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sealed (C# Reference)

Updated: September 2008

When applied to a class, the sealed modifier prevents other classes from inheriting from it. In the following example, class B inherits from class A, but no class can inherit from class B.

    class A {}    
    sealed class B : A {}

You can also use the sealed modifier on a method or 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 them from overriding specific virtual methods or properties.

In the following example, Z inherits from Y but Z cannot override the virtual function F that is declared in X and sealed in Y.

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"); }
}

When you define new methods or properties in a class, you can prevent deriving classes from overriding them by not declaring them as virtual.

It is an error to use the abstract modifier with a sealed class, because an abstract class must be inherited by a class that provides an implementation of the abstract methods or properties.

When applied to a method or property, the sealed modifier must always be used with override.

Because structs are implicitly sealed, they cannot be inherited.

For more information, see Inheritance (C# Programming Guide).

sealed class SealedClass
{
    public int x;
    public int y;
}

class SealedTest2
{
    static void Main()
    {
        SealedClass sc = new SealedClass();
        sc.x = 110;
        sc.y = 150;
        Console.WriteLine("x = {0}, y = {1}", sc.x, sc.y);
    }
}
// Output: x = 110, y = 150

In the previous example, you might try to inherit from the sealed class by using the following statement:

class MyDerivedC: SealedClass {} // Error

The result is an error message:

'MyDerivedC' cannot inherit from sealed class 'SealedClass'.

For more information, see the following sections in the C# Language Specification:

  • 10.1.1.2 Sealed classes

  • 10.6.5 Sealed methods

To determine whether to seal a class, method, or property, you should generally consider the following two points:

  • The potential benefits that deriving classes might gain through the ability to customize your class.

  • The potential that deriving classes could modify your classes in such a way that they would no longer work correctly or as expected.

Date

History

Reason

September 2008

Corrected variable names in text description of code example.

Customer feedback.

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