Virtual method dispatch mechanism in C Sharp (C#) - Mini Project: Build & Apply

Virtual method dispatch mechanism

📖 Scenario: Imagine you are creating a simple program to show how different animals make sounds. Each animal can make a sound, but the exact sound depends on the type of animal.

🎯 Goal: You will build a small program with a base class Animal and two derived classes Dog and Cat. You will use virtual methods to show how the program decides which sound to make when calling the method on different animals.

📋 What You'll Learn

Create a base class called Animal with a virtual method MakeSound() that prints "Some sound".

Create two classes Dog and Cat that inherit from Animal and override the MakeSound() method to print "Bark" and "Meow" respectively.

Create a list of Animal objects containing one Dog and one Cat.

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

💡 Why This Matters

🌍 Real World

Virtual method dispatch is used in many programs to allow different objects to behave differently while sharing the same interface. For example, in games, different characters can move or attack differently but use the same method names.

💼 Career

Understanding virtual methods and polymorphism is essential for object-oriented programming jobs. It helps write flexible and reusable code that can handle many types of objects easily.

1

Create the base class Animal with a virtual method

Create a public class called Animal with a public virtual method MakeSound() that prints "Some sound" using Console.WriteLine.

C Sharp (C#)

// Your code here

Hint

Use the virtual keyword to allow the method to be overridden in child classes.

2

Create Dog and Cat classes overriding MakeSound

Create two public classes Dog and Cat that inherit from Animal. Override the MakeSound() method in Dog to print "Bark" and in Cat to print "Meow" using Console.WriteLine.

C Sharp (C#)

using System;

public class Animal
{
    public virtual void MakeSound()
    {
        Console.WriteLine("Some sound");
    }
}

// Your code here

Hint

Use the override keyword to change the behavior of the virtual method in child classes.

3

Create a list of Animal objects with Dog and Cat

Create a List<Animal> called animals and add one Dog object and one Cat object to it. Remember to include using System.Collections.Generic; at the top.

C Sharp (C#)

using System;
using System.Collections.Generic;

public class Animal
{
    public virtual void MakeSound()
    {
        Console.WriteLine("Some sound");
    }
}

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

public class Cat : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Meow");
    }
}

// Your code here

Hint

Use List<Animal> to hold different animal objects because they share the same base class.

4

Use foreach loop to call MakeSound on each animal

Add a foreach loop inside Main() to go through each animal in animals and call animal.MakeSound(). This will print the correct sound for each animal.

C Sharp (C#)

using System;
using System.Collections.Generic;

public class Animal
{
    public virtual void MakeSound()
    {
        Console.WriteLine("Some sound");
    }
}

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

public class Cat : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Meow");
    }
}

class Program
{
    static void Main()
    {
        List animals = new List();
        animals.Add(new Dog());
        animals.Add(new Cat());

        // Use foreach loop to call MakeSound on each animal
        // Your code here
    }
}

Hint

The foreach loop calls the overridden MakeSound() method for each animal, showing how virtual method dispatch works.

Practice

1. What does the virtual method dispatch mechanism in C# primarily allow?
virtual methods let child classes provide their own version of a method. What is the main benefit?

easy

A. It forces the program to call the base class method only.

B. It makes all methods static by default.

C. It disables method overriding in child classes.

D. It allows the program to decide at runtime which method version to call.

Solution

1. Step 1: Understand virtual method purpose

   Virtual methods allow child classes to override a method and provide their own implementation.

2. Step 2: Identify when method is chosen

   The actual method called is decided at runtime, based on the object's real type, not the variable's type.

3. Final Answer:

   It allows the program to decide at runtime which method version to call. -> Option D

4. Quick Check:

   Virtual method dispatch = runtime method choice [OK]

Hint: Virtual means runtime method choice, not compile-time [OK]

Common Mistakes:

• Thinking method is fixed at compile time
• Confusing virtual with static methods
• Assuming base method always runs

2. Which of the following is the correct syntax to declare a virtual method in a C# class?

easy

A. virtual public void Display() { }

B. public void virtual Display() { }

C. public virtual void Display() { }

D. public override void Display() { }

Solution

1. Step 1: Recall virtual method syntax

   The keyword virtual comes after the access modifier and before the return type and method name.

2. Step 2: Check each option

   public virtual void Display() { } matches correct syntax: public virtual void Display() { }. Options B and C have wrong order, D uses override which is for overriding, not declaring virtual.

3. Final Answer:

   public virtual void Display() { } -> Option C

4. Quick Check:

   virtual keyword after access modifier [OK]

Hint: virtual keyword goes right after access modifier [OK]

Common Mistakes:

• Placing virtual after method name
• Using override instead of virtual to declare
• Wrong keyword order

3. Consider the following code:

class Base {
    public virtual string GetName() => "Base";
}
class Derived : Base {
    public override string GetName() => "Derived";
}

Base obj = new Derived();
Console.WriteLine(obj.GetName());

What will be the output?

medium

A. Base

B. Derived

C. Compile-time error

D. Runtime exception

Solution

1. Step 1: Identify method overriding

   The Derived class overrides the virtual method GetName from Base.

2. Step 2: Understand virtual dispatch

   The variable obj is of type Base but holds a Derived object. Virtual dispatch calls the Derived version at runtime.

3. Final Answer:

   Derived -> Option B

4. Quick Check:

   Virtual method calls child's override [OK]

Hint: Virtual calls run child's method if overridden [OK]

Common Mistakes:

• Assuming base method runs due to variable type
• Confusing compile-time and runtime binding
• Expecting errors from override

4. What is wrong with this code snippet?

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

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

medium

A. Dog's Speak method should be marked override to override base virtual method.

B. Animal's Speak method should not be virtual.

C. Dog's Speak method should be static.

D. No error; code runs and prints "Dog barks".

Solution

1. Step 1: Check method overriding rules

   To override a virtual method, the child method must use override keyword.

2. Step 2: Analyze given code

   Dog's Speak method lacks override, so it hides base method instead of overriding. Virtual dispatch calls base method.

3. Final Answer:

   Dog's Speak method should be marked override to override base virtual method. -> Option A

4. Quick Check:

   Override keyword needed to override virtual method [OK]

Hint: Override keyword required to override virtual method [OK]

Common Mistakes:

• Forgetting override keyword in child method
• Assuming method hides base automatically
• Confusing virtual and override keywords

5. You have a base class Shape with a virtual method Draw(). Two derived classes Circle and Square override Draw(). You want to write a method that takes a list of Shape objects and calls Draw() on each, ensuring the correct derived method runs.

Which approach correctly uses virtual method dispatch to achieve this?

hard

A. Declare Draw() as virtual in Shape, override in derived classes, then call Draw() on each Shape reference in the list.

B. Declare Draw() as static in Shape and call it directly on the class.

C. Do not use virtual; instead, use type checking and cast each object to call the correct method.

D. Override Draw() in derived classes but call Shape.Draw() explicitly for all objects.

Solution

1. Step 1: Understand virtual method usage

   Declaring Draw() as virtual in base allows derived classes to override it.

2. Step 2: Use polymorphism in list iteration

   Calling Draw() on each Shape reference triggers virtual dispatch, running the correct derived method.

3. Final Answer:

   Declare Draw() as virtual in Shape, override in derived, call Draw() on each Shape reference. -> Option A

4. Quick Check:

   Virtual + override + call on base type = correct method run [OK]

Hint: Use virtual + override, call on base type for correct method [OK]

Common Mistakes:

• Using static methods which don't support polymorphism
• Manually casting instead of relying on virtual dispatch
• Calling base method explicitly ignoring overrides