
Design patterns are among the major tools in the toolboxes of those who espouse the generalist approach. If you look at samples from a broad spectrum of software solutions, you will find that though the specifics may vary widely, there is often an underlying structural similarity. (Searching a filesystem for a file with a particular attribute is in some sense structurally similar to searching an annotated parse tree for a symbol with a particular type.) Design patterns codify general solutions to common problems.

The ultimate example of the generalist approach is of course the design and implementation of programming languages themselves. As problem solving tools go, it is hard to get more general than a programming language like C#. When designing new programming languages (or new versions of old programming languages), we think about common problems that are faced every day by real developers and figure out how to create a language which solves them in a general, aesthetically pleasing, and powerful way that is broadly applicable.

We want to embed the most useful and powerful abstractions so deeply into the language infrastructure that you barely even consciously register them as being there anymore. Patterns like “local variable”or “procedure call”or “while loop”are so much a part of the air we all breathe that we don’t even think of them as patterns anymore.

Furthermore, we want to make a language in which patterns which are useful but perhaps not quite so fundamental are nevertheless relatively straightforward to implement clearly and elegantly. A class in C# may be marked as “static,”or “abstract,”or “sealed,”but not as “singleton.”That was a deliberate choice of the language designers. However, implementing a singleton class in C# is still relatively easy.

The gray zone in between “clearly foundational”and “occasionally useful”is where the interesting design challenges lie. Our observations of design patterns used by real-world developers in C# (and other languages) strongly drive the design process for new versions.

Consider for example how you would implement an iterator pattern on a linked list in C# 1.0. You would end up defining an enumerator class to represent a position in the list containing a lot of boring boilerplate code (which impedes readability), and the solution would not be very reusable. The notion of “enumerate a set of things”is sufficiently applicable to a wide variety of problems that it met the bar for inclusion as a first class language concept. In C# 2.0 with its yield return statement the compiler can generate all the boring code for you, and the generic type system makes iterating over a set of things typesafe no matter what the “things” are.

All of this is a long way to say just why it is that I am so very excited about Language Integrated Query (LINQ) in C# 3.0. We believed that iterating over collections of things was a great start, but that we could do so much more. Sorting, filtering, grouping, joining, projecting, and transforming data are also fundamental operations that are useful in pretty much every domain. Whether you are writing a ray tracer, a compiler, an XML reader, or an online banking security system, odds are good that you are going to need to manipulate collections of something in a rich way.

By moving these concepts out of domain-specific object models and into a generalpurpose programming language, we hopefully solve those more general problems. We additionally hope, though, that by adding C# 3.0’s query expressions, lambda expressions, extension methods, initializer expressions, expression trees, and so on to the already rich set of C# 2.0 and 1.0 features, we make it easier to elegantly implement all sorts of other useful design patterns.

And that is also why I am excited about this book. C# 3.0 Design Patterns brings the frequently abstruse world of design patterns into sharp focus with pragmatic C# 3.0 implementations. I look forward to seeing where developers can go with these tools and this language, and what useful patterns we can build into the infrastructures of future languages.

—Eric Lippert
Senior Developer
C# Compiler Team
Seattle, Washington
November 30, 2007

Erik Meijer Technical Lead Microsoft WebData Team

31 July 2003