Sealed Classes in C#

It makes a big difference when you are working on a large project (say 50 developers) and some of them are not "so good" developers are working on an application that uses a class library. As an architect, you do not want them to use some of the classes in the class library by the application developers b/c you are afraid they may use classes in a wrong way or these classes may affect resources and miuse of them will create serious problems. That's one reason I can see why Microsoft would implement sealed classes.

Read this article here:

http://www.c-sharpcorner.com/UploadFile/srimanigvenkataraman/ComparisonJavaandCs11292005035430AM/ComparisonJavaandCs.aspx

The purpose of a sealed class is, when a component/library developer does not want it's user applications to call a class. This class is designed for library only.

Static classes on the other hand, are designed for user applications.

The sealed modifier is primarily used to prevent unintended derivation, but it also enables certain run-time optimizations. In particular, because a sealed class is known to never have any derived classes, it is possible to transform virtual function member invocations on sealed class instances into non-virtual invocations.

In C# structs are implicitly sealed; therefore, they cannot be inherited.

using System;
sealed class MyClass 
{
   public int x; 
   public int y;
}

class MainClass 
{
   public static void Main() 
   {
      MyClass mC = new MyClass(); 
      mC.x = 110;
      mC.y = 150;
      Console.WriteLine("x = {0}, y = {1}", mC.x, mC.y); 
   }
}

In the preceding example, if you attempt to inherit from the sealed class by using a statement like this:

class MyDerivedC: MyClass {} // Error

You will get the error message:
'MyDerivedC' cannot inherit from sealed class 'MyBaseC'.

In C# a method can't be declared as sealed.However when we override a method in a derived class, we can declare an overrided method as sealed as shown below. By declaring it as sealed, we can avoid further overriding of this method.

using System;

class Class1
{
public int x;
public int y;

public virtual void Method()
{
Console.WriteLine("virtual method");
}
}

class Class2 : Class1
{
public override sealed void Method()
{
Console.WriteLine("sealed method");
}
}

class MainClass
{
public static void Main()
{
Class1 c1 = new Class2();
c1.x = 110;
c1.y = 150;
Console.WriteLine("x = {0}, y = {1}", c1.x, c1.y);
c1.Method();
}
}