Inheritance is one of the three key concepts in an object-oriented programming. We can use inheritance to avoid repetition when different classes have a number of features in common and are related to each other.

In this post, we are going to talk about C# inheritance, why is it important and what we can use it for.

If you want to see complete navigation of this tutorial, you can do that here C# Intermediate Tutorial.

To download the source code, you can visit Inheritance in C# Source Code. 

We are going to split this article into the following sections:

Using Inheritance

We can define inheritance between two classes by using the following syntax:

What this means is that DerivedSubClass inherits from the DerivedClass and from the BaseClass as well, because DerivedClass inherits from the BaseClass. That way, we can share the class features between multiple classes, even though the one class can inherit only from one base class.

So, let’s create some basic inheritance structure:

In this example, the XMLWriter and JSONWriter classes have they own methods but both of them share the Write() method from the base Writer class.

So, if we create an object of type XMLWriter, we will be able to access its own method and the method from the base class:

It goes the same for the JSONWriter class.

Calling Constructors from the Base Class

From the derived classes, we can access the constructor of a base class. This is used quite common, due to initialization of some properties that are shared between derived classes. We can use the base keyword to execute that:

As we can see, we pass a string value to the derived class’s constructors and by using the basekeyword, we are passing that string value to the constructor of the base class. In there, we set up the value for the Name property.

Accessing Classes

The inheritance hierarchy means that our XMLWriter (or JSONWriter) class is a special type of the Writer, it has all the Writer’s non-private members, and additional features declared inside the XML(JSON)Writer class. But there are some limitations to this hierarchy.

Let’s look at the following example:

This means if we refer to the XMLWriter or JSONWriter object with the Writer object, we can just access the methods declared inside the Writer class.

There is one more limitation. We can’t assign a higher rank object to a lower rank object:

But we can solve this problem by using the “as” keyword:

Declaring Methods with the New Keyword

In the real world project, we often need to have so many different functionalities, and that usually leads to the existence of many different methods, properties etc. Sometimes it is pretty hard to come up with the unique and meaningful name for our identifiers, especially if we have the inheritance hierarchy. Sooner or later we are going to try to reuse a name that is already in use by one of the classes in the higher hierarchy level. If it comes to that (we have two methods with the same name in derived and base class) we are going to receive a warning:

Hiding implementation - C# Inheritance

Using the New Keyword

A method in a derived class hides a method in a base class with the same signature. So, as you can see in the picture above, our method SetName exists in the XMLWriter class and Writer class. Since the XMLWriter class inherits from the Writer class it hides an implementation of the SetName method from the Writer class.

Although our code will compile and run, we should take this warning seriously. It can happen that another class inherits from the XMLWriter class and implements the SetName method. The developer may expect to execute the SetName method from the Writer class (because XMLWriter inherits from the Writer) but this is not a case. The SetName method from the Writer class is hidden by the SetName method from the XMLWriter class.

If we find ourselves in this kind of situation the best way is to change the method signatures. But if we are sure that we want a behavior like this, we can use the new keyword. The new keyword will simply tell the compiler that we are hundred percent sure in what we are doing and that we don’t want a warning message to appear anymore:

Now we don’t have a warning message anymore.

Declaring Methods with the Virtual Keyword

Sometimes, we don’t want to hide an implementation of a method from a base class with the same signature as a method from a derived class. What we want is to provide an opportunity for a different implementation of a method with the same signature in a derived class. So, we want to override our method from a base class with the method inside a derived class.

A method that is intended to be overridden is called a virtual method. When we talk about overriding and hiding, we need to be clear with those terms. The hide means that we want completely to hide the implementation of a method from the base class, but the override means that we want a different implementation of a method from a base class.

To create a virtual method we use the virtual keyword:

Declaring Methods with the Override Keyword

If we declare a method as a virtual in our base class, we can create a method in a derived class with the keyword override to declare another implementation of that method:

If we want, we can call an original implementation of that method in a derived class by using the base keyword:

All these inheritance actions and different method implementations with the mentioned keywords has its own unique name polymorphism.

Rules to Follow While Working With Polymorphic Methods

There are some important rules which we need to follow when declaring polymorphic methods by using the virtual and override keywords:

  • We can’t declare a virtual method as private. Its purpose is to be exposed to a derived class, so making it private is meaningless. Similarly, overridden methods can’t be private because a derived class can’t change the protection level of a method that it inherits
  • The signatures of virtual and overridden methods must be identical
  • We can override only a virtual method. If we try to override a method that has no virtual keyword, we will get an error
  • If we don’t use the override keyword we are not overriding the method we are just hiding it. If this is the behavior we want, we should use the new keyword
  • An overridden method is a virtual one as well, so it can be overridden in a further derived class

Conclusion

In this article, we have learned:

  • What is inheritance and how to use it
  • How to use the new, virtual and override keywords
  • Rules of polymorphism in the C# language

In the next article, we are going to talk about Interfaces in C#Interfaces in C#.

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