Sealed Class
In C#, when we don't want a class to be inherited by another class, we can declare the class as a sealed class.
A sealed class cannot have a derived class. We use the sealed keyword to create a sealed class. For example,
using System;
namespace SealedClass {
sealed class Animal {
}
// trying to inherit sealed class
// Error Code
class Dog : Animal {
}
class Program {
static void Main (string [] args) {
// create an object of Dog class
Dog d1 = new Dog();
Console.ReadLine();
}
}
}
In the above example, we have created a sealed class Animal. Here, we are trying to derive Dog class from the Animal class.
Since a sealed class cannot be inherited, the program generates the following error:
error CS0509: 'Dog': cannot derive from sealed type 'Animal'
Sealed Method
During method overriding, if we don't want an overridden method to be further overridden by another class, we can declare it as a sealed method.
We use a sealed keyword with an overridden method to create a sealed method. For example,
using System;
namespace SealedClass {
class Animal {
public virtual void makeSound() {
Console.WriteLine("Animal Sound");
}
}
class Dog : Animal {
// sealed method
sealed public override void makeSound() {
Console.WriteLine("Dog Sound");
}
}
class Puppy : Dog {
// trying to override sealed method
public override void makeSound() {
Console.WriteLine("Puppy Sound");
}
}
class Program {
static void Main (string [] args) {
// create an object of Puppy class
Puppy d1 = new Puppy();
Console.ReadLine();
}
}
}
In the above example, we have overridden the makeSound() method inside the Dog class.
// Inside the Dog class
sealed public override void makeSound() {
Console.WriteLine("Dog Sound");
}
Notice that we have used the sealed keyword with makeSound(). This means the Puppy class that inherits the Dog class is not allowed to override makeSound().
Hence, we get an error
error CS0239: 'Puppy.makeSound()': cannot override inherited member 'Dog.makeSound()' because it is sealed
when we try to further override the makeSound() method inside the Puppy class.
Note: Sealing an overridden method prevents method overriding in multilevel inheritance.
Why Sealed Class?
1. We use sealed classes to prevent inheritance. As we cannot inherit from a sealed class, the methods in the sealed class cannot be manipulated from other classes.
It helps to prevent security issues. For example,
sealed class A {
...
}
// error code
class B : A {
...
}
As class A cannot be inherited, class B cannot override and manipulate the methods of class A.
2. One of the best uses of sealed classes is when you have a class with static members.
The Pens class of the System.Drawing namespace is one of the examples of the sealed class. The Pens class has static members that represent the pens with standard colors. Pens.Blue represents a pen with blue color.