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For example the object-oriented programmer defines an abstract data type by encapsulating its implementation and its interface in a class.
One or more instances of the class can then be created or instantiated .
An instance of a class is known as an object .
Every object has state and behavior where the state is determined by the current values stored in the object's instance variables and the behavior is determined by the instance methods belonging to the object.
Inherited abstract data types are derived classes or subclasses of base classes or super classes. We extend super classes to create subclasses.
Within the program the code instantiates objects (creates instances of classes) and sends messages to the objects by calling the class's methods (or member functions ).
If a program is "object oriented", it uses encapsulation , inheritance , and polymorphism . It defines abstract data types, encapsulates those abstract data types into classes, instantiates objects fromthe classes, and sends messages to the objects.
The members of a class fall generally into one of the following categories:
The individual members of a class can be public , private , or protected .
To make things even more confusing, almost every item or action used in the OOP jargon has evolved to be described by several different terms. For example, we can cause an object to change its state by sending it a message, calling its methods, or calling its member functions. The term being used oftendepends on the author who wrote the specific book that you happen to be reading at the time.
Hopefully most of this terminology will become clear as we pursue these modules
I usually try to provide some code in each module that you can compile and execute. Listing 2 contains the C# code for a simple program that will display the words "Hello C# World" on the system console screen when youcompile and run it. (The system console screen will probably appear as a black window when you run the program.)
Listing 2 . Hello World in C#.
//File Program.cs
using System;namespace Hello01{
class Hello01{static void Main(string[] args){Console.WriteLine("Hello C# World");
//Press any key to dismiss console screen.Console.ReadKey();
}//end Main}//end class definition
}//end namespace You may be surprised at how much you can learn about the structure and syntax of an object-oriented program in C# by taking this simple program apart andexamining the elements of the program. Note, however, that this program is strictly class based. It does not instantiate any objects .
The central block of code beginning with the word class defines a new class named Hello01 .
You probably learned that every C++ program requires a function named main . Execution of the C++ program begins and ends in the main function.
The same is true in C#. Every C# program requires a method named Main (note the upper-case "M") . However, unlike in C++, the Main method in C# must be defined inside of a class definition.
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