Base class and derived class in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a base class named Animal.

C Sharp (C#)

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

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

ADog

BAnimal

CCat

DBird

Attempts:

3 left

💡 Hint

Common Mistakes

Using a derived class name like Dog instead of the base class Animal.

The base class should be named Animal as it represents the general class.

2fill in blank

medium

Complete the code to declare a derived class Dog that inherits from Animal.

C Sharp (C#)

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

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

AAnimal

BFish

CBird

DCat

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class name other than Animal after the colon.

The derived class Dog inherits from the base class Animal.

3fill in blank

hard

Fix the error in the code to call the Speak method from the Dog class instance.

C Sharp (C#)

Dog myDog = new Dog();
myDog.[1]();

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

AFly

BBark

CRun

DSpeak

Attempts:

3 left

💡 Hint

Common Mistakes

Calling Bark instead of Speak when the goal is to call the base class method.

The Speak method is defined in the base class Animal and can be called on the Dog instance.

4fill in blank

hard

Fill both blanks to override the Speak method in the Dog class.

C Sharp (C#)

public class Dog : Animal 
{
    public override void [1]() 
    {
        Console.WriteLine([2]);
    }
}

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

ASpeak

B"Woof!"

C"Meow!"

DBark

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name or wrong string inside Console.WriteLine.

To override the base class method, use Speak and print "Woof!" for the Dog class.

5fill in blank

hard

Fill all three blanks to create a derived class Cat that overrides Speak and adds a new method Purr.

C Sharp (C#)

public class Cat : [1] 
{
    public override void [2]() 
    {
        Console.WriteLine("Meow!");
    }

    public void [3]() 
    {
        Console.WriteLine("Purr...");
    }
}

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

AAnimal

BSpeak

CPurr

DDog

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong base class or method names.

The class Cat inherits from Animal, overrides Speak, and adds a new method Purr.

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