Sealed Classes and Sealed Methods

Sealed Class

Sealed class is a special kind of class in C# which is used to prevent the inheritance to next level. Any class with sealed keyword cannot be derived to any subclass. We will get a compile error if we try to define base class as sealed class.


sealed class A { }
class B: A { } //CompileError

In the above example if we see, Class B tries to inherit from class A which is sealed. We will get a compiler error.

Now if we move the sealed keyword from Class A to Class B and try to inherit from class B we will get a compile error ('C': cannot derive from sealed type 'B').


class A { }
sealed class B: A { }
class C : B { } //Compile Error

Below code should be fine, as Sealed class should be the last in the inheritance hierarchy.

class A { }
class B : A { }
sealed class C : B { }

• Sealed and Abstract modifiers cannot be used together as Abstract Class has to provide the functionality and Sealed will prevent that.
• Sealed class cannot be a base class, it can only be the last derived class in the hierarchy.
• Structs are implicitly sealed.
• Sealed class provides performance benefits also, it depends on execution environments.

Sealed Methods

We can use sealed modifier on methods or properties also which override a virtual method or property of a base class. Sealed keyword helps us to allow classes to derive from our class and prevent them from overriding the virtual methods/properties.

In the below example if we see, Class A has 2 virtual methods Display1 and Display2. Class B inherits from Class A, in Class B we have sealed Display1 method as we don't want this method to be overridden in the deriving class. Now Class C inherits from Class B, if we try to override Display1 method of Class B, we get a compile error saying ( 'C.Display1()': cannot override inherited member 'B.Display1()' because it is sealed) but we can still override Display2 method as we didn't Seal it.

class A {

protected virtual void Display1()

{
Console.WriteLine("A.Display1");

}

protected virtual void Display2()

{

Console.WriteLine("A.Display2");

}

}

class B : A    {        
sealed protected override void Display1()        
{            
Console.WriteLine("B.Display1");        
}        
protected override void Display2()        
{            
Console.WriteLine("B.Display2");        
}    
}    

class C : B    {        
//protected override void Display1() //Compile Error        
//{        
//     Console.WriteLine("A.Display1");        
//} 
protected override void Display2()        
{            
Console.WriteLine("A.Display2");        
}    
}

• Sealed keyword is always used with Override keyword.
• Sealed method is not used with Sealed Class.
• Sealed method is used to define the overriding level of a virtual method.

Hope this article helps you in understanding the concept of Sealed identifier.
For any queries and suggestions, comment below :)