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

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;