight confusion with the meaning of 'multiple inheritance'?

Multiple inheritance is not when 'B inherits from A, and A inherits from O'. That's just a simple inheritance hierarchy -- which is a feature of C++, Java and C#.

In the above case, we'd say that B inherits directly from A, and inherits indirectly from O. B inherits the (non-private) members from A, and, indirectly, the (non-private) members of O.

C++ in addition supports true multiple inheritance, which can sometimes be called 'mix-in inheritance': An example of multiple inheritance would be

 class A : public O {};
 class B : A, O {};

Here B inherits directly from O, and also inherits directly from A -- In this case two copies of O's members exist in B, and to access the members of B that come from O, you need to specify which of those copies you want:

e.g. b.O::omember; or b.A::omember;

With large C++ class frameworks, you can often get undesired copies of base classes in your derived classes when you use multiple inheritance. To get round this, C++ provides virtual inheritance, which forces only one copy of the virtual base class to be inherited: The following example should make this clear (or make it even more confusing!)

// Note, a struct in C++ is simply a class with everything public
struct O { int omember; };
struct A1 :  O {};
struct B1 :  O, A1 {};

struct A2 :  virtual O {};
struct B2 :  virtual O, A2 {};

B1 b1;
B2 b2;

// There are two 'omember's in b1
b1.omember; // Compiler error - ambiguous
b1.A1::omember = 1;
b1.O::omember = 2;

// There's only one 'omember' in b2, so all the following three lines
// all set the same member
b2.A2::omember = 1;
b2.O::omember = 2;
b2.omember = 3;