Abstract classes and methods in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract class method call

What is the output of this C# program?

C Sharp (C#)

using System;
abstract class Animal {
    public abstract void Speak();
}
class Dog : Animal {
    public override void Speak() {
        Console.WriteLine("Woof");
    }
}
class Program {
    static void Main() {
        Animal a = new Dog();
        a.Speak();
    }
}

A Woof

B Animal speaks

C Compilation error: Cannot create instance of abstract class

D Runtime error: Method not implemented

💡 Hint

Remember that abstract classes cannot be instantiated directly, but you can create instances of derived classes.

The variable 'a' is of type Animal but holds a Dog instance. The Dog class overrides the abstract Speak method, so calling a.Speak() prints 'Woof'.

🧠 Conceptual

intermediate

Abstract method characteristics

Which statement about abstract methods in C# is correct?

A Abstract methods can have a body with implementation.

B Abstract methods can be static.

C Abstract methods must be declared in abstract classes.

D Abstract methods can be private.

💡 Hint

Think about where abstract methods are allowed and their purpose.

Abstract methods cannot have a body and must be declared inside abstract classes. They cannot be static or private because they require overriding in derived classes.

🔧 Debug

advanced

Identify the compilation error

What is the cause of the compilation error in this code?

C Sharp (C#)

using System;
abstract class Shape {
    public abstract double Area();
}
class Circle : Shape {
    public double Radius;
}
class Program {
    static void Main() {
        Circle c = new Circle();
        c.Radius = 5;
        Console.WriteLine(c.Area());
    }
}

A Area method must be static

B Circle does not implement the abstract method Area()

C Radius field must be private

D Cannot instantiate abstract class Shape

💡 Hint

Check if all abstract methods are implemented in derived classes.

The Circle class inherits from Shape but does not provide an implementation for the abstract method Area(), causing a compilation error.

📝 Syntax

advanced

Correct syntax for abstract method override

Which option shows the correct way to override an abstract method in C#?

C Sharp (C#)

abstract class Vehicle {
    public abstract void Start();
}
class Car : Vehicle {
    // Choose the correct override syntax here
}

A public abstract override void Start() { Console.WriteLine("Car started"); }

B public void Start() { Console.WriteLine("Car started"); }

C override public void Start() { Console.WriteLine("Car started"); }

D public override void Start() { Console.WriteLine("Car started"); }

💡 Hint

Remember the order and keywords required for overriding methods.

The correct syntax uses 'public override' followed by the method signature and body. Option D misses 'override', C has wrong keyword order, and A wrongly declares the method abstract with a body.

🚀 Application

expert

Number of methods to implement

Given the following code, how many methods must the class 'SmartPhone' implement to compile successfully?

C Sharp (C#)

abstract class Device {
    public abstract void PowerOn();
    public abstract void PowerOff();
}
abstract class Phone : Device {
    public abstract void Call(string number);
}
class SmartPhone : Phone {
    // Implementations here
}

A 3

B 2

C 1

D 4

💡 Hint

Count all abstract methods inherited from base classes that must be implemented.

SmartPhone inherits from Phone which inherits from Device. Device has 2 abstract methods (PowerOn, PowerOff), Phone adds 1 (Call). SmartPhone must implement all 3.