Community Involvement and Roslyn

By now, I’m sure you’ve heard the big news: There’s a new public preview of the Roslyn C# and VB.NET Compilers.  And, even bigger news: The Roslyn Compilers are Open Source.

This is an important milestone for these projects. The C# compiler is now written in C#; the VB.NET compiler is written in VB.  (See below). They also now do have full support for all the features in C# version 5 (and then some).  The rest of this blog post walks through some of the important ideas in the Roslyn ecosystem.

TL;DR version: Go to and get started.

Using the Roslyn Compilers (and IDE updates)

You can get the Roslyn compiler from a NuGet package, or by signing up and downloading the End User Preview. You will want to do this before you explore the Roslyn Source. That’s because the Roslyn compiler source makes use of new language features only available in the Roslyn compiler. That means you can’t build the Roslyn source with the compilers that ship with VS 2013.

Once you have the end user preview. You’ll find quite a few new features in the IDE. Most of these are covered in the //build talks, available online. My one recommendation: train yourself to look for the light bulb in the left margin of source. There’s lots of new information there.

If you want to dive deeper into Roslyn, get the SDK Preview. That provides templates and examples for writing extensions based on the Roslyn Language services API. One design goal for the Roslyn compilers is to provide the same language services to the compiler, the IDE, and 3rd Party add-ins. Those APIs let you view program source at its syntactic and semantic tree level, and manipulate those trees. If you are interested in analysis tools, or Refactorings, that’s where you start.

Learning about the source (and more about the languages)

The Roslyn source is hosted on CodePlex. It is a git – based repository. You can clone it using your favorite git tools. In my case, I cloned it from the command line using posh-git. Once cloned, I added it to my list of local repositories in GitHub for Windows. That’s worked well for me.

Once you’ve gotten the source, follow the instructions here to build it. Congratulations, you’ve built a compiler.

Feel like learning more, get involved in the discussions.  First, read the language design notes for C# (or VB if that’s your favorite) and then check the Language Feature Implementation status. The language design notes will give you the history and background for the current language features. It’s very illuminating. Personally, I’m thrilled that the teams have the confidence to share this level of detail with the public. You’ll learn an incredible amount by reading about the alternative implementations and the thoughts and discussions around each decision.

For many of the proposed new features, discussions are ongoing. In particular, the discussion on the null propagating operator is rather lively. If you have an opinion, join in.

Do you have ideas?

Well, we all do. And here’s where life gets very tricky. The Roslyn compilers are licensed under the Apache v2 license.  You, me, and everyone else on the planet can fork them and add our own new language features. On the one hand, it’s a fantastic move to release the compilers as open source. On the other hand, it could lead to chaos.

On the plus side, academics (and their students) can teach and learn compiler construction by working with a world class commercial compiler for two of the most popular programming languages on the planet.

Also on the plus side, community members can prototype ideas for language features and submit them for review. That will do a lot to keep the C# and VB communities vibrant and involved.

And, by looking at the compiler source, you’ll better understand how to build extensions and addins that leverage the Roslyn APIs.

But there is a dark side.

Part of the success of VB.NET and C# is that they are consistent languages. The Microsoft and Xamarin compilers both track the standard very closely, and have very few divergences. That’s a positive for the language.

I worry that it’s possible to have several dialects of C# in the future that all behave differently. If that happens, this global experiment to open source Roslyn will have been a mistake.

Rather than worry, here’s my recommendation to anyone looking at creating new language features: Get involved in the feature discussions first. Talk with the team. They are already active in the discussion forums. Get buy-in for the new feature.

If that happens, this is a marvelous new era. If community members create and discuss great new features, and those features become part of the standard C# Language, the global experiment to open source Roslyn will come to be seen as one of the defining positive moments in computing history. That’s what I want to see happen.

Created: 4/9/2014 3:00:03 PM

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