.NET Type Design Guidelines—Static Class Design

Visual C# Tutorials

.NET Framework Tutorials

.NET Type Design Guidelines Types and Namespaces Class or Struct? Class or Interface? Abstract Class Design Static Class Design Interface Design Struct Design Enum Design Nested Types Summary

© 2006 Microsoft Corp.

Static Class Design

A static class is defined as a class that contains only static members (of course besides the instance members inherited from System.Object and possibly a private constructor). Some languages provide built-in support for static classes. In C# 2.0, when a class is declared to be static, it is sealed, abstract, and no instance members can be overridden or declared.

public static class File {
   ...
}

If your language does not have built-in support for static classes, you can declare such classes manually as in the following C++ example:

public class File abstract sealed {
   ...
}

Static classes are a compromise between pure object-oriented design and simplicity. They are commonly used to provide shortcuts to other operations (such as System.IO.File), or functionality for which a full object-oriented wrapper is unwarranted (such as System.Environment).

DO use static classes sparingly.

Static classes should be used only as supporting classes for the object-oriented core of the framework.

DO NOT treat static classes as a miscellaneous bucket.

There should be a clear charter for the class.

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.

BRIAN GRUNKEMEYER

In the .NET Framework 1.0, I wrote the code for the System.Environment class, which is an excellent example of a static class. I messed up and accidentally added a property to this class that wasn't static (HasShutdownStarted). Because it was an instance method on a class that could never be instantiated, no one could call it. We didn't discover the problem early enough in the product cycle to fix it before releasing version 1.0. If I were inventing a new language, I would explicitly add the concept of a static class into the language to help people avoid falling into this trap. And in fact, in C# 2.0 did add support for static classes!

JEFFREY RICHTER

Make sure that you do not attempt to define a static structure, because structures (value types) can always be instantiated no matter what. Only classes can be static.