The Roslyn Preview Is Now Available

The Roslyn Preview Is Now Available

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I am super excited to announce that the Roslyn project code is now sufficiently coherent that we can start showing it to customers!

But I am getting ahead of myself somewhat. What is this "Roslyn" project?

Here's the deal. We've got these great premiere languages for .NET development, C# and Visual Basic. Obviously the compilers need to do considerable lexical, syntactic and semantic analysis of the code in order to first off, produce IL out the back end of the compiler, and second, produce all of that on-the-fly analysis needed for syntax colourization, IntelliSense, automatic refactorings, and so on. We do all this work to produce an analysis of the code, but we do not let you, the customer, take advantage of that analysis engine directly. Rather, you just get to see how the compiler and IDE teams make use of that analysis engine.

This is unfortunate. When I look around just Microsoft I see a dozen little C# and VB language lexers, parsers and semantic analyzers that different teams have written to meet their own needs. There have got to be many more out there in the wild. This is a problem we can solve once for you, and then let you make use of the analysis engine for your own purposes.

Furthermore, it became clear as we were designing the next versions of C# and VB that the existing compiler infrastructure built in unmanaged C++ was not going to meet our needs without a major overhaul at some point. We want a compiler infrastructure that supports both new interesting language features, and more dynamic ways to interact with your code as you're developing it.

To achieve all those goals we decided to join the C# and VB teams together, and then split the resulting team apart into two subteams: one concentrating on upcoming C# and VB features like async/await, and one concentrating on the long-term future of the compiler and tools infrastructure. The latter team is codenamed "Roslyn", and that's the team I've been working on for some time now.

We are now at the point where the Roslyn codebase is sufficiently fully-featured and coherent that it makes sense to start getting feedback from customers. You can download the Roslyn Community Technology Preview as of right now; it installs as an extension to Visual Studio 2010 SP1. We would love to get as much constructive feedback from you as possible; rather than leaving comments here, please leave comments on the Roslyn Forum. That way we'll be sure that our crack team of program managers sees the feedback and can aggregate it all appropriately. We're making a huge investment here and want to know that we're heading in the right direction to make something insanely great for you guys.

What we're looking for feedback on primarily is the set of code analysis APIs we're exposing to you guys. The APIs themselves as a set of classes with methods and so on, are now reasonably complete; we do not anticipate making major changes to the infrastructure for Symbols, SyntaxNodes and so on unless we hear loud feedback that we have gone with the wrong model. Does this  set of APIs meet your needs? What parts do you like, and what would you like to be different?

I want to make sure that we're setting expectations appropriately here. This is a very early pre-beta-quality release of some extremely complicated software. Keep that in mind as you are using it. The lexical and syntactic analysis engines that underlie those APIs are pretty much complete. The semantic analysis engine that sits behind the semantic analysis APIs is right now nowhere near done; on the C# side we are still missing semantic analysis of major feature areas like query comprehensions, attributes, iterator blocks, optional arguments, dynamic, async/await and unsafe as well as a bunch of control flows; VB is in a similar state. Extension methods, method type inference, lambdas, and generics are working, so LINQ queries in the "method call" syntactic form should work.

Also, the performance of the entire analysis engine is not as high as we would like; we have done some performance tuning but there is more to come.

In short, you're getting the earliest possible build we could reasonably show to people to get good feedback; we are still a long way from being done. We are absolutely not announcing any dates or "ship vehicles" for a final release; even if I knew - which I don't - I couldn't tell you, so don't ask.

We are also as a part of this release shipping a preview of a new "interactive scripting" environment for C# that will allow you to play around with code in a more experimental and free-form way, like you can do in F# and other "Read-Eval-Print Loop" languages. Give it a spin and let us know what you think! (Sorry, no VB version of this feature is available with the CTP.)

Over the next few months I'll do some more posts describing the interesting technology we've built behind-the-scenes to make the C# and VB Roslyn code analysis engines work. For now, you can get more information on Roslyn at the following places:

Some helpful blog posts:

Fun stuff:




  • It's a nice achievement but I'm wondering who the target audience is?

    C# already has some nice refactoring features and this whould merely allow some third party developers to come with their own plug in like resharper. Few developers would have an interest developing these features. Maybe others will make use of a different analysis when dealing with certain third party types, but that's breaking the familiar user experience.

    Your question is basically "who is the target audience for your library of code analysis tools?" The answer is "people who like to write programs that analyze code". If you're not interested in writing programs that analyze code then Roslyn is probably not the library for you. -- Eric

  • Eric, this is a great CTP which I was awaiting for a long time. I am a promoter of using pragmatic aspect oriented approaches with dynamically created proxies and interceptors. However, in the .NET area (unlike Java) up to now I have to fall back to 3rd party tools (e.g. Springframework) or use System.Reflection.Emit directly. The 3rd party tools often generate proxies which use reflection at runtime etc. This makes it inappropriate for performance reasons at times. Now with Roslyn I am able to very easily generate my dynamic proxies myself in the way I want them. I know that this may be not the originally intented use but it works quite nicely so far...

    Thanks again!


  • Probably publish the top 10 use-case scenario of Roslyn would help to understand why we should use it. Or what is it and what is not would be even  better.

  • Any response to the comment on comparing Roslyn to Clang? How are they different? How are they similar? Thanks!

  • Duplicate code detection would be a helpful outcome and use of the new API.  Find all token sequences over X long that match treating an indentifier as 'identifier' and literals as 'number literal' would be great.

  • Just wrote my first VS plugin using roslyn. It shows a warning when a pure method is used as a statement.

    Unfortunately the contract reference assemblies aren't redistributable, and as a non lawyer I don't know if I may distribute a list of pure functions with my plugin, so for now the full functionality will only be available when Code Contracts is installed.


    Didn't come across any big issues in the API. I missed an equivalent of `Type.FullName`, but that wasn't hard to work around.

    Loading the contract reference assemblies for the framework libraries was a bit more work. Reflection-only assembly loading is very annoying(typeload exceptions,...), so I had to use MonoCecil instead.

  • FYI, your wiki link is currently to the city in Washington.

    This is the link I think you had intended.

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