Sealed classes and methods in C Sharp (C#)

Introduction

Sealed classes and methods stop other classes from changing or extending them. This helps keep your code safe and clear.

When you want to stop a class from being inherited by others.

When you want to prevent a method from being changed in child classes.

When you want to make sure your code works exactly as you wrote it.

When you want to improve performance by avoiding extra method lookups.

When you want to protect important parts of your code from accidental changes.

Syntax

C Sharp (C#)

sealed class ClassName
{
    // class members
}

// class DerivedClass : ClassName // This will cause an error
// {
// }

class BaseClass
{
    public virtual void MethodName() { }
}

class DerivedClass : BaseClass
{
    public sealed override void MethodName() { }
}

class MoreDerivedClass : DerivedClass
{
    // Cannot override MethodName here
}

A sealed class cannot be inherited by any other class.

A sealed method cannot be overridden by any further derived classes.

Examples

This shows a sealed class Animal. No class can inherit from it.

C Sharp (C#)

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

// Trying to inherit Animal will cause an error
// class Dog : Animal { }

This shows a sealed method Start in class Car. It cannot be changed in SportsCar.

C Sharp (C#)

class Vehicle
{
    public virtual void Start() {
        Console.WriteLine("Vehicle starts");
    }
}

class Car : Vehicle
{
    public sealed override void Start() {
        Console.WriteLine("Car starts");
    }
}

class SportsCar : Car
{
    // Cannot override Start here because it is sealed in Car
}

Sample Program

This program shows a sealed class Fruit which cannot be inherited. It also shows a sealed method Grow in class Tree that cannot be overridden by Oak.

C Sharp (C#)

using System;

sealed class Fruit
{
    public void ShowType() {
        Console.WriteLine("This is a fruit.");
    }
}

// Uncommenting the following line will cause a compile error
// class Apple : Fruit { }

class Plant
{
    public virtual void Grow() {
        Console.WriteLine("Plant grows");
    }
}

class Tree : Plant
{
    public sealed override void Grow() {
        Console.WriteLine("Tree grows tall");
    }
}

class Oak : Tree
{
    // Cannot override Grow() here because it is sealed in Tree
}

class Program
{
    static void Main() {
        Fruit fruit = new Fruit();
        fruit.ShowType();

        Plant plant = new Plant();
        plant.Grow();

        Tree tree = new Tree();
        tree.Grow();

        Oak oak = new Oak();
        oak.Grow();
    }
}

Important Notes

Sealed classes and methods help keep your code safe from unwanted changes.

Trying to inherit a sealed class or override a sealed method will cause a compile error.

Sealing methods can also improve performance slightly by avoiding virtual method calls.

Summary

Sealed classes cannot be inherited.

Sealed methods cannot be overridden further.

Use sealing to protect your code and improve clarity.

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