Runtime polymorphism execution in C Sharp (C#) - Mini Project: Build & Apply

Runtime Polymorphism Execution

📖 Scenario: Imagine you are creating a simple program to show how different animals make sounds. Each animal has its own way of making a sound, but you want to use the same method name to call their sounds. This is called runtime polymorphism.

🎯 Goal: You will build a program with a base class Animal and two derived classes Dog and Cat. Each derived class will have its own version of the MakeSound method. You will then create a list of animals and call MakeSound on each one to see the different sounds.

📋 What You'll Learn

Create a base class called Animal with a virtual method MakeSound.

Create two derived classes called Dog and Cat that override MakeSound.

Create a list of Animal objects containing both Dog and Cat instances.

Use a foreach loop to call MakeSound on each animal and print the result.

💡 Why This Matters

🌍 Real World

Runtime polymorphism is used in many programs to handle different objects with a common interface, like different shapes in a drawing app or different payment methods in a shopping app.

💼 Career

Understanding runtime polymorphism is essential for object-oriented programming jobs, as it helps write flexible and reusable code.

1

Create the base class Animal

Create a public class called Animal with a public virtual method MakeSound that returns a string "Some sound".

C Sharp (C#)

// Your code here

Hint

Use the virtual keyword to allow derived classes to override the MakeSound method.

2

Create derived classes Dog and Cat

Create two public classes called Dog and Cat that inherit from Animal. Override the MakeSound method in Dog to return "Bark" and in Cat to return "Meow".

C Sharp (C#)

public class Animal
{
    public virtual string MakeSound()
    {
        return "Some sound";
    }
}

// Your code here

Hint

Use the override keyword to provide a new version of MakeSound in each derived class.

3

Create a list of animals

Create a List<Animal> called animals and add one Dog object and one Cat object to it.

C Sharp (C#)

public class Animal
{
    public virtual string MakeSound()
    {
        return "Some sound";
    }
}

public class Dog : Animal
{
    public override string MakeSound()
    {
        return "Bark";
    }
}

public class Cat : Animal
{
    public override string MakeSound()
    {
        return "Meow";
    }
}

// Your code here

Hint

Remember to include using System.Collections.Generic; at the top for the list.

4

Call MakeSound on each animal and print

Use a foreach loop with variable animal to go through animals and print the result of animal.MakeSound().

C Sharp (C#)

using System.Collections.Generic;

public class Animal
{
    public virtual string MakeSound()
    {
        return "Some sound";
    }
}

public class Dog : Animal
{
    public override string MakeSound()
    {
        return "Bark";
    }
}

public class Cat : Animal
{
    public override string MakeSound()
    {
        return "Meow";
    }
}

List animals = new List();
animals.Add(new Dog());
animals.Add(new Cat());

// Your code here

Hint

Use Console.WriteLine(animal.MakeSound()) inside the foreach loop.

Practice

1. What does runtime polymorphism in C# allow you to do?

easy

A. Create multiple instances of the same class

B. Change variable types at runtime

C. Call derived class methods through a base class reference

D. Use static methods without creating objects

Solution

1. Step 1: Understand runtime polymorphism concept

   Runtime polymorphism allows a base class reference to call methods overridden in derived classes.

2. Step 2: Identify correct behavior

   This means the actual method called depends on the object's real type, not the reference type.

3. Final Answer:

   Call derived class methods through a base class reference -> Option C

4. Quick Check:

   Runtime polymorphism = base ref calls derived method [OK]

Hint: Think: base class ref calls derived method at runtime [OK]

Common Mistakes:

• Confusing polymorphism with changing variable types
• Thinking static methods are polymorphic
• Believing polymorphism creates multiple instances

2. Which keyword is used in C# to allow a method to be overridden in a derived class?

easy

A. virtual

B. override

C. new

D. abstract

Solution

1. Step 1: Identify keyword to enable overriding

   The base class method must be marked with virtual to allow overriding.

2. Step 2: Understand roles of keywords

   override is used in derived classes, virtual in base classes.

3. Final Answer:

   virtual -> Option A

4. Quick Check:

   Base method uses virtual to allow override [OK]

Hint: Base method uses virtual; derived uses override [OK]

Common Mistakes:

• Using override in base class instead of virtual
• Confusing new keyword with override
• Thinking abstract is required for all overrides

3. What is the output of this C# code?

class Animal {
  public virtual string Speak() => "Animal sound";
}
class Dog : Animal {
  public override string Speak() => "Bark";
}
class Cat : Animal {
  public override string Speak() => "Meow";
}

Animal a = new Dog();
Console.WriteLine(a.Speak());

medium

A. Animal sound

B. Bark

C. Meow

D. Compile error

Solution

1. Step 1: Identify object type and method called

   Variable a is of type Animal but references a Dog object.

2. Step 2: Apply runtime polymorphism

   Since Speak is virtual and overridden in Dog, the Dog version runs, printing "Bark".

3. Final Answer:

   Bark -> Option B

4. Quick Check:

   Base ref calls Dog's Speak() = Bark [OK]

Hint: Base ref calls derived override method at runtime [OK]

Common Mistakes:

• Expecting base class method output
• Confusing object type with reference type
• Thinking compile error due to override

4. Identify the error in this C# code related to runtime polymorphism:

class Base {
  public override void Show() {
    Console.WriteLine("Base Show");
  }
}
class Derived : Base {
  public override void Show() {
    Console.WriteLine("Derived Show");
  }
}

medium

A. Base class method must be virtual, not override

B. Derived class method cannot override base method

C. Missing semicolon after method declaration

D. No error, code is correct

Solution

1. Step 1: Check base class method declaration

   Base class method incorrectly uses override instead of virtual.

2. Step 2: Understand override rules

   Only derived classes use override; base class must use virtual to allow overriding.

3. Final Answer:

   Base class method must be virtual, not override -> Option A

4. Quick Check:

   Base method needs virtual keyword [OK]

Hint: Base method uses virtual, not override [OK]

Common Mistakes:

• Using override in base class method
• Thinking override is allowed without virtual
• Ignoring method signature correctness

5. Given these classes:

class Vehicle {
  public virtual string Describe() => "Vehicle";
}
class Car : Vehicle {
  public override string Describe() => "Car";
}
class SportsCar : Car {
  public override string Describe() => "SportsCar";
}

Vehicle v = new SportsCar();
Car c = new SportsCar();
SportsCar s = new SportsCar();

Console.WriteLine(v.Describe());
Console.WriteLine(c.Describe());
Console.WriteLine(s.Describe());

What is the output?

hard

A. Vehicle\nCar\nSportsCar

B. Car\nCar\nCar

C. Vehicle\nVehicle\nVehicle

D. SportsCar\nSportsCar\nSportsCar

Solution

1. Step 1: Identify actual object type for all references

   All variables v, c, and s reference a SportsCar object.

2. Step 2: Apply runtime polymorphism for Describe()

   Since Describe is overridden in SportsCar, all calls print "SportsCar" regardless of reference type.

3. Final Answer:

   SportsCar\nSportsCar\nSportsCar -> Option D

4. Quick Check:

   All calls use SportsCar override [OK]

Hint: Actual object type decides method, not reference type [OK]

Common Mistakes:

• Assuming base class method runs for base type variable
• Confusing reference type with object type
• Ignoring override in most derived class