Roslyn and Mono

Hello Internet! I wanted to share some updates of Roslyn and Mono.

We have been working towards using Roslyn in two scenarios. As the compiler you get when you use Mono, and as the engine that powers code completion and refactoring in the IDE.

This post is a status update on the work that we have been doing here.

Roslyn on MonoDevelop/XamarinStudio

For the past year, we have been working on replacing the IDE's engine that gives us code completion, refactoring capabilities and formatting capabilities with one powered by Roslyn.

The current engine is powered by a combination of NRefactory and the Mono C# compiler. It is not as powerful, comprehensive or reliable as Roslyn.

Feature-wise, we completed the effort, and we now have a Roslyn-powered branch that uses Roslyn for code completion, refactoring, suggestions and code formatting.

In addition, we ported most of the refactoring capabilities from NRefactory to work on top of Roslyn. These were quite significant. Visual Studio users can try them out by installing the Refactoring Essentials for Visual Studio extension.

While our Roslyn branch is working great and is a pleasure to use, it also consumes more memory and by extension, runs a little slower. This is not Roslyn's fault, but the side effects of leaks and limitations in our code.

Our original plan was to release this for our September release (what we internally call "Cycle 6"), but we decided to pull the feature out from the release to give us time to fix the leaks that affected the Roslyn engine and tune the performance of Roslyn running on Mono.

Our revisited plan is to ship an update to our tooling in Cycle 6 (the regular feature update) but without Roslyn. In parallel, we will ship a Roslyn-enabled preview of MonoDevelop/XamarinStudio. This will give us time to collect your feedback on performance and memory usage regressions, and time to fix the issues before we make Roslyn the default.

Roslyn as a Compiler in Mono

One of the major roadblocks for the adoption of Roslyn in Mono was the requirement to generate debugging information that Mono could consume on Unix (the other one is that our C# batch compiler is still faster than Roslyn).

The initial Roslyn release only had support for generating debug information through a proprietary/native library on Windows, which meant that while Roslyn could be used to compile code on Unix, the result would not contain any debug information - this prevented Roslyn from being useful for most compilation uses.

Recently, Roslyn got support for Portable Program Database (PPDB) files. This is a fully documented, open, compact and efficient format for storing debug information.

Mono's master release contains now support for using PPDB files as its debug information. This means that Roslyn can produce debug information that Mono can consume.

That said, we still need more work in the Mono ecosystem to fully support PPDB files. The Cecil library is used extensively to manipulate IL images as well as their associated debug information. Our Reflection.Emit implementation will need to get a backend to generate PPDBs (for third party compilers, dynamic code generators) and support in IKVM to produce PPDB files (this is used by Mono's C# compiler and other third party compilers).

Additionally, many features in Roslyn surfaced bloat and bugs in Mono's class libraries. We have been fixing those bugs (and in many cases, the bugs have gone away by replacing Mono's implementation with implementations from Microsoft's Reference Source).

Posted on 21 Jul 2015 by Miguel de Icaza
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