Strong-typed Metadata

Your code is code and your code is data.

Metaprogramming opens up worlds where you care very much that your code is data. Editor enhancements open up worlds where you care very much that your code is data. Visualizations open up worlds where you care very much that your code is data. And I think that’s only the beginning.

There’s nothing really new about thinking of code as data. Your compiler does it, metaprogramming techniques do it, and delegates and functional programming do it.

So, let’s make your code data. Living breathing strongly-typed data. Strong typing means describing the code in terms of the underlying problem and providing this view as a first class citizen rather than a passing convenience.

Describing the Underlying Problem

I’ll use logging as an example, because the simpler problem of PropertyChanged just happens to have an underlying problem of classes and properties, making it nearly impossible to think about with appropriate abstractions. Class/property/method is only interesting if the underlying problem is about classes, properties and methods.

The logging problem is not class/method – it’s log/log event. When you strongly type the metadata to classes that describe the problem being solved you can reason about code in a much more effective manner. Alternate examples would be classes that express a service, a UI, a stream or an input device like a machine.

I use EventSource for logging, but my metadata describes the problem in a more generalized way – it describes it as a SemanticLog. A SemanticLog looks like a class, and once you create metadata from it, you can create any logging system you want.

Your application has a handful of conceptual groups like this. Each conceptual group has a finite appropriate types of customization. Your application problem also has a small number of truly unique classes.

Treating Metadata as a First Class Citizen

In the past, metadata has been a messy affair. The actual metadata description of the underlying patterns of your application have been sufficiently difficult to extract that you’ve had no reason to care,. Thus, tools like the compiler that treated your code as data simply created the data view it needed and tossed in out as rubbish when it was done.

The .NET Compiler Platform, Roslyn, stops throwing away its data view. It exposes it for us to play with.

Usage Examples

I’m interested in strongly typed metadata to write templates for metaprogramming. I want these template to be independent of how you are running them – whether they are part of code generation, metaprogramming, a code refactoring or whatever. I also want these templates to be independent of how the metadata is loaded.

Strongly typed metadata works today in T4 templates. My CodeFirstMetadata project has examples.

I’m starting work on expansion first templates and there are many other ways to use strong-typed metadata – both for other metaprogramming techniques and completely different uses. One of the reasons I’m so excited about this project is to see what interesting things people do, once their code is in a strong-typed form. At the very least, I think it will be an approach to visualizations and ensuring your code follows expected patterns. It will be better at ensuring large scale patterns than code analysis rules. Whew! So much fun work to do!!!

Strong-typed Metadata in a T4 Template

Here’s a sample of strong typing in a T4 template


There’s some gunk at the top to add some assemblies and some using statements for the template itself. The important piece at the top is that the class created by this template is a generic type with a type argument – CodeFirstSemanticLog – that is a strong-typed metadata class. Thus the Meta property of the CodeFirstT4CSharpBase class is a SemanticLog class and understands concepts specific to the SemanticLog, like IncludesInterface. I’ve removed a few variable declarations that are specific to the included T4 files.

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