Abstract classes and methods in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an abstract class named Animal.

C Sharp (C#)

public [1] class Animal {
    public abstract void Speak();
}

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

Asealed

Babstract

Cstatic

Dpartial

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sealed' instead of 'abstract' which prevents inheritance.

Using 'static' which makes the class non-instantiable and non-inheritable.

The keyword abstract is used to declare an abstract class in C#.

2fill in blank

medium

Complete the code to declare an abstract method Speak inside the abstract class Animal.

C Sharp (C#)

public abstract class Animal {
    public [1] void Speak();
}

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

Aoverride

Bvirtual

Cabstract

Dsealed

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'virtual' which requires a method body.

Using 'override' which is for overriding base class methods.

The abstract keyword declares a method without implementation inside an abstract class.

3fill in blank

hard

Fix the error in the derived class Dog that does not implement the abstract method Speak.

C Sharp (C#)

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

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

Aoverride

Babstract

Csealed

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'abstract' in the derived class method which is not allowed.

Using 'virtual' which does not fulfill the abstract method requirement.

The derived class must use override to implement the abstract method from the base class.

4fill in blank

hard

Fill both blanks to declare an abstract class Vehicle and an abstract method Move.

C Sharp (C#)

public [1] class Vehicle {
    public [2] void Move();
}

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

Aabstract

Bvirtual

Dsealed

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'virtual' for the method without a body causes a syntax error.

Using 'sealed' for the class prevents inheritance.

The class must be declared abstract and the method abstract to be incomplete and require implementation in derived classes.

5fill in blank

hard

Fill all three blanks to implement an abstract class Shape with an abstract method Area and a derived class Circle overriding Area.

C Sharp (C#)

public [1] class Shape {
    public [2] double Area();
}

public class Circle : Shape {
    private double radius;
    public Circle(double r) { radius = r; }
    public [3] double Area() {
        return Math.PI * radius * radius;
    }
}

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

Aabstract

Coverride

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Not using 'override' in the derived class method causes a compile error.

Using 'virtual' instead of 'abstract' for the base method without a body.

The class Shape and its method Area are declared abstract. The Circle class overrides the Area method to provide implementation.

Practice

1. What is true about an abstract class in C#?

easy

A. It can be instantiated like any other class.

B. It must have only abstract methods.

C. It cannot be instantiated directly.

D. It cannot have any methods.

Solution

1. Step 1: Understand abstract class instantiation rules

   An abstract class is designed as a base template and cannot be created as an object directly.

2. Step 2: Check other options for correctness

   Abstract classes can have both abstract and non-abstract methods, so options A, B, and D are incorrect.

3. Final Answer:

   It cannot be instantiated directly. -> Option C

4. Quick Check:

   Abstract class = no direct instantiation [OK]

Hint: Remember: abstract classes are blueprints, not objects. [OK]

Common Mistakes:

• Thinking abstract classes can be instantiated.
• Believing abstract classes must have only abstract methods.
• Confusing abstract classes with interfaces.

2. Which of the following is the correct way to declare an abstract method in C#?

easy

A. public abstract void Display() {}

B. public abstract void Display();

C. abstract public void Display() {}

D. public void abstract Display() {}

Solution

1. Step 1: Recall abstract method syntax

   Abstract methods have no body and end with a semicolon, declared with the 'abstract' keyword before the return type.

2. Step 2: Validate each option

   public abstract void Display(); matches the correct syntax. public void abstract Display() {} and C have wrong keyword order or include a body. public abstract void Display() {} incorrectly includes a method body.

3. Final Answer:

   public abstract void Display(); -> Option B

4. Quick Check:

   Abstract method = declaration only, no body [OK]

Hint: Abstract methods end with semicolon, no braces. [OK]

Common Mistakes:

• Adding method body to abstract methods.
• Wrong keyword order in declaration.
• Using braces {} with abstract methods.

3. What will be the output of the following code?

abstract class Animal {
    public abstract string Speak();
}

class Dog : Animal {
    public override string Speak() {
        return "Woof";
    }
}

class Program {
    static void Main() {
        Animal myDog = new Dog();
        System.Console.WriteLine(myDog.Speak());
    }
}

medium

A. Woof

B. Animal

C. Compile-time error

D. Runtime error

Solution

1. Step 1: Understand class inheritance and method override

   Dog inherits from abstract Animal and implements the abstract Speak method returning "Woof".

2. Step 2: Trace program execution

   Main creates a Dog object as Animal type and calls Speak(), which runs Dog's override returning "Woof".

3. Final Answer:

   Woof -> Option A

4. Quick Check:

   Override abstract method = Dog's Speak() output [OK]

Hint: Abstract method calls run subclass override. [OK]

Common Mistakes:

• Expecting abstract class method output.
• Thinking abstract classes can be instantiated.
• Confusing compile-time and runtime errors.

4. Identify the error in this code snippet:

abstract class Shape {
    public abstract double Area();
}

class Circle : Shape {
    public double Area() {
        return 3.14 * 5 * 5;
    }
}

medium

A. Circle class cannot inherit from Shape.

B. Area() method cannot return double.

C. Shape class cannot have abstract methods.

D. Circle must declare Area() as override.

Solution

1. Step 1: Check method overriding rules

   When a subclass implements an abstract method, it must use the 'override' keyword.

2. Step 2: Identify missing override keyword

   Circle's Area() method lacks 'override', causing a compile error.

3. Final Answer:

   Circle must declare Area() as override. -> Option D

4. Quick Check:

   Override abstract method = must use 'override' keyword [OK]

Hint: Override abstract methods with 'override' keyword. [OK]

Common Mistakes:

• Omitting 'override' keyword in subclass method.
• Thinking abstract methods can be implemented without override.
• Confusing return types.

5. You want to create a base class Vehicle with an abstract method StartEngine(). You also want to ensure every subclass implements StartEngine() differently. Which is the best approach?

hard

A. Make Vehicle an abstract class with an abstract StartEngine() method.

B. Make Vehicle a normal class and provide a default StartEngine() implementation.

C. Make Vehicle an interface with StartEngine() method.

D. Make Vehicle a sealed class with StartEngine() method.

Solution

1. Step 1: Understand requirement for different implementations

   Each subclass must implement StartEngine() differently, so a base method without body is needed.

2. Step 2: Choose correct class type and method declaration

   Abstract class Vehicle with abstract StartEngine() enforces subclasses to implement it uniquely.

3. Final Answer:

   Make Vehicle an abstract class with an abstract StartEngine() method. -> Option A

4. Quick Check:

   Abstract class + abstract method = enforced subclass implementation [OK]

Hint: Use abstract class + abstract method for enforced overrides. [OK]

Common Mistakes:

• Using sealed class which prevents inheritance.
• Using interface when base class behavior is needed.
• Providing default method when unique implementations required.