How type casting is done between base class and derived class?

The point is that the Object class doesn't contain a reference to Button. In fact it contains no fields at all!

However, the Button class inherits from Object and you are therefore allowed to do this because the compiler knows that a Button contains (amongst others) all the members that Object has:

  Object o = new Button();

but you're not allowed to do this because Button contains some members which Object doesn't have:

  Button b = new Object();

Moreover, if you know that 'o' refers to a Button, then you can get back to a Button reference using a cast:

  Button b = (Button)o;

In .NET, all classes ultimately inherit from the System.Object class ('object' in C#).

That means that objects of any class can be assigned to an 'object' variable because they will  inherit all the members of the object class. So this is perfectly legal:

Button btn = new Button();

btn.Name = "Click me";

object obj = btn;

string s = obj.ToString(); // ToString method inherited by Button from object

However, you can't do this because the object class doesn't have a Name property:

string name = obj.Name; // compiler error

but you can do this:

Button b = (Button)obj;

string name = b.Name;

because you've now converted the object reference to a Button reference which does have a Name property.

An object is a bit like an onion - the inner layer(s) contain the members inherited from its base class(es) and the outer layers contain the members which it adds itself.

Note however that constructors and destructors are not inherited. Technically, private members are inherited but are not accessible within a derived class.