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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()
protected virtual void Display2()

class B : A    {        
sealed protected override void Display1()        
protected override void Display2()        

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


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