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.
Example
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("Y.F2"); }
}
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).
For more examples, see Abstract and Sealed Classes and Class Members (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'.
C# Language Specification
For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.
Remarks
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.
See Also
Reference
Static Classes and Static Class Members (C# Programming Guide)
Abstract and Sealed Classes and Class Members (C# Programming Guide)
Access Modifiers (C# Programming Guide)