Why polymorphism matters in C Sharp (C#) - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a base class named Animal.

C Sharp (C#)

public class [1] {
    public void Speak() {
        Console.WriteLine("Animal sound");
    }
}

Drag options to blanks, or click blank then click option'

AAnimal

BDog

CCat

DBird

Attempts:

3 left

💡 Hint

Common Mistakes

Using a specific animal name like Dog or Cat for the base class.

The base class should be named Animal to represent a general animal.

2fill in blank

medium

Complete the code to override the Speak method in the Dog class.

C Sharp (C#)

public class Dog : Animal {
    public override void [1]() {
        Console.WriteLine("Bark");
    }
}

Drag options to blanks, or click blank then click option'

ASpeak

BRun

CEat

DSleep

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method name that does not exist in the base class.

The method Speak is overridden to provide a specific sound for Dog.

3fill in blank

hard

Fix the error in the code by completing the method declaration to allow polymorphism.

C Sharp (C#)

public class Animal {
    public [1] void Speak() {
        Console.WriteLine("Animal sound");
    }
}

Drag options to blanks, or click blank then click option'

Astatic

Babstract

Cvirtual

Dsealed

Attempts:

3 left

💡 Hint

Common Mistakes

Using static which prevents overriding.

Using sealed which prevents inheritance.

Using abstract without making the class abstract.

The virtual keyword allows derived classes to override the method, enabling polymorphism.

4fill in blank

hard

Fill both blanks to create a list of animals and call their Speak method polymorphically.

C Sharp (C#)

List<[1]> animals = new List<[2]> {
    new Dog(),
    new Cat()
};

foreach(var animal in animals) {
    animal.Speak();
}

Drag options to blanks, or click blank then click option'

AAnimal

BDog

CCat

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using a specific animal type like Dog or Cat for the list type.

The list should be of type Animal to hold different animal objects and call their overridden methods.

5fill in blank

hard

Fill all three blanks to implement polymorphism with an interface and classes.

C Sharp (C#)

public interface I[1] {
    void Speak();
}

public class [2] : I[1] {
    public void Speak() {
        Console.WriteLine("Meow");
    }
}

I[1] animal = new [2]();
animal.Speak();

Drag options to blanks, or click blank then click option'

AAnimal

BCat

CDog

DPet

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching interface and class names.

Not using the interface type for the variable.

The interface is named IAnimal, the class Cat implements it, and the variable uses the interface type to call Speak polymorphically.

Practice

1. What is the main benefit of polymorphism in C# programming?

easy

A. It forces all classes to have the same properties.

B. It makes the program run faster by using less memory.

C. It prevents any changes to the code once compiled.

D. It allows one method to work with different types of objects.

Solution

1. Step 1: Understand polymorphism concept

   Polymorphism means one method name can work with different object types.

2. Step 2: Identify the main benefit

   This makes code easier to write and maintain by reusing method names for different classes.

3. Final Answer:

   It allows one method to work with different types of objects. -> Option D

4. Quick Check:

   Polymorphism = One method, many types [OK]

Hint: Polymorphism means one method, many object types [OK]

Common Mistakes:

• Thinking polymorphism speeds up code automatically
• Confusing polymorphism with code immutability
• Believing polymorphism forces identical class properties

2. Which of the following is the correct way to declare a method that demonstrates polymorphism in C#?

easy

A. public void Draw() { }

B. public void Draw(int x) { }

C. public virtual void Draw() { }

D. public static void Draw() { }

Solution

1. Step 1: Identify polymorphic method declaration

   In C#, polymorphism is often shown using virtual methods that can be overridden.

2. Step 2: Check method options

   Only 'public virtual void Draw()' allows derived classes to override and show polymorphism.

3. Final Answer:

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

4. Quick Check:

   Virtual method enables polymorphism [OK]

Hint: Use 'virtual' keyword to enable polymorphism [OK]

Common Mistakes:

• Choosing static methods which cannot be overridden
• Ignoring the virtual keyword for polymorphism
• Confusing method overloading with polymorphism

3. Consider the following C# code:

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

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

What is the output?

medium

A. Woof

B. ...

C. Meow

D. Compilation error

Solution

1. Step 1: Understand polymorphism with virtual and override

   The variable 'a' is of type Animal but holds a Dog object. The Speak method is virtual and overridden in Dog.

2. Step 2: Determine which Speak method runs

   At runtime, the Dog's Speak method runs, returning "Woof".

3. Final Answer:

   Woof -> Option A

4. Quick Check:

   Virtual method calls overridden version [OK]

Hint: Virtual method calls override in actual object type [OK]

Common Mistakes:

• Expecting base class method output
• Confusing variable type with object type
• Thinking code causes compile error

4. What is wrong with this C# code snippet that tries to use polymorphism?

class Shape { public void Draw() { Console.WriteLine("Shape"); } }
class Circle : Shape { public void Draw() { Console.WriteLine("Circle"); } }

Shape s = new Circle();
s.Draw();

medium

A. Shape cannot be assigned a Circle object.

B. Draw method in Shape should be virtual to enable polymorphism.

C. Circle class must not have a Draw method.

D. Draw method must be static.

Solution

1. Step 1: Check method declarations for polymorphism

   Polymorphism requires the base method to be marked 'virtual' and the derived method to 'override'.

2. Step 2: Identify missing virtual keyword

   Here, Shape's Draw is not virtual, so Circle's Draw hides it but does not override.

3. Final Answer:

   Draw method in Shape should be virtual to enable polymorphism. -> Option B

4. Quick Check:

   Base method must be virtual for polymorphism [OK]

Hint: Base method needs 'virtual' for polymorphism [OK]

Common Mistakes:

• Thinking method hiding equals polymorphism
• Believing derived method must be removed
• Assuming static methods support polymorphism

5. You have a list of different shapes (Circle, Square, Triangle) all inheriting from Shape with a virtual method Draw(). How does polymorphism help you when you want to draw all shapes without checking their types?

hard

A. You can call Draw() on each Shape reference and the correct shape's Draw runs automatically.

B. You must check each shape's type and call its specific Draw method manually.

C. You need to cast each shape to its exact type before calling Draw().

D. You cannot use polymorphism with collections of different shapes.

Solution

1. Step 1: Understand polymorphism with collections

   Polymorphism allows calling the same method on base class references that point to different derived objects.

2. Step 2: Apply to drawing shapes

   Calling Draw() on each Shape in the list runs the correct overridden Draw method for each shape automatically.

3. Final Answer:

   You can call Draw() on each Shape reference and the correct shape's Draw runs automatically. -> Option A

4. Quick Check:

   Polymorphism enables method calls without type checks [OK]

Hint: Call base method; derived version runs automatically [OK]

Common Mistakes:

• Thinking you must check types before calling methods
• Trying to cast objects unnecessarily
• Believing polymorphism doesn't work with lists