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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a sealed class named 'Vehicle'.

C Sharp (C#)

public [1] class Vehicle
{
    public void Start() { }
}

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

Asealed

Babstract

Cstatic

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

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

Using 'static' which makes the class static, not sealed.

The keyword sealed prevents other classes from inheriting from this class.

2fill in blank

medium

Complete the code to prevent overriding of the 'Drive' method in derived classes.

C Sharp (C#)

public class Car
{
    public virtual void Drive() { }
}

public class SportsCar : Car
{
    public [1] override void Drive() { }
}

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

Aabstract

Bsealed

Cstatic

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'abstract' which requires the method to be overridden.

Using 'virtual' which allows overriding.

The sealed keyword on an override method stops further overriding in subclasses.

3fill in blank

hard

Fix the error in the code by completing the class declaration to prevent inheritance.

C Sharp (C#)

public class BaseClass { }

public [1] class DerivedClass : BaseClass { }

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

Astatic

Babstract

Csealed

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'abstract' which allows inheritance.

Using 'virtual' which is invalid on classes.

Marking 'DerivedClass' as sealed prevents other classes from inheriting from it.

4fill in blank

hard

Fill both blanks to declare a sealed class and a sealed override method.

C Sharp (C#)

public class Animal
{
    public virtual void Speak() { }
}

public [1] class Dog : Animal
{
    public [2] override void Speak() { }
}

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

Asealed

Babstract

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'abstract' instead of 'sealed' for the class.

Using 'virtual' instead of 'sealed' for the method.

The class 'Dog' is sealed to prevent inheritance, and the 'Speak' method in 'Dog' is sealed to prevent further overrides.

5fill in blank

hard

Fill all three blanks to create a sealed class, a sealed override method, and call the base method.

C Sharp (C#)

public class Shape
{
    public virtual void Draw() { Console.WriteLine("Drawing shape"); }
}

public [1] class Circle : Shape
{
    public [2] override void Draw()
    {
        base.[3]();
        Console.WriteLine("Drawing circle");
    }
}

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

Asealed

BDraw

Dvirtual

Attempts:

3 left

💡 Hint

Common Mistakes

Not sealing the class or method properly.

Calling a wrong base method name.

The class 'Circle' is sealed to prevent inheritance. The 'Draw' method in 'Circle' is sealed to prevent further overrides. The base method 'Draw' is called inside the override.

Practice

1. What does the sealed keyword do when applied to a class in C#?

easy

A. Prevents the class from being inherited by other classes.

B. Allows the class to be inherited multiple times.

C. Makes the class abstract and uninstantiable.

D. Enables the class to override methods from its base class.

Solution

1. Step 1: Understand the sealed keyword on classes

   The sealed keyword on a class means no other class can inherit from it.

2. Step 2: Compare options with this meaning

   Only Prevents the class from being inherited by other classes. correctly states that the class cannot be inherited.

3. Final Answer:

   Prevents the class from being inherited by other classes. -> Option A

4. Quick Check:

   Sealed class = no inheritance [OK]

Hint: Sealed class means no child classes allowed [OK]

Common Mistakes:

• Thinking sealed classes can be inherited
• Confusing sealed with abstract
• Assuming sealed allows overriding

2. Which of the following is the correct syntax to declare a sealed method in C#?

easy

A. sealed public void MyMethod() { }

B. public override sealed void MyMethod() { }

C. public sealed void MyMethod() { }

D. override sealed public void MyMethod() { }

Solution

1. Step 1: Recall sealed method syntax

   A method can only be sealed if it overrides a base method, so it must have override sealed modifiers.

2. Step 2: Check options for correct order and modifiers

   public override sealed void MyMethod() { } correctly uses public override sealed void MyMethod(). Other options miss override or have wrong order.

3. Final Answer:

   public override sealed void MyMethod() { } -> Option B

4. Quick Check:

   Sealed method = override + sealed [OK]

Hint: Sealed methods must override and use 'override sealed' [OK]

Common Mistakes:

• Declaring sealed method without override
• Wrong order of modifiers
• Missing override keyword

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

class Base {
    public virtual void Show() { Console.WriteLine("Base"); }
}
class Derived : Base {
    public sealed override void Show() { Console.WriteLine("Derived"); }
}
class MoreDerived : Derived {
    public override void Show() { Console.WriteLine("MoreDerived"); }
}

var obj = new MoreDerived();
obj.Show();

medium

A. Base

B. Derived

C. MoreDerived

D. Compilation error

Solution

1. Step 1: Understand sealed override effect

   The method Show in Derived is sealed, so it cannot be overridden in MoreDerived.

2. Step 2: Check the code for override in MoreDerived

   MoreDerived tries to override Show, which causes a compilation error.

3. Final Answer:

   Compilation error -> Option D

4. Quick Check:

   Sealed method cannot be overridden [OK]

Hint: Sealed override blocks further overrides causing errors [OK]

Common Mistakes:

• Assuming MoreDerived.Show runs
• Ignoring sealed keyword effect
• Thinking output is Derived

4. Identify the error in this code snippet:

sealed class Animal {
    public void Speak() { Console.WriteLine("Animal speaks"); }
}
class Dog : Animal {
    public void Speak() { Console.WriteLine("Dog barks"); }
}

medium

A. Method Speak must be virtual in Animal.

B. Cannot declare method Speak in Dog class.

C. Cannot inherit from sealed class Animal.

D. No error, code runs fine.

Solution

1. Step 1: Check sealed class inheritance rules

   A sealed class cannot be inherited by any other class.

2. Step 2: Analyze Dog class inheritance

   Dog tries to inherit from sealed Animal, which causes a compilation error.

3. Final Answer:

   Cannot inherit from sealed class Animal. -> Option C

4. Quick Check:

   Sealed class blocks inheritance [OK]

Hint: Sealed class cannot have child classes [OK]

Common Mistakes:

• Thinking method override causes error
• Ignoring sealed class inheritance rule
• Assuming code compiles fine

5. You have a base class Vehicle with a virtual method Start(). You want to create a class Car that overrides Start() but prevents any further subclass from overriding it. How should you declare Start() in Car?

hard

A. public override sealed void Start() { }

B. public sealed void Start() { }

C. public override void Start() sealed { }

D. sealed public override void Start() { }

Solution

1. Step 1: Understand method sealing rules

   To prevent further overrides, the method must be both override and sealed.

2. Step 2: Check correct syntax for sealed override

   The correct syntax is public override sealed void Start(). Other options have wrong order or missing keywords.

3. Final Answer:

   public override sealed void Start() { } -> Option A

4. Quick Check:

   Sealed override method syntax = override sealed [OK]

Hint: Use 'override sealed' to block further overrides [OK]

Common Mistakes:

• Omitting override keyword
• Wrong order of sealed and override
• Trying to seal without overriding