Using reflection for code gen?

Question

I'm writing a console tool to generate some C# code for objects in a class library. The best/easiest way I can actual generate the code is to use reflection after the library has been built. It works great, but this seems like a haphazard approch at best. Since the generated code will be compiled with the library, after making a change I'll need to build the solution twice to get the final result, etc. Some of these issues could be mitigated with a build script, but it still feels like a bit too much of a hack to me.

My question is, are there any high-level best practices for this sort of thing?

Answer 1

Have you considered using T4 templates for performing the code generation? It looks like it's getting much more publicity and attention now and more support in VS2010.

This tutorial seems database centric but it may give you some pointers: http://www.olegsych.com/2008/09/t4-tutorial-creatating-your-first-code-generator/ in addition there was a recent Hanselminutes on T4 here: http://www.hanselminutes.com/default.aspx?showID=170.

Edit: Another great place is the T4 tag here on StackOverflow: https://stackoverflow.com/questions/tagged/t4

EDIT: (By asker, new developments)

As of VS2012, T4 now supports reflection over an active project in a single step. This means you can make a change to your code, and the compiled output of the T4 template will reflect the newest version, without requiring you to perform a second reflect/build step. With this capability, I'm marking this as the accepted answer.

Answer 2

Its pretty unclear what you are doing, but what does seem clear is that you have some base line code, and based on some its properties, you want to generate more code.

So the key issue here are, given the base line code, how do you extract interesting properties, and how do you generate code from those properties?

Reflection is a way to extract properties of code running (well, at least loaded) into the same execution enviroment as the reflection user code. The problem with reflection is it only provides a very limited set of properties, typically lists of classes, methods, or perhaps names of arguments. IF all the code generation you want to do can be done with just that, well, then reflection seems just fine. But if you want more detailed properties about the code, reflection won't cut it.

In fact, the only artifact from which truly arbitrary code properties can be extracted is the the source code as a character string (how else could you answer, is the number of characters between the add operator and T in middle of the variable name is a prime number?). As a practical matter, properties you can get from character strings are generally not very helpful (see the example I just gave :).

The compiler guys have spent the last 60 years figuring out how to extract interesting program properties and you'd be a complete idiot to ignore what they've learned in that half century.

They have settled on a number of relatively standard "compiler data structures": abstract syntax trees (ASTs), symbol tables (STs), control flow graphs (CFGs), data flow facts (DFFs), program triples, ponter analyses, etc. If you want to analyze or generate code, your best bet is to process it first into such standard compiler data structures and then do the job. If you have ASTs, you can answer all kinds of question about what operators and operands are used. If you have STs, you can answer questions about where-defined, where-visible and what-type. If you have CFGs, you can answer questions about "this-before-that", "what conditions does statement X depend upon". If you have DFFs, you can determine which assignments affect the actions at a point in the code. Reflection will never provide this IMHO, because it will always be limited to what the runtime system developers are willing to keep around when running a program. (Maybe someday they'll keep all the compiler data structures around, but then it won't be reflection; it will just finally be compiler support).

Now, after you have determined the properties of interest, what do you do for code generation? Here the compiler guys have been so focused on generation of machine code that they don't offer standard answers. The guys that do are the program transformation community (http://en.wikipedia.org/wiki/Program_transformation). Here the idea is to keep at least one representation of your program as ASTs, and to provide special support for matching source code syntax (by constructing pattern-match ASTs from the code fragments of interest), and provide "rewrite" rules that say in effect, "when you see this pattern, then replace it by that pattern under this condition". By connecting the condition to various property-extracting mechanisms from the compiler guys, you get relatively easy way to say what you want backed up by that 50 years of experience. Such program transformation systems have the ability to read in source code, carry out analysis and transformations, and generally to regenerate code after transformation.

For your code generation task, you'd read in the base line code into ASTs, apply analyses to determine properties of interesting, use transformations to generate new ASTs, and then spit out the answer.

For such a system to be useful, it also has to be able to parse and prettyprint a wide variety of source code langauges, so that folks other than C# lovers can also have the benefits of code analysis and generation.

These ideas are all reified in the DMS Software Reengineering Toolkit. DMS handles C, C++, C#, Java, COBOL, JavaScript, PHP, Verilog, ... and a lot of other langauges.

(I'm the architect of DMS, so I have a rather biased view. YMMV).

Answer 3

You may wish to use CodeDom, so that you only have to build once.

First, I would read this CodeProject article to make sure there are not language-specific features you'd be unable to support without using Reflection.