Base class and derived class in C Sharp (C#)

Introduction

We use base and derived classes to organize code by sharing common features and adding new ones. It helps avoid repeating code and makes programs easier to understand.

When you have different types of animals that share common traits but also have unique behaviors.

When creating a game with various characters that share basic actions but have special abilities.

When building a system with different types of employees who all have names and IDs but different roles.

When you want to reuse code for shapes like circles and rectangles that all have an area but calculate it differently.

Syntax

C Sharp (C#)

class BaseClass {
    // common properties and methods
}

class DerivedClass : BaseClass {
    // additional properties and methods
}

The base class is the parent that holds shared code.

The derived class inherits from the base and can add or change features.

Examples

Animal is the base class with a method Eat(). Dog is the derived class that adds Bark().

C Sharp (C#)

class Animal {
    public void Eat() {
        Console.WriteLine("Eating food");
    }
}

class Dog : Animal {
    public void Bark() {
        Console.WriteLine("Barking");
    }
}

Vehicle is the base class. Car inherits Start() and adds Honk().

C Sharp (C#)

class Vehicle {
    public void Start() {
        Console.WriteLine("Starting vehicle");
    }
}

class Car : Vehicle {
    public void Honk() {
        Console.WriteLine("Honking horn");
    }
}

Sample Program

This program shows a Dog class inheriting from Animal. Dog can use Eat() from Animal and also Bark() on its own.

C Sharp (C#)

using System;

class Animal {
    public void Eat() {
        Console.WriteLine("Animal is eating");
    }
}

class Dog : Animal {
    public void Bark() {
        Console.WriteLine("Dog is barking");
    }
}

class Program {
    static void Main() {
        Dog myDog = new Dog();
        myDog.Eat();  // inherited from Animal
        myDog.Bark(); // own method
    }
}

Important Notes

Derived classes get all public and protected members from the base class automatically.

You can create many derived classes from one base class to share common code.

Use inheritance to keep code clean and avoid repeating the same code in multiple places.

Summary

Base class holds shared code for other classes.

Derived class inherits from base and can add or change features.

This helps organize code and reuse common parts easily.

Practice

1. What is the main purpose of a base class in C#?

easy

A. To hold common code that other classes can reuse

B. To create objects directly without inheritance

C. To prevent other classes from inheriting

D. To store data only without any methods

Solution

1. Step 1: Understand the role of base class

   A base class contains common code that multiple classes can share to avoid repetition.

2. Step 2: Compare options with this role

   To hold common code that other classes can reuse matches this purpose exactly, while others describe incorrect or unrelated uses.

3. Final Answer:

   To hold common code that other classes can reuse -> Option A

4. Quick Check:

   Base class = shared code [OK]

Hint: Base class shares code for reuse [OK]

Common Mistakes:

• Thinking base class cannot be instantiated
• Confusing base class with interface
• Believing base class only stores data

2. Which of the following is the correct syntax to declare a derived class Car that inherits from a base class Vehicle in C#?

easy

A. class Car inherits Vehicle { }

B. class Car : Vehicle { }

C. class Car extends Vehicle { }

D. class Car -> Vehicle { }

Solution

1. Step 1: Recall C# inheritance syntax

   In C#, a derived class uses a colon (:) followed by the base class name.

2. Step 2: Match options with correct syntax

   class Car : Vehicle { } uses the correct syntax: class Car : Vehicle { }. Others use incorrect keywords or symbols.

3. Final Answer:

   class Car : Vehicle { } -> Option B

4. Quick Check:

   Inheritance syntax = colon (:) [OK]

Hint: Use colon (:) to inherit in C# [OK]

Common Mistakes:

• Using 'inherits' instead of ':'
• Using 'extends' like in Java
• Using arrow '->' symbol

3. What will be the output of this C# code?

class Animal {
    public void Speak() {
        Console.WriteLine("Animal speaks");
    }
}
class Dog : Animal {
    public void Speak() {
        Console.WriteLine("Dog barks");
    }
}

class Program {
    static void Main() {
        Animal a = new Dog();
        a.Speak();
    }
}

medium

A. Compilation error

B. Dog barks

C. Animal speaks

D. Runtime error

Solution

1. Step 1: Understand method hiding vs overriding

   The Speak method in Dog hides the base method but is not marked virtual or override.

2. Step 2: Check method call behavior

   Since a is of type Animal, it calls Animal.Speak() ignoring Dog's method.

3. Final Answer:

   Animal speaks -> Option C

4. Quick Check:

   Non-virtual method call = base method [OK]

Hint: Non-virtual methods call base version [OK]

Common Mistakes:

• Assuming derived method runs without override
• Confusing method hiding with overriding
• Expecting polymorphism without virtual keyword

4. Identify the error in this code snippet:

class Person {
    public string Name;
}
class Student : Person {
    public string Name;
}

class Program {
    static void Main() {
        Student s = new Student();
        s.Name = "Alice";
        Console.WriteLine(s.Name);
    }
}

medium

A. No error, code runs and prints 'Alice'

B. Missing constructor in derived class

C. Derived class cannot declare a field with the same name as base class

D. Must use override keyword for Name field

Solution

1. Step 1: Analyze the code execution

   The code declares a public field Name in both Person and Student, causing the derived field to hide the base one. This is allowed in C#.

2. Step 2: Determine if there is an error

   The code compiles (with a compiler warning about hiding), executes successfully, and prints 'Alice' as it accesses the derived class's Name field.

3. Final Answer:

   No error, code runs and prints 'Alice' -> Option A

4. Quick Check:

   Field hiding allowed, no hard error [OK]

Hint: Field hiding is allowed but generates a warning [OK]

Common Mistakes:

• Thinking derived class cannot declare same field name
• Believing missing constructor causes issue
• Mistaking fields for methods that need override

5. You want to create a base class Shape with a method Area() that derived classes Circle and Rectangle must implement differently. Which approach is best in C#?

hard

A. Use interface instead of base class for Area()

B. Make Area() a virtual method in Shape and override it in derived classes

C. Define Area() only in derived classes without base declaration

D. Declare Area() as an abstract method in Shape and implement in derived classes

Solution

1. Step 1: Understand requirement for method implementation in derived classes

   The base class Shape should force derived classes to provide their own Area() implementation.

2. Step 2: Choose correct C# feature

   Declaring Area() as abstract in Shape requires derived classes to implement it, matching the requirement.

3. Final Answer:

   Declare Area() as an abstract method in Shape and implement in derived classes -> Option D

4. Quick Check:

   Abstract method = must implement in derived [OK]

Hint: Use abstract method to force implementation [OK]

Common Mistakes:

• Using virtual without override
• Not declaring method in base class
• Confusing interface with abstract class