A static class is defined as a class that contains only static members (of course besides the instance members inherited from Object and possibly a private constructor). Some languages provide built-in support for static classes. In C# 2.0 and later, when a class is declared to be static, it is sealed, abstract, and no instance members can be overridden or declared.
Static classes are a compromise between pure object-oriented design and simplicity. They are commonly used to provide shortcuts to other operations (such as File), holders of extension methods, or functionality for which a full object-oriented wrapper is unwarranted (such as Environment).
✓ DO use static classes sparingly.
Static classes should be used only as supporting classes for the object-oriented core of the framework.
X DO NOT treat static classes as a miscellaneous bucket.
X DO NOT declare or override instance members in static classes.
✓ DO declare static classes as sealed, abstract, and add a private instance constructor if your programming language does not have built-in support for static classes.
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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.