When to use abstract vs concrete in C Sharp (C#) - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of abstract and concrete class usage

What is the output of this C# code when run?

C Sharp (C#)

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());
    }
}

A Woof!

B Animal

C Compile-time error: Cannot create instance of abstract class

D Runtime error: Method not implemented

💡 Hint

Look at how the abstract method Speak is implemented in the Dog class.

The abstract class Animal defines an abstract method Speak. The Dog class overrides Speak and returns "Woof!". The program creates a Dog instance and calls Speak, printing "Woof!".

🧠 Conceptual

intermediate

When to use abstract classes vs concrete classes

Which situation best describes when you should use an abstract class instead of a concrete class?

A When you want to provide a base class with some methods that must be implemented by subclasses

B When you want to create objects directly without any subclassing

C When you want to prevent any class from inheriting your class

D When you want to create a class with only static methods

💡 Hint

Think about the purpose of abstract classes in design.

Abstract classes are used to define a common base with some methods that subclasses must implement. Concrete classes can be instantiated directly.

🔧 Debug

advanced

Identify the error in abstract class usage

What error will this code produce when compiled?

C Sharp (C#)

abstract class Vehicle {
    public abstract void Drive();
}

class Car : Vehicle {
    // Missing override of Drive method
}

class Program {
    static void Main() {
        Vehicle myCar = new Car();
        myCar.Drive();
    }
}

A No error, outputs nothing

B Runtime error: Drive method not found

C Compile-time error: Car does not implement abstract method Drive

D Compile-time error: Cannot instantiate abstract class Vehicle

💡 Hint

Check if all abstract methods are implemented in subclasses.

The Car class does not implement the abstract method Drive, so the compiler will raise an error.

📝 Syntax

advanced

Correct syntax for abstract method in C#

Which option shows the correct syntax to declare an abstract method in an abstract class?

A public void abstract Run();

B public abstract void Run() {}

C abstract public void Run() {}

D public abstract void Run();

💡 Hint

Abstract methods do not have a body and use the 'abstract' keyword before the return type.

The correct syntax is 'public abstract void Run();' without a method body.

🚀 Application

expert

Choosing abstract vs concrete class design

You are designing a system for different types of payment methods. You want to ensure every payment method implements a 'ProcessPayment' method but also share some common code for logging. Which design is best?

A Create a concrete class 'PaymentMethod' with all methods implemented and no inheritance

B Create an abstract class 'PaymentMethod' with an abstract 'ProcessPayment' method and concrete logging methods

C Create a concrete class 'PaymentMethod' with an empty 'ProcessPayment' method to override

D Create an interface 'PaymentMethod' with 'ProcessPayment' and implement logging in each class

💡 Hint

Think about sharing code and forcing implementation of certain methods.

An abstract class allows sharing common code (like logging) and forces subclasses to implement 'ProcessPayment'. Interfaces cannot have shared code.