This book, Framework Design Guidelines , presents best practices for designing frameworks, which are reusable object-oriented libraries. The guidelines are applicable to frameworks ranging in size and in their scale of reuse:
- Large system frameworks, such as the .NET Framework, usually consisting of thousands of types and used by millions of developers.
- Medium-size reusable layers of large distributed applications
- or extensions to system frameworks, such as the Web Services
- Enhancements.
- Small components shared among several applications; for example, a grid control library.
It is worth noting that this book focuses on design issues that directly affect the programmability of a framework (publicly accessible APIs). As a result, we generally do not cover much in terms of implementation details. Just like a user interface design book doesn't cover the details of how to implement hit testing, this book does not describe how to implement a binary sort, for example. This scope allows us to provide a definitive guide for framework designers instead of being yet another book about programming.
These guidelines were created in the early days of .NET Framework development. They started as a small set of naming and design conventions but have been enhanced, scrutinized, and refined to a point where they are generally considered the canonical way to design frameworks at Microsoft. They carry the experience and cumulative wisdom of thousands of developer hours over three versions of the .NET Framework. We tried to avoid basing the text purely on some idealistic design philosophies, and we think its day-to-day use by development teams at Microsoft has made it an intensely pragmatic book.
The book contains many annotations that explain trade-offs, explain history, amplify, or provide critiquing views on the guidelines. These annotations are written by experienced framework designers, industry experts, and users. They are the stories from the trenches that add color and setting for many of the guidelines presented.
To make them more easily distinguished in text, namespace names, classes, interfaces, methods, properties, and types are set in monospace font. The book assumes basic familiarity with .NET Framework programming. A few guidelines assume familiarity with features introduced in version 2.0 of the Framework. If you are looking for a good introduction to Framework programming, there are some excellent suggestions in the Suggested Reading List at the end of the book.
Guideline Presentation
The guidelines are organized as simple recommendations using Do, Consider, Avoid, and Do not. Each guideline describes either a good or bad practice and all have a consistent presentation. Good practices have a check mark in front of them, and bad practices have an X in front of them. The wording of each guideline also indicates how strong the recommendation is. For example, a Do guideline is one that should always1 be followed (all examples are from this book):
DO name custom attribute classes with the suffix "Attribute."
public class ObsoleteAttribute : Attribute { ... }
On the other hand, Consider guidelines should generally be followed, but if you fully understand the reasoning behind a guideline and have a good reason to not follow it anyway, you should not feel bad about breaking the rules:
CONSIDER defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.
Similarly, Do not guidelines indicate something you should almost never do:
DO NOT assign instances of mutable types to read-only fields.
Less strong, Avoid guidelines indicate that something is generally not a good idea, but there are known cases where breaking the rule makes sense:
AVOID using ICollection or ICollection as a parameter just to access the Count property.
Some more complex guidelines are followed with additional background information, illustrative code samples, and rationale:
DO implement IEquatable on value types. The Object.Equals method on value types causes boxing and its default implementation is not very efficient because it uses reflection. IEquatable.Equals can offer much better performance and can be implemented so it does not cause boxing.
public struct Int32 : IEquatable {
public bool Equals(Int32 other){ ... }
}
Language Choice and Code Examples
One of the goals of the Common Language Runtime is to support a variety of programming languages: those provided by Microsoft, such as C++, VB, and C#, as well as third-party languages such as Eiffel, COBOL, Python, and others. Therefore, this book was written to be applicable to a broad set of languages that can be used to develop and consume modern frameworks. To reinforce the message of multilanguage framework design, we considered writing code examples using several different programming languages. However, we decided against this. We felt that using different languages would help to carry the philosophical message, but it could force readers to learn several new languages, which is not the objective of this book.
We decided to choose a single language that is most likely to be readable to the broadest range of developers. We picked C#, because it is a simple language from the C family of languages (C, C++, Java, and C#), a family with a rich history in framework development.
Choice of language is close to the hearts of many developers, and we offer apologies to those who are uncomfortable with our choice.
About This Book
This book offers guidelines for framework design from the top down.
Chapter 1 is a brief introduction to the book, describing the general philosophy of framework design. This is the only chapter without guidelines.
Chapter 2, "Framework Design Fundamentals," offers principles and guidelines that are fundamental to overall framework design.
Chapter 3, "Naming Guidelines," contains naming guidelines for various parts of a framework, such as namespaces, types, members, and common design idioms.
Chapter 4, "Type Design Guidelines," provides guidelines for the general design of types.
Chapter 5,"Member Design," takes it a step further and presents guidelines for the design of members of types. Chapter 6, "Designing for Extensibility," presents issues and guidelines that are important to ensure appropriate extensibility in your framework.
Chapter 7, "Exceptions," presents guidelines for working with exceptions, the preferred error reporting mechanisms.
Chapter 8, "Usage Guidelines," contains guidelines for extending and using types that commonly appear in frameworks.
Chapter 9, "Common Design Patterns," offers guidelines and examples of common framework design patterns.
Appendix A contains a short description of coding conventions used in this book. Appendix B describes a tool called FxCop. The tool can be used to analyze framework binaries for compliance with the guidelines described in this book. A link to the tool is included on the DVD that accompanies this book.
Appendix C is an example of an API specification that framework designers within Microsoft create when designing APIs.
Included with the book is a DVD that contains several hours of video presentations covering topics presented in this book by the authors, a sample API specification, and other useful resources.
1.Always might be a bit too strong a word. There are guidelines that should literally be always followed, but they are extremely rare. On the other hand, you probably need to have a really unusual case for breaking a "Do" guideline and still have it be beneficial to the users of the framework.
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