Sealed classes cannot be inherited from, and they prevent extensibility. In contrast, classes that can be inherited from are called unsealed classes.
✓ CONSIDER using unsealed classes with no added virtual or protected members as a great way to provide inexpensive yet much appreciated extensibility to a framework.
Developers often want to inherit from unsealed classes so as to add convenience members such as custom constructors, new methods, or method overloads. For example, System.Messaging.MessageQueue is unsealed and thus allows users to create custom queues that default to a particular queue path or to add custom methods that simplify the API for specific scenarios.
Classes are unsealed by default in most programming languages, and this is also the recommended default for most classes in frameworks. The extensibility afforded by unsealed types is much appreciated by framework users and quite inexpensive to provide because of relatively low test costs associated with unsealed types.
Portions © 2005, 2009 Microsoft Corporation. All rights reserved.
Reprinted by permission of Pearson Education, Inc. from Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries, 2nd Edition by Krzysztof Cwalina and Brad Abrams, published Oct 22, 2008 by Addison-Wesley Professional as part of the Microsoft Windows Development Series.