Sealed Class

What?

Sealed classes are used to restrict the inheritance feature of object oriented programming. Once a class is defined as a sealed class, this class cannot be inherited.

Why?

Sealed classes are used to restrict the inheritance feature of object oriented programming. Once a class is defined as a sealed class, this class cannot be inherited.

Features

1. Sealed class can be instantiated.
2. It can inherit from other classes.
3. It cannot be inherited.

Example

Consider the following example in which class SealedClass inherited from class BaseClass but as we have marked SealedClass sealed using sealed modifier, it cannot be used as a base class by other classes.

Class BaseClass  
{  

}   
sealed class SealedClass : BaseClass  
{  

}

using System;  
class Class1  
{  
    static void Main(string[] args)  
    {  
        SealedClass sealedCls = new SealedClass();  
        int total = sealedCls.Add(4, 5);  
        Console.WriteLine("Total = " + total.ToString());  
    }  
}  
// Sealed class  
sealed class SealedClass  
{  
    public int Add(int x, int y)  
    {  
        return x + y;  
    }  
}

You can also use the sealed modifier on a method or a property that overrides a virtual method or property in a base class. This enables you to allow classes to derive from your class and prevent other developers that are using your classes from overriding specific virtual methods and properties.

class X  
{  
    protected virtual void F()  
    {   
        Console.WriteLine("X.F");   
    }  
    protected virtual void F2()  
    {  
        Console.WriteLine("X.F2");   
    }  
}  
class Y : X  
{  
    sealed protected override void F()  
    {  
        Console.WriteLine("Y.F");  
    }  
    protected override void F2()  
    {  
        Console.WriteLine("X.F3");  
    }  
}  
class Z : Y  
{  
    // Attempting to override F causes compiler error CS0239.  
    //   
    protected override void F()  
    {  
         Console.WriteLine("C.F");   
    }  
    // Overriding F2 is allowed.   
    protected override void F2()  
    {  
        Console.WriteLine("Z.F2");   
    }  
}