how to create abstract method

Using the same logic, one could say that an abstract class is not a class if it provides no implementations ?

Even without spotting this flawed logic, Mehdi's statements are false. I explained it below.
—SA

This is not true too. 1) First of all, there are abstract methods and property; "abstractness" of the class is a secondary non-required thing; 2) if you don't mark class "abstract", you still can write abstract methods/properties in it adding an abstract method will require to add "abstract" to the class, too; the compiler will issue an error. But you can mark the class "abstract" without adding abstract members. In both cases, instantiation of an abstract class will be prevented by the compiler.

An abstract class is not a placeholder at all. This is an "incomplete class". Its main purpose is to serve as a base class representing some common features of some set of derived classes which are used in polymorphic sets of object. If virtual methods/property are not used or if they used but late binding is not used, abstract classes defeat the purpose. But the common sense dictate that such abstract classes without abstract members should be allowed. This is the same as allowing methods with empty bodies, and so one. The compiler should not prevent compiling incomplete but formally correct code.

Please, check up thoroughly all the statements you post as "answer", to avoid confusing readers.

—SA