Base class and derived class in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Base class and derived class

Define Base Class

↓

Define Derived Class : Base Class

↓

Create Derived Class Object

↓

Access Base and Derived Members

↓

Use Derived Object with Base Class Features

First, we define a base class. Then, we create a derived class that inherits from the base. We create an object of the derived class and access both base and derived members.

Execution Sample

C Sharp (C#)

using System;

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

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

class Program {
  static void Main() {
    var dog = new Dog();
    dog.Eat();
    dog.Bark();
  }
}

This code defines a base class Animal with method Eat, a derived class Dog with method Bark, then creates a Dog object and calls both methods.

Execution Table

Step	Action	Object	Method Called	Output
1	Create Dog object	dog	N/A	N/A
2	Call Eat method	dog	Eat()	Eating
3	Call Bark method	dog	Bark()	Barking
4	End of program	N/A	N/A	N/A

💡 Program ends after calling both base and derived class methods on dog object.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
dog	null	Dog object created	Dog object unchanged	Dog object unchanged	Dog object unchanged

Key Moments - 2 Insights

Why can the dog object call the Eat() method even though Eat() is defined in Animal, not Dog?

Because Dog inherits from Animal, it gets all Animal's public methods. See execution_table step 2 where dog calls Eat() successfully.

What happens if we try to call Bark() on an Animal object?

Animal does not have Bark() method, so calling Bark() on Animal object causes a compile error. Only Dog objects have Bark().

Visual Quiz - 3 Questions

Look at the execution table, what is the output when dog.Eat() is called at step 2?

A Barking

B Eating

C Error

D No output

💡 Hint

Check the Output column at step 2 in execution_table.

At which step is the Dog object created?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the Action column in execution_table for object creation.

If we remove inheritance (Dog no longer inherits Animal), what happens when calling dog.Eat()?

A It works as before

B Runs but no output

C Compile error: Eat() not found

D Runtime error

💡 Hint

Inheritance allows Dog to use Animal's methods; without it, Eat() is unknown.

Concept Snapshot

Base class defines common features.
Derived class inherits base features and adds new ones.
Create derived object to use both base and derived methods.
Inheritance enables code reuse and organization.
Derived class syntax: class Derived : Base {}

Full Transcript

In C#, a base class is a class that provides common methods or properties. A derived class inherits from the base class, gaining its features and adding new ones. In the example, Animal is the base class with method Eat. Dog is the derived class inheriting Animal and adding Bark. When we create a Dog object, it can call both Eat (from Animal) and Bark (from Dog). This shows how inheritance works: derived classes reuse base class code. If Dog did not inherit Animal, calling Eat on Dog would cause an error. This concept helps organize code and avoid repetition.

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