Kevin Hazzard - Metaprogramming in .NET

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When I think about metaprogramming I view it through three sets of experience: as a computer scientist, a business developer, and a .NET framework author.

From a computer science perspective, it is clear that our industry has been largely stagnant from a language perspective for an extremely long time. The slow evolution of 3GLs (third-generation languages) from C to C++ to Java to C# has resulted in incremental improvements, but no major leaps in terms of developer productivity, maintainability of code, reduction of complexity, or other meaningful metrics.

(I chose the C language progression in my example because it is perhaps the most widely known. Comparable progressions exist for BASIC, Pascal, and many other language families.)

Metaprogramming offers interesting possibilities around the creation of domain-specific languages and other abstraction concepts that could eventually break us out of the 3GL world weve lived in for the past 20-30 years. Although this book doesnt focus on such a long-term goal, I think you can use Metaprogramming in .NET as a starting point to gain valuable perspective on myriad core ideas that might inspire you to think more about the future of our industry.

As someone whos been a business developer for over 25 years, Ive watched as metaprogramming has become one of the most mainstream and important tools for software development. Metaprogramming enables development time code generation as well as software that can dynamically adapt its behaviors at runtime.

In the mid-1990s people mocked attempts by Microsoft and others to create wizards that generated code for various business application scenarios. Today, such code generation tools are considered invaluable in environments as varied as Ruby on Rails, Eclipse, and Visual Studio. Most business developers rely daily on massive amounts of code generated by their tools during the development and build process.

Similarly, developers rely on runtime-generated code created by test mocking frameworks, dynamic UI generation tools, rules engines, and more. Even more subtle aspects of metaprogramming, such as the use of introspection (reflection) to create data binding frameworks, are pervasive.

This book explores a number of the underlying technologies and techniques used to implement code generation and dynamic applications during the development, build, and runtime phases of an applications lifecycle. Understanding these concepts is important for effective use of existing tools, and critical for creating your own or improving those that exist.

Finally, I am the author of the widely used CSLA .NET business objects framework. Within my framework I make extensive use of many of the techniques discussed in this book, including reflection, dynamic type loading, and expression trees.

A framework such as CSLA .NET couldnt exist without these technologies, and without the basic concepts of metaprogramming. Nor is CSLA .NET unique in this regard. Many frameworks in the data layer, business layer, and presentation layer make heavy use of metaprogramming techniques to provide broad and flexible support for object-relational mapping, business rules, validation rules, data binding, and dynamic UI generation.

In my view, metaprogramming is extremely important because its core concepts are used in popular development and testing frameworks and tools, as well as to enable code generation tooling and dynamic application behaviors. It is also one of the most promising areas of focus for the future of our industry as we look for ways to improve maintainability and reduce the cost of software over its lifetime.

This book is an excellent way to get started down the road of understanding and fully using the power of metaprogramming.

R OCKFORD L HOTKA
CTO AT M AGENIC
C REATOR OF THE CSLA .NET F RAMEWORK

In software development, metaprogramming is one of those words that sounds fancy and sophisticatedand somewhat intimidating at the same time. But what does it mean to be doing metaprogramming? The meta prefix can mean changed or higher. It can also mean after or beside, depending on the context. All of those terms describe the various forms of metaprogramming that we cover in this book.

You may choose to do metaprogramming in order to change code to support a higher level of abstraction within your system or to inject some new behavior that suits your particular needs. You may choose to do these things at compile time, between compile time and deployment, or even at runtime. Because of the flexible nature of the meta prefix, all of these scenarios qualify as metaprogramming.

No matter your reasons for doing metaprogramming, you must have a firm grip on the larger architectural picture of your project to do it effectively. Thats why metaprogramming is sometimes considered a dark art, to be practiced only by senior developers and architects. Nothing could be further from the truth. Everyone can do some form of metaprogramming. By manipulating code with other code the metaprogramming way, you can suddenly tackle classes of coding problems that you were never able to overcome before.

Your foray into metaprogramming may be to improve code reuse through simple templating or reflection. But soon you might also find yourself doing it to reduce the complexity of your systems. For example, weaving the code that does logging, performance monitoring, or transaction handling into a class library after its been compiled can greatly increase developer comprehension by reducing code complexity. Hiding all of that plumbing with metaprogramming can benefit everyone on the team.