The sealed class

One of the principles of OOP is inheritance, but sometimes you may need to restrict inheritance in your code for the sake of your application's architecture. C# provides a keyword called sealed. If this keyword is placed before a class's signature, the class is considered a sealed class. If a class is sealed, that particular class can't be inherited by other classes. If any class tries to inherit a sealed class, the compiler will throw an error. Structs can also be sealed, and in that case, no class can inherit that struct.

Let's look at an example of a sealed class:

sealed class Animal {
    public string name;
    public int ageInMonths;
    public void Move(){
        Console.WriteLine("Moving");
    }
    public void Eat(){
        Console.WriteLine("Eating");
    }
}
public static void Main(){
    Animal dog = new Animal();
    dog.name = "Doggy";
    dog.ageInMonths = 1;
    
    dog.Move();
    dog.Eat();
}

In the preceding example, we can see how we can create a sealed class. Just using the sealed keyword before the class keyword makes the class a sealed class. In the preceding example, we created an Animal sealed class, and in the main method, we instantiated the class and used it. This is now working fine. However, if we try to create a Dog class that will inherit the Animal class, as in the following code, then the compiler will throw an error, saying that the sealed Animal class can't be inherited:

class Dog : Animal {
    public char gender;
}

Here is a screenshot of what the compiler will show: