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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an abstract class in C#.

C Sharp (C#)

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

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

Astatic

Bconcrete

Csealed

Dabstract

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'concrete' instead of 'abstract' to declare an abstract class.

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

2fill in blank

medium

Complete the code to create a concrete class that inherits from an abstract class.

C Sharp (C#)

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

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

ASpeak

BRun

CJump

DBark

Attempts:

3 left

💡 Hint

Common Mistakes

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

The concrete class Dog must override the abstract method Speak from the base class.

3fill in blank

hard

Fix the error in the code by choosing the correct keyword for the class.

C Sharp (C#)

public [1] class Vehicle {
    public void Start() {
        Console.WriteLine("Starting vehicle");
    }
}

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

Asealed

Bconcrete

Cabstract

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'abstract' when the class has no abstract members.

The class is intended to be a concrete class that cannot be inherited, so sealed is the correct keyword.

4fill in blank

hard

Fill both blanks to define an abstract method and a concrete method in an abstract class.

C Sharp (C#)

public abstract class Shape {
    public abstract double [1]();
    public double [2]() {
        return 0;
    }
}

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

AArea

BPerimeter

CDraw

DCalculate

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to make a concrete method abstract or vice versa.

The abstract method Area must be implemented by subclasses, while Perimeter is a concrete method with a default implementation.

5fill in blank

hard

Fill all three blanks to create a concrete class that implements abstract methods and adds a new method.

C Sharp (C#)

public class Circle : Shape {
    public override double [1]() {
        return 3.14 * radius * radius;
    }
    public override double [2]() {
        return 2 * 3.14 * radius;
    }
    public double [3]() {
        return radius;
    }
    private double radius;
    public Circle(double r) {
        radius = r;
    }
}

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

AArea

BPerimeter

CGetRadius

DCalculate

Attempts:

3 left

💡 Hint

Common Mistakes

Not overriding all abstract methods or using wrong method names.

The class Circle overrides the abstract methods Area and Perimeter from Shape and adds a new method GetRadius.

Practice

1. Which statement best describes when to use an abstract class in C#?

easy

A. When you want to define a common plan without providing full implementation.

B. When you want to create a fully working class that can be instantiated.

C. When you want to prevent inheritance altogether.

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

Solution

1. Step 1: Understand abstract class purpose

   An abstract class defines methods or properties that must be implemented by subclasses but does not provide full implementation itself.

2. Step 2: Compare with concrete class

   Concrete classes provide full working code and can be instantiated, unlike abstract classes.

3. Final Answer:

   When you want to define a common plan without providing full implementation. -> Option A

4. Quick Check:

   Abstract class = common plan without full code [OK]

Hint: Abstract = plan only, Concrete = full working code [OK]

Common Mistakes:

• Confusing abstract with concrete classes
• Thinking abstract classes can be instantiated
• Believing abstract classes provide full method bodies

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

easy

A. class abstract Vehicle { }

B. public abstract class Vehicle { }

C. abstract public Vehicle { }

D. public class abstract Vehicle { }

Solution

1. Step 1: Recall C# syntax for abstract classes

   The correct syntax places the keyword abstract before class and then the class name.

2. Step 2: Check each option

   public abstract class Vehicle { } uses public abstract class Vehicle { }, which is correct. Other options have incorrect keyword order or missing keywords.

3. Final Answer:

   public abstract class Vehicle { } -> Option B

4. Quick Check:

   abstract class syntax = 'public abstract class' [OK]

Hint: Use 'abstract' before 'class' keyword [OK]

Common Mistakes:

• Placing 'abstract' after 'class'
• Omitting 'class' keyword
• Incorrect keyword order

3. What will be the output of this C# code?

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();
    }
}

medium

A. Runtime error because Speak is abstract

B. Compile-time error because Animal is abstract

C. Woof

D. No output

Solution

1. Step 1: Understand abstract method implementation

   The abstract method Speak in Animal is overridden in Dog with a concrete implementation that prints "Woof".

2. Step 2: Analyze runtime behavior

   Creating an Animal reference to a Dog object and calling Speak() calls the overridden method, printing "Woof".

3. Final Answer:

   Woof -> Option C

4. Quick Check:

   Abstract method overridden = prints 'Woof' [OK]

Hint: Abstract method must be overridden to run [OK]

Common Mistakes:

• Thinking abstract class cannot be referenced
• Expecting compile or runtime error
• Assuming abstract methods have bodies

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 must declare Area() as override

B. Shape cannot have abstract methods

C. Circle cannot inherit from Shape

D. Area method should return int, not double

Solution

1. Step 1: Check abstract method override rules

   When a class inherits an abstract method, it must override it using the override keyword.

2. Step 2: Identify missing override keyword

   The Circle class defines Area() but misses override, causing a compile error.

3. Final Answer:

   Circle must declare Area() as override -> Option A

4. Quick Check:

   Override keyword required for abstract methods [OK]

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

Common Mistakes:

• Omitting override keyword
• Thinking abstract methods can be ignored
• Confusing return types

5. You want to design a system where all vehicles must have a method StartEngine(), but the way engines start differs by vehicle type. Which approach is best in C#?

hard

A. Create a static class Vehicle with static StartEngine() method.

B. Create a concrete class Vehicle with a fully implemented StartEngine() method for all vehicles.

C. Use an interface with a concrete StartEngine() method and inherit it in all vehicle classes.

D. Create an abstract class Vehicle with abstract method StartEngine(), then implement it in subclasses.

Solution

1. Step 1: Analyze requirement for different implementations

   Since StartEngine() differs by vehicle type, it should be declared abstract to force subclasses to provide their own version.

2. Step 2: Choose correct class design

   An abstract class Vehicle with an abstract StartEngine() method fits best, allowing subclasses to implement specific behavior.

3. Final Answer:

   Create an abstract class Vehicle with abstract method StartEngine(), then implement it in subclasses. -> Option D

4. Quick Check:

   Abstract class for common plan, concrete for specifics [OK]

Hint: Abstract method for varying behavior, concrete class for details [OK]

Common Mistakes:

• Using concrete class with one method for all vehicles
• Trying to put method body in interface (not allowed)
• Using static class which can't be inherited