What is sealed class?

Ajitender Vijay

13y2.4kVisual Basic .NET

Hi Friends,

What is Sealed Class? and in what scenario we go for Sealed Class.

12 Replies

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Raj Kumar13y ago
Sealed classes are used to restrict the inheritance feature of object oriented programming. Once a class is defined as sealed class, this class cannot be inherited.
In C#, the sealed modifier is used to define a class as sealed. In Visual Basic .NET,NotInheritable keyword serves the purpose of sealed. If a class is derived from a sealed class, compiler throws an error.

// 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, it will work fine. But if you try to derive a class from sealed class, 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;
    }
}
Jignesh Trivedi13y ago
yes

agree with above answer.
you can create instance of sealed class.
Harshal Shah13y ago
sealed class means that doesn't allow you to inherit it's members..
Vulpes13y ago
Yes, you can create an object of a sealed class and access its instance members provided it has an accessible constructor or, failing that, has a static 'factory' method for creating objects of that class.
Suma M13y ago
Thanks Jignesh, A sealed class cannot be inherited but its objects can be created and its members can be used right?
Jignesh Trivedi13y ago
some more examples

ObsoleteAttribute
System.Data.SqlClient.SqlCommand
System.Data.SqlClient.SqlConnection
System.Data.SqlClient.SqlDataAdapter
System.Data.SqlClient.SqlError
Vulpes13y ago
Apart from String, here are some other sealed classes in the .NET Framework:

Thread
StringBuilder
Activator
AppDomain
DBNull
TimeZoneInfo
Version
Suma M13y ago
Hi,

Please give some existing examples of sealed class in .net assembly
Jignesh Trivedi13y ago
hi,
A sealed class cannot be inherited by other class. we can used sealed calss to prevent inheritance of the class.
The design intent of a sealed class is to indicate that the class is specialized and there is no need to extend it to provide any additional functionality.
A sealed class is mostly used for security reasons by preventing unintended derivation by which the derived class may corrupt the implementation provided in the sealed class.
hope this will help you.
Vulpes13y ago
A private class can only be nested within another class and is only accessible within the 'outer' class. Also, unless it is sealed, it can technically be inherited from by other private nested classes. Consider for example this program which compiles fine:

using System;

class A
{
private class B
{

}

private class C : B
{

}
}

class D
{
static void Main()
{

}
}

So, it's a different thing altogether to a 'normal' sealed class which is usually a public or internal 'top level' class though it doesn't have to be.

In the first version of C#, classes which contained nothing but static members were usually sealed and contained private constructors to prevent instantiation.

However, with the advent of static classes in C# 2.0 which are automatically sealed and cannot be inherited from, it's no longer necessary to 'roll your own'.
Ajitender Vijay13y ago
Hi Vuples,

we can do same with private class also then why we go for sealed Class
Vulpes13y ago
A sealed class is a class which cannot be inherited from.

There are no hard and fast rules on this but the writer of the class would make it sealed if (s)he felt that it would not be appropriate for it to act as a base class.

Perhaps the best known example of a sealed class is the System.String class in the .NET framework.