Announcing .NET Core and ASP.NET 5 RC

Today, we are announcing .NET Core and ASP.NET 5 Release Candidate, supported on Windows, OS X and Linux. This release is “Go Live”, meaning you can deploy apps into production and call Microsoft Support if you need help. Please check out the Announcing ASP.NET 5 RC blog post to learn more about the updates to ASP.NET 5.

The best way to get the RC is to go to the site. It has everything you need: downloads, instructions and samples. If you already have one of the betas installed, you can upgrade your environment to RC from the command-line.

We also have exciting news to share about what’s coming next for .NET Core, specifically new commandline tools and .NET Native. Check out the updated .NET Core Roadmap for more details.

The team will be hanging out in the .NET Foundation Forums to answer your questions about the RC or anything else about .NET Core and ASP.NET 5. We’re here to help!

.NET Core and ASP.NET 5 RC

.NET Core and ASP.NET 5 are now RC. They are ready for you to start building web apps and services with ASP.NET 5. You can build apps and services that scale, that work on-prem and in the Cloud and that can be run on Windows, Linux and OS X. .NET apps are portable. You can take an app running on Windows and move it to Linux, or vice-versa, without code modification. That’s a lot of flexibility in the way you build, deploy and manage apps.

For RC, we’ve added many features and now have largely feature complete Linux and OS X implementations in place. You can read Announcing ASP.NET 5 RC to learn about ASP.NET 5 RC in detail. You can also check out the release notes to see a list of the product changes and a milestone of commits: ASP.NET, .NET Core. The following are some of the key features that we’ve added since Preview:

ASP.NET 5 Visual Studio Experience

  • Integrating Bootstrap snippets
  • Updated Bower package UI
  • MVC scaffolding is enable for ASP.NET 5 projects

ASP.NET 5 Runtime

  • Transparent DNX app hosting model
  • Configurable webroot folder
  • Strong-named framework assemblies

.NET Core Runtime and BCL

.NET Core Libraries

Feature Complete

We often say, “shipping is a feature”. Getting to “feature complete” is a very important step towards shipping a final version.

.NET Core and ASP.NET 5 are now largely feature complete on Linux, OS X and Windows. For the most part, we have transitioned our focus to performance, reliability and overal quality. We still have a few features left to build, but that is now the exception. On that note, Exceptions are fully implemented!

People often ask us about baseline performance. Linux performance was much slower than our Windows implementation at first. The team has done a great job getting the Linux and OS X implementations to be at parity with the existing Windows implementation. The team hasn’t really started more serious performance work. That’s still coming.

Explicit DNX app hosting model

DNX has included a host that provides a variety of services to your app. You opt into using this host when you launch your app with dnx run, dnx web or any other dnx launch commands. The way that the host was called and wired up into your app was a bit magic. Some scenarios require providing a different host or doing work before the host initialized your app. There are lots of reasons why you might want a bit more control.

Starting with RC1, the host is now an explicit part of your app. This approach removes the magic, making it more obvious to you how your app functions on top of the platform. In addition, you now have more control. By default, the host is called through the following code, using the new C# 6 expression method bodies syntax.

You can also call the host the old fashioned way. This approach is important if you want to do work before calling the host. You shouldn’t do work after the host.

RyuJIT now supported on Linux and OS X

RyuJIT can generate code for Windows, Linux and OS X. Even though RyuJIT can generate machine code for X64 on Windows, it needed to be taught how to generate code for the same chip on Linux and OS X. The calling convention and other aspects are different on different OSes. We’ve made changes to accomodate that.

RyuJIT is also the compiler that is used for CrossGen, the ahead-of-time compiler for CoreCLR. It performs the same task as NGEN for the .NET Framework on Windows. CrossGen had to be taught to generate native images for Linux and OS X. CrossGen produces PE images on all OSes, since that’s the executable binary format CoreCLR knows how to load. It generates PDB images on Windows and Textual Symbol Tables on Linux and OS X in order to integrate with platform-specific debugging tools. CrossGen is currently broken on Linux and will be fixed.

Long file names (AKA “MaxPath”)

Windows has had a “MaxPath” restriction on the length of filenames, including directories. The limit is around 260 characters. That’s probably fine for most photo collections, but can be a problem for things like developer tools. The .NET Framework, being born on Windows, inhereted this same limitation.

Fast forward to now. The MaxPath restriction has been removed from .NET Core. You can use long filenames in .NET Core apps. Windows still has limited support for long filenames, however the .NET Core APIs won’t get in your way of operating on whatever files you want to create.

For more information and a code sample, see this gist.

Cross Platform SQL Client

An initial version of SqlClient is now working on .NET Core. The major update to SQL client was to move to the .NET Core networking libraries instead of the native Windows-only library that it uses on .NET Framework. By moving the networking logic to C# and .NET Core APIs, the SqlClient library is now able to run anywhere that .NET Core runs.

SqlClient is not yet feature complete, but it does work on Windows, OS X and Linux. To use SqlClient on OS X or Linux, Multiple Active Result Sets (MARS) must be disabled in the connection string (it is enabled by default). Set it to false by including MultipleActiveResultSets=False;, such as in the following example.

Additionally, connectivity to SQL Server is only possible via TCP. 

Communication over Named Pipes, Shared Memory, or LocalDB is not supported yet.

Example connection string:

connectionString="Data Source=my-data-source;Initial Catalog=mydataBase;
 MultipleActiveResultSets=False;User Id=myUserName; Password=myPassword;"

For more information and a code sample, see this gist.

Go Live Support

The .NET Core 5 and ASP.NET 5 Release Candidates is a “Go Live” release. That means if your ASP.NET 5 app passes your tests and you are happy with it, you can host your app in production.

You can engage with the .NET Core CLR, .NET Core FX and ASP.NET 5 teams directly on GitHub, or, if you encounter issues that are preventing you from deploying in a production environment, please contact Microsoft Product Support. Support is English only and U.S. business hours only (M-F 6a-6p PST), business day response.

CoreFX Open Source Progress

.NET Core open source was launched one year ago, November 12th, 2014. The team released a few libraries on that day with a promise of many more libraries. The team maintained a repo for some time that marked the progress of components making their way from closed source to open source. We’ve since closed that repo, after making good on the promise.

The repo included a pie chart graphic that was automatically generated by a script, based on data files that described the progres. You can see how close we were the last time the graphic was generated. We didn’t both updating the files, and therefore the graphic, since we’re now done.



We have released .NET Core and ASP.NET 5 RC. You can use these in production and call on Microsoft for support via the regular Microsoft Support channels. Get ASP.NET, read the ASP.NET 5 RC announcement, learn from ASP.NET Docs and play with some simple ASP.NET samples. Get started with RC!

The team will be monitoring StackOverlow ( and .net tags) and the .NET Foundation Forums for questions.