In C#, the sealed modifier is used to define a class as sealed. In Visual Basic .NET the NotInheritable keyword serves the purpose of sealed. If a class is derived from a sealed class then the compiler throws an error.
If you have ever noticed, structs are sealed. You cannot derive a class from a struct.
The following class definition defines a sealed class in C#:
// Sealed class
sealed class SealedClass
{
}
In the following code, I create a sealed class SealedClass and use it from Class1. If you run this code then it will work fine. But if you try to derive a class from the SealedClass, you will get an error.
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;
}
}
MSDN Updated
Sealed Methods and Properties
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");
}
}
Why Sealed Classes?
We just saw how to create and use a sealed class. The main purpose of a sealed class is to take away the inheritance feature from the user so they cannot derive a class from a sealed class. One of the best usage of sealed classes is when you have a class with static members. For example, the "Pens" and "Brushes" classes of the "System.Drawing" namespace.
The Pens class represents the pens for standard colors. This class has only static members. For example, "Pens.Blue" represents a pen with the blue color. Similarly, the "Brushes" class represents standard brushes. "Brushes.Blue" represents a brush with blue color.
So when you're designing your application, you may keep in mind that you have sealed classes to seal the user's boundaries.
In the next article of this series, I will discuss some usage of abstract classes.
Note: When this article was originally written the C# language did not have static classes. Now C# has static classes.