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C# Intermediate > 5 The Mystery of the old server > Introduction to inheritance

Integrated Emotional Binding

Noname is scanning files at Codeasy

It was incredibly annoying to wait for Noname to scan the files on the hidden server. Bored with waiting, I began trawling a local Wonderland network to see if I could learn anything new.

The first five to ten pages that I looked at was just mindless drivel. A few articles about resource gathering in Wonderland, a political one describing the importance of resistance, a pity story about a family where she was a resistance fighter and he decided to work for the machines. (In the end, she accidentally shot him.) Getting discouraged, I started clicking through faster and faster.

Click, click, click, wait - go back! On one of the pages, a bold headline that said "Learn About Mind-Reading Chips" attracted my attention. Without hesitation, I followed it.

The article began by describing newly discovered chips that the resistance managed to steal from machines and which they hoped to one day reproduce. The chips were first found embedded into former human prisoners' heads and acted to connect humans to humans and humans to ... machines. The first type of connection was described in all possible aspects: for instance, a mother could connect with her child to know in-depth how he or she felt. In other scenarios, couples connected via one of these chips were discovering something new in their relationships; they called it "integrated emotional binding." The second type of connection, between humans and machines, was scarcely explained at all. It seemed a bit shady that this article would omit the part where enemies were forced to be mentally in sync with one another. Why omit the part that we were all fighting to prevent? Aren't there enough secrets in this place already?

I wondered if there was a history available for this page. Fortunately, there was a log of all changes to this article for the last 5 years. There was quite a bit of material, but the most significant change appeared to have happened the day before I first arrived at Wonderland. It was titled "Final cleanup." I opened it, eager to catch every word.

Of course, I should have known that getting any useful information wouldn't be that easy. There was protection on the change log. It required knowing something, called "inheritance." The only thing left to do was to search for that term in the system and learn enough to get access to the change log. The timing of the massive change couldn't be a coincidence, and I had a gut feeling something important was hiding in there.

Inheritance

Inheritance is a mechanism of basing an object or class upon another object or class, retaining similar implementation. Inheritance allows you to re-use functionality that is already implemented in another class, and also to add custom logic on top of it. For example, let's say you have a class Pet.

class Pet
{
    public string Name { get; set; }

    public void PrintName()
    {
        Console.WriteLine($"My name is {Name}");
    }
}

This class really is as simple as it looks. It prints a pet's name to the console when you call the PrintName method. Now, let's imagine that your customer made a new request - to create a class specifically for a pet cat. It should have the same method, PrintName, as well as an additional one, PrintBreed, which prints the cat's breed. To solve this, you could create such a class as follows:

class Cat
{
    public string Name { get; set; }
    public string Breed { get; set; }

    public void PrintName()
    {
        Console.WriteLine($"My name is {Name}");
    }

    public void PrintBreed()
    {
        Console.WriteLine($"My breed is {Breed}");
    }
}

If you look closer, you can see that the class Cat just duplicated the functionality of the class Pet. The code for the property Name and method PrintName are copied from the class Pet. This approach has a couple key disadvantages:

  1. The cost of support. When you duplicate code, you have more code in general, and you need to support all of it. Any change/bugfix that needs to be implemented must be added in each copy of the code snippet.
  2. The cost of complexity. A new programmer in your team would have a harder time understanding the structure of the project and dependencies if there is a lot of duplicate code.

To avoid duplication in the class Cat, we can use inheritance. The following is an example of what the code looks like if the class is derived from another one:

class Pet
{
    public string Name { get; set; }

    public void PrintName()
    {
        Console.WriteLine($"My name is {Name}");
    }
}

class Cat : Pet     // Class Cat derives from the class Pet
{
    public string Breed { get; set; }

    public void PrintBreed()
    {
        Console.WriteLine($"My breed is {Breed}");
    }
}

public class ClassWithMain
{
    public static void Main()
    {
        Cat mrMartin = new Cat
        {
            Name = "mr Martin", // Property Name is derived from the class Pet
            Breed = "Siamese"
        };

        mrMartin.PrintName();  // PrintName is derived from the class Pet
        mrMartin.PrintBreed();
    }
}
// Outputs:
// My name is mrMartin
// My breed is Siamese

When you re-use the functionality of one class in another one by using inheritance, you are deriving one class from another. The class whose members are inherited is called the base class, and the class that inherits those members is called the derived class. A derived class can have only one direct base class. In our example, class Pet is the base class and the class Cat is a derived one. To derive class A from class B, use a colon (:) after the class A name, and then append the name for class B. You can treat it as a copy of the code from the class B to class A.

In the example above, we derived class Cat from class Pet. Having done that, we can now use the property Name and the method PrintName on every object of type Cat.

As you can see, there is no code duplication when you use inheritance. It all happens behind the scenes.

Inheritance in C# at Codeasy

Make the program compile by deriving class Flower from the class Plant. Set flower's property Type to Flowering, and then output it to the screen.

Derive the class Programmer from the class Employee. Inside class Programmer, implement a public method IsDepartmentCorrect that returns a bool and takes no arguments. The method should return true if the department is IT and false otherwise.

Conceptually, the connection between classes that are represented by inheritance can be thought of as an "is" relationship. A derived class is a specialization of the base class. In our example: a cat is a pet; that's why Cat derives from Pet. Beginners often wonder when one class should or should not derive from another one. A good check would be to apply the "is" rule. If one of them "is" another, you can apply inheritance, otherwise - don't. Here are some more examples:

  • Minivan derives from Vehicle. This is the proper usage of inheritance: a minivan is a vehicle.
  • Student derives from Pet. This is NOT a proper usage of inheritance. A student is NOT a Pet, even though they both have names.

When writing code, always check whether inheritance makes sense in the current context with the is rule.

From the following list, derive some classes from others: Building, Wall, SkyScraper and ClimbingWall. Apply inheritance only in those cases where it makes sense. Use the is rule.

From the following list, derive some classes from others: Company, Department, MarketingDepartment and EmailCampaignsDepartment. Apply inheritance only in those cases where it makes sense. Use the is rule.

Inheritance is transitive. This means if B is derived from C, and A is derived from B, then A is also derived from C:

class C
{
    public void PrintCName()
    {
        Console.WriteLine("I'm class C");
    }
}

class B : C
{
    public void PrintBName()
    {
        Console.WriteLine("I'm class B");
    }
}

class A : B
{
    public void PrintAName()
    {
        Console.WriteLine("I'm class A");
    }
}

public class ClassWithMain
{
    public static void Main()
    {
        var a = new A();

        a.PrintAName();
        a.PrintBName(); // Inherited from class B
        a.PrintCName(); // Inherited from class C
    }
}
// Outputs:
// I'm class A
// I'm class B
// I'm class C

As you can see, class A derives from B and B derives from C. Therefore, B gets all functionality from C and A gets all functionality from both B and C.

Inherit classes Parrot, Bird, and Animal from one another in the correct order. Use the is rule. The code should compile if you do it correctly.
Implement a method SayHi in the class Parrot. If the parrot is dead it should just return. If the parrot is flying, it is quite hard to say anything, and the program should output "Fleeeeeee". If the parrot is alive and not flying, it should output "Hi".

Note that it is NOT possible to derive a class from more than one class.

class C
{
    public void PrintCName()
    {
        Console.WriteLine("I'm class C");
    }
}

class B
{
    public void PrintBName()
    {
        Console.WriteLine("I'm class B");
    }
}

class A : B, C // ERROR!!! Can't derive from more than one class
{
    public void PrintAName()
    {
        Console.WriteLine("I'm class A");
    }
}

Fix the code to make it compile by adding or replacing an inheritance. It should output the text about the square to the screen.

Fix the code to make it compile by adding or replacing an inheritance. It should output the text about John to the screen.

Noname was taking so long that I fell asleep while trying to commit all the vageries of inheritance to memory. If you have any trouble sleeping, I can heartily confirm that reading about inheritance in OOP for twenty minutes will do the trick. You are welcome!

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C# Intermediate > 5 The Mystery of the old server > Introduction to inheritance

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