C# OOP in Details

Note: Sometimes users might be unclear about what a base class is and what a derived class is. A base class is the super-class and a derived class is the class(es) that inherit the base class.

Here we can see a simple inheritance where the base class is the “GuestVist” and the derived class is “HouseOwnerClass”. Here the “HouseOwnerClass” class inherits the base class of “GuestVist”.

Here the HouseOwnerClass class inherits the base class GuestVist:
Multi level Inheritance: With more than one Derived Class for example. Here we can see an example. The first base class is derived in DerivedClass1 and then the DerivedClass1 is derived in DerivedClass2. Now from DerivedClass2 we can access both BaseClass's and DerivedClass1's variabled and methods.
Multiple Inheritance:

Will .Net Support Multiple Inheritance?

The answer to this question is No. In C#, it's not possible to use Multiple Inheritance in a class.

What is Multiple Inheritance? Multiple Inheritance is nothing but having more than one class and we can inherit both classes in our derived class.

What will happen if I write a Multiple Class Inheritance Using C#?

Let's take our example House. Here we have the derived class “HouseOwnerClass” with the two additional classes “GuestVist” and “FriendsandRelationsClass”.

Now suppose for our house both Guest and Friend have visited. For this, we write the preceding three classes and inherit the two classes in our derived class.

When I write the Multiple Inheritance in C #, it will display the warning message during our code and execute our program. The Warning message will be “Expected and interface”.

See the following image that shows the warning error message while I write Multiple Inheritance.



Then how can we use Multiple Inheritance?

Interface will be used for Multiple Inheritance.

9. Polymorphism

Poly means more than one form. In the beginning of the article in the Method Section, we have already seen an example of Polymorphism. The same method name with a different parameter is an example of polymorphism.

Method Overloading and Method Overriding will be used in polymorphism. Polymorphism has the two types of execution, one is Compile Time Polymorphism and the other one is called Run time Polymorphism.

• Method Overloading
  
  Method overloading is nothing but the same method name used for more than one method with different argument(s).
  
  Here is an example program of Method Overloading. As we can see here, the method name “BedRoom” has been used for two methods but the parameter for both methods are different.
  
  1. class HouseOwnerClass
  2. {
  3. //Function with parameter
  4. public void BedRoom(String nameandColor)
  5. {
  6. Console.WriteLine(nameandColor);
  7. }
  9. // Same Function Name with Different Paramenter
  10. public void BedRoom(String MemberName, String Color)
  11. {
  12. Console.WriteLine(MemberName + " Like " + Color + "Color");
  13. }
  15. static void Main(string[] args)
  16. {
  17. HouseOwnerClass objHouseOwner = new HouseOwnerClass();
  19. objHouseOwner.BedRoom("My Name is Shanu I like Lavender color");
  20. objHouseOwner.BedRoom("My Name is Afraz I like Light Blue color");
  21. objHouseOwner.BedRoom("SHANU", "Lavender");
  22. Console.ReadLine();
  24. }
  25. }
• Method Overriding
  
  The difference between Method Overloading and Overriding are that in Method Overloading we will have the same method name with a different argument.
  
  In Method Overriding we will have the same Method Name and same argument and same type but method overriding can only be used in the derived class. Method Overriding cannot be done in the same class.
  
  We will see how Method Overriding can be used in Abstract Method, Virtual Method and in a Sealed Method. Kindly refer to that section in this article.

10. Abstract Class/Method

Abstract Class: An Abstract class will have the keyword “abstract”.
The Abstract class will be a super class for all our classes. An Abstract class cannot be accessed by an object. That means we cannot create an object for an abstract class.

What will happen when we create an object for an Abstract Class

Here we can see the error warning message “An instance of an abstract class cannot be created” when I try to create an object for my abstract class.



An Abstract class can have both an Abstract Method and a normal method. In an Abstract Class at least one Abstract Method should be declared. In addition, the derived class should override the abstract method. To access the abstract method we should use the “override” keyword in our derived class.

What will happen if we create an abstract method not overriden in the derived class?

Here we can see an abstract class with an abstract method, but the abstract method is not overriden in the derived class. See the following image for the warning message displaying as class must implement the abstract member.



Here we can see an example of an Abstract class and an Abstract method in detail.

In this example, we can see an Abstract class that has a normal method and Abstract method. Abstract methods do not have a body part in the Abstract class that means we can only declare an Abstract method in an Abstract class. There should be a minimum of one Abstract Method in an Abstract Class.
11. Virtual Class/Method

A Virtual method is very useful in our day-to-day programming.

What a Virtual method is and what the use of a Virtual method is

Take our House example. A guest confirms, since today a total of five people will visit your home. For this, we write a function to display a message since there are five guests visiting our home. Once a guest visits, we see there are a total of 20 persons that have visited. In some cases it might increase or decrease; we will know when they reach us.

In that case, the guest will be a separate class and House will be a separate class. Without changing the message in the Guest class, how can we change the data in our Derived class?

The difference between an Abstract method and a Virtual method

Both similarities use the override keyword. An Abstract Method can only be declared in an Abstract Class. That means no body part for an abstract method in an Abstract class. However, for virtual it can have a body part.

See the example program below. Here we have both an Abstract Method and a Virtual Method. In the Abstract class, the virtual method says total five guests but in the derived Class program, the message was overriden as 20 guests. See the final output in the following. Guess what will be displayed for the Virtual method? Will the result be 5 Guests or 20 Guests? Check the output below the program.
The Complete Program
Output



12. Sealed Class/Method

Sealed Class: As the name indicates, this class cannot be inherited by other classes.

Take our House example. In a house, the house owner can have a secret room, perhaps an official or a financial room. The owner doesn't want others to access his official room. The sealed class will be useful in those cases.

A Sealed class can be declared using the keyword Sealed. If one class is declared Sealed, it cannot be inherited in other derived classes.

What will happen when we inherit a sealed class in our derived class

Let's see an example of attempting to inherit my sealed class from my derived class. It shows me the following warning message.



Here we can see an example program of the Sealed Class.
Sealed Method: If we declared a method as sealed then that specific method cannot be overriden in the derived class.

Let's see our house class here I have base class with Virtual Method and virtual Sealed methods.

The Virtual method can be overriden in the derived class. But the Virtual Sealed Method cannot be overriden in the sealed class.
13. Static Class/Method

We have already learned about what a Sealed Class is in this article. Now let's see what a Static class and Static method are.
Both Static and Sealed Class cannot be inherited.

The Difference between a Static Class and a Sealed Class

We can create an object (instance) of the Sealed Class. We can see in my sealed class section I have created a sample Sealed class and in the Main Method I created an object to access the sealed Class. And in a Sealed Class both Static and non-Static methods can be written.

But for a Static Class it's not possible to create an Object. In a Static Class only Static members are allowed. That means in a static class it's not possible to write non-static methods.

We can say our Main method is an example of a Static method. When we create a console application in C# we can see each class has a default Main method. In my article I have explained that when a Console or Windows application begins execution, first the Main method will be executed. There is no need to create an object of the Main method since it was been declared as a static method.
Something interesting about a Static class is that memory will be allocated for all static variables and methods during execution but for non-static variables and methods memory will be allocated only when the object for the class is created.

Let's take our same sealed class example for our static class and method.

What happens when we inherit a Static class in a derived class

Let's see an example when I try to inherit my static class from my derived class. It shows me the following warning message.



What will happen when we declare a non-Static method in a Static class

Let's see an example of attemting to create a non-Static method at my Static Class.



What will happen when we create an object for the Static class

Let's see an example of attemting to create an object for my Static Class.



When we run the program we get the error message as “Cannot create an instance of a static class”. Sorry, I'm using the Korean Version of Visual Studio so the error message is in the Korean language.

How to call the Static Class Method and variable without creating the Object

It's simple. We can just use the “ClassName.Variable or Method Name” for example “OwnerofficialRoom.AllMyPersonalItems();”
See the following example with a Static class:
The output for the preceding program is as in the following:



It is possible to create a Static Method in a non-Static Class

Yes, we can create a Static Method to the non Static class. No need to create an object to access the static method(s) in a non-static class. We can directly use the class name to access the Static method.

See the following example with Static method in a non-static Class.
The output of the preceding program is the following:



14. Interface

An interface is same like an abstract class but in an interface we only have a method declaration. In an abstract class however we can have both, a method declaration and a method definition. Methods of an interface must be implemented in an implementing class.

See the following example program for a interface. All the methods of the interface have been implemented in the class. As I have already said, C# doesn't support multiple inheritance. To get multiple inheritance we can use an interface. This following program is an example for multiple inheritance using interface.

1. interface GuestInterface
2. {
3. void GuestWelcomeMessage();
4. void NoofGuestes();
5. }
6. interface FriendsandRelationsInterface
7. {
8. void friendwelcomemessage();
9. void FriendName();
10. }
12. class HouseOwnerClass : GuestInterface, FriendsandRelationsInterface
13. {
14. public void GuestWelcomeMessage()
15. {
16. Console.WriteLine("All guests are well come to our home");
17. }
19. public void NoofGuestes()
20. {
21. Console.WriteLine("Total 15 Guestes has visited");
22. }
24. public void friendwelcomemessage()
25. {
26. Console.WriteLine("Welcome to our Home");
27. }
28. public void FriendName()
29. {
30. Console.WriteLine("Friend name is: Afraz");
31. }
32. static void Main(string[] args)
33. {
34. HouseOwnerClass obj = new HouseOwnerClass();
35. obj.GuestWelcomeMessage();
36. obj.NoofGuestes();
37. obj.friendwelcomemessage();
38. obj.FriendName();
39. Console.ReadLine();
40. }
41. }

The output will be like this.



In some cases we need to have certain methods that will be used in many derived classes. Each derived can implement different functionality for those Methods. In These cases, we can use the Interface.

We can use our Guest and House example. For the Guest the “Welcome Message” and “No of Guest” Functions are common, but it will be different for different owners in the same house. A Guest might a fathers guest, a Mothers Guest, a Children's Guest or a Family Guest. Each guest can have a different welcome message subject, but the functions are the same as a message. Let's consider now that Father is a class, Mother is a class and Children are one class. Both the guestWelcome Message and Noofguest method are the same for all. In this case, we can create an interface and declare both methods in the interface. All father, mother and children, classes can inherit the interface and write their own method details.

An interface is similar to an Abstract class but the major difference between the Abstract Class and the Interface are that in an Abstract class there can be both an Abstract method and Non-Abstract methods. But in an interface all methods are abstract by default. That means there is not a non-Abstract type method in the interface. All the Methods declared in the interface should be overriden in the derived class.

What will happen when non-abstract methods with body parts are declared in an Interface

It will display the warning message as “unexpected modifier” in the Access modifier part and the “Unexpected Method body” error warning in the message body. See the following image for the details.



Example program for interface: