Question

It is well known that C# class can inherit only from a Single Base class
How does it make sense because all classes derive from System.Object?

Also, If I look into class ValueType definition (for example), which i know that inherits directly from System.Object, i see that it is just an abstract class. i expect to see:
public abstract class ValueType : System.Object

Answer 1

Inheritance is most easily visualized as a hierarchy or tree. See if this makes sense:

                     System.Object
                     /           \
                    /             \
                   /               \
MyProject.SomeClass                 MyProject.SomeBaseClass
                                         /           \
                                        /             \
                                       /               \
                   MyProject.SomeDerivedClass     MyProject.SomeOtherDerivedClass

Looking at MyProject.SomeDerivedClass, you can clearly see that it has exactly one parent in the hierarchy, its base class, MyProject.SomeBaseClass. Now, MyProject.SomeBaseClass has exactly one parent as well, System.Object.

While MyProject.SomeDerivedClass doesn’t directly inherit from System.Object, its parent does; everything the parent either inherits or provides is passed on to the derived object. So, even though MyProject.SomeDerivedClass only inherits directly from MyProject.SomeBaseClass, it still inherits from System.Object because its parent does so.