Announcing .NET Core RC2 and .NET Core SDK Preview 1

Rich Lander [MSFT]

Updated (2017): See .NET Core Releases to learn about newer releases. This release is not supported.

Today, we are announcing the release of .NET Core RC2. You can use it to build ASP.NET Core, console apps and class libraries for Windows, OS X and Linux. RC2 is a major update from the November RC1 release, including new APIs, performance and reliability improvements and a new set of tools.

You can install .NET Core SDK 1.0 Preview 1 now (including .NET Core RC2), on Windows, OS X and Linux. You can also use it with Docker.

You can use .NET Core RC2 with a variety of editors and IDEs:

Coming soon:

  • Deploy .NET Core RC2 apps to Azure Websites
  • Acquire .NET Core RC2 with RedHat Enterprise Linux yum installer

We’ve been working directly with a number of customers who are hosting RC1 in production today, on Windows and Linux. Thanks for using RC1! Some of those customers have already moved to RC2, also in production. We appreciate all the feedback we’ve received since we released RC1. Please continue that feedback with the RC2 release.


Today, we are releasing three components (release notes below).

We made major changes to the .NET Core SDK, formerly called DNX, since RC1. These changes were fairly significant, and we plan to make additional changes before the final release of the SDK. As a result, we opted to call the SDK a preview release. While that may sound like a concern, the SDK is typically only used and development time, not in production, and so changes to the SDK should not affect running applications.

.NET Core and ASP.NET Core have improved significantly since RC1. We’ve added features and improved performance and reliability. RC1 was “Go Live” and so is RC2. “Go Live” means you can call Microsoft Support for help with issues.

Note that the .NET Core SDK includes a new telemetry feature. See the end of the post for more information on .NET Core Tools data collection.

Platform Support

We’ve been adding support for a growing number of operating systems. We started out the project with a plan to support .NET Core on Windows, OS X and Linux. Close watchers of the coreclr and corefx projects will have noticed that the platform support has been growing steadily. .NET Core RC2 is supported on the following platforms.

  • Red Hat Enterprise Linux 7.2
  • Centos 7.1
  • Debian 8.2+
  • Ubuntu 14.04 (16.04 support is coming at RTM)
  • Linux Mint 17+
  • OS X 10.11
  • Windows 7+ / Windows Server 2012 R2+
  • Windows Nano Server TP5

.NET Core RC2 will soon be showing up in the Red Hat Enterprise Linux software collection. You will be able to install it with yum, following instructions which will soon be posted to the site.

Ubuntu was the first distro that we supported. We heard feedback that it made more sense to start with Debian, given that it is the parent of Ubuntu and many more distros. More recently, we added support for Debian, enabling .NET Core to be used in a larger set of Debian-based distros.

We intend .NET Core to be an open and flexible development platform. We’ll publish instructions in the next couple weeks on how to test .NET Core on arbitrary distros. You can see how Linux Mint is supported in runtimes.json, for example.

.NET Core Tools

You typically start .NET Core development by installing the .NET Core SDK. The SDK includes enough software to build an app. The SDK gives you both the .NET Core Tools and a copy of .NET Core. As new versions of .NET Core are made available, you can download and install them without needing to get a new version of the tools.

Apps specify their dependence on a particular .NET Core version via the project.json project file. The tools help you acquire and use that .NET Core version. You can switch between multiple apps on your machine in Visual Studio, Visual Studio Code or at a command prompt and the .NET Core tools will always pick the right version of .NET Core to use within the context of each app.

You can also have multiple versions of the .NET Core tools on your machine, too, which can be important for continuous integration and other scenarios. Most of the time, you will just have one copy of the tools, since doing so provides a simpler experience.

The dotnet Tool

Your .NET Core experience will start with the dotnet tool. It exposes a set of commands for common operations, including restoring packages, building your project and unit testing. It also includes a command to create an empty new project to make it easy to get started.

There are many tools that come with the .NET Core Tools. You won’t see most of them since they all expose themselves through the dotnet tool. dotnet has a very simple extensibility model, so it’s also easy to add more commands as needed.

The following list provides a partial list of the commands.

  • dotnet new – Initializes a sample console C# project.
  • dotnet restore – Restores the dependencies for a given application.
  • dotnet build – Builds a .NET Core application.
  • dotnet publish – Publishes a .NET portable or self-contained application.
  • dotnet run – Runs the application from source.
  • dotnet test – Runs tests using a test runner specified in the project.json.
  • dotnet pack – Creates a NuGet package of your code.

dotnet new also supports F#. You can type dotnet new --lang F# to create a new F# app. The .NET Core Tools will download F# tools as part of dotnet restore. VB is not yet supported.

Development Workflow

The .NET Core tools enable multiple workflows.

The simplest one is the following, which uses the tools to create a new project, restore its package dependencies, and then build and run the app.

dotnet new
dotnet restore
dotnet run

You can separate the run command into two steps. In the example below, the app is called “test-app”, hence the dll name.

dotnet build
dotnet bin/Debug/netcoreapp1.0/test-app.dll

The tools also enable producing NuGet packages, Unit testing and other scenarios. You can learn more in the .NET Core docs.

For Unit testing with xUnit, see the Getting started with xUnit (.NET Core / ASP.NET Core) page.

Comparison to DNX

We learned a lot from our experience building DNX, with RC1 and prior .NET Core releases. DNX was actually three things at once:

  • a set of tools
  • a set of framework libraries
  • a set of services available to apps

The tools part of DNX lines up best with the .NET Core Tools that are part of today’s release.

DNX was great if you wanted all three of those things, but could be a problem if you only wanted one or two of them. This problem became obvious to us as we experimented with the corert native compilation project, which required a different set of tools than DNX provided. DNX also relied on environment variables to set an “in use” version. That made it hard to use multiple .NET Core apps from the same command prompt.

Those challenges provided us with a set of issues to resolve as we started our .NET Core RC2 project. They were the impetus to building the .NET Core RC2 tools.

The dotnet tool replaces the dnx and dnu tools that came with RC1. The dnvm tool doesn’t have a replacement yet. That’s something that might come in a later release.

.NET Core App Types

We’ve talked to many customers about how they want to deploy apps. We heard two main models:

  • Deploy smaller apps that have a dependency on a centrally installed .NET Core version, perhaps used by multiple apps. We call this model “portable”.
  • Deploy larger self-contained apps that have no .NET Core dependencies. We call this model “self-contained”.

Both of these app deployment models are supported and are a good choice, depending on the scenario. As of RC2, we have focussed most on the first scenario. We will continue to improve both scenarios.

Portable Apps

Portable applications are the default application type in .NET Core. They require .NET Core to be installed on the targeted machine in order for them to run. This means that your application is portable between installations of .NET Core, including on multiple OSes.

This type of application will only carry its own code and dependencies that are outside of .NET Core libraries. As long as .NET Core is installed on a given machine, the app will typically work. You do not need to decide upfront which OSes your app will run on.

Self-contained Apps

Self-contained applications contain all of their app dependencies, including the .NET Core runtime, as part of the application. This makes the app larger, but also makes it capable of running on any .NET Core supported platforms with the correct native dependencies, whether it has .NET Core installed or not. This makes it that much easier to deploy to the target machine, since you only deploy your application.

Since the application carries the runtime, you need to make an explicit choice which platforms your application needs to run on. For instance, if you publish a self-contained application for Windows 10, that same application will not work on OS X or Linux and vice versa. Of course, you can add or remove platforms during development at any given time.

Framework APIs

We added over a 1000 new APIs in .NET Core RC2. You can see the release notes and the complete API diff for .NET Core RC2 relative to RC1.

We added APIs to the following existing namespaces and classes:

New .NET Core APIs

  • System.Drawing
  • System.IO.BufferedStream
  • System.IO.Packaging

APIs added in RC2 to existing classes and namespaces

  • System.Console
  • System.Data
  • System.Diagnostics.ProcessStartInfo
  • System.Environment
  • System.Linq
  • System.Reflection
  • System.Runtime.InteropServices
  • System.Runtime.InteropServices.RuntimeInformation
  • System.Security.Cryptography
  • System.Security.Principal
  • System.Security.WindowsPrincipal
  • System.ServiceModel

More detailed description of System.Data additions:

  • SqlBulkCopy was added to System.Data.SqlClient for bulk operations.
  • System.Data interfaces were re-introduced.
  • DbDataReader API added to retrieve the Schema information for the tables being queried.
  • MARS support.

We are committed to adding many more APIs in future releases.

Runtime Improvements

There were many performance and reliability improvements since RC1. There were also some larger improvements, described below.

JIT Compiler

There were three major improvements to the RyuJIT JIT compiler.

  • RyuJIT has been updated to include all the RyuJIT improvements that were included in the .NET Framework 4.6.1.
  • RyuJIT-provided SIMD code generation for System.Numerics on all platforms.
  • Code-quality improvements (more optimized code) in some cases.

Garbage Collection

The garbage collector now supports background server garbage collection (GC) on Unix operating systems. Server GC was available in RC1, but was not optimized on Unix. In RC2, the server GC has been optimized for Unix and we have also enabled background garbage collection. This should result in lower pause times.

You can read the original introduction of background server GC in .NET Framework 4.5 blog post to learn more about it.

Runtime Configuration

.NET Core supports a new way of specifying runtime configuration. This is conceptually similar to the app.config files that are used with .NET Framework apps.

The runtime configuration files store the dependencies of an application (formerly stored in the .deps file). They also include runtime configuration options, such as the Garbage Collector mode. Optionally they can also include data for runtime compilation (compilation settings used to compile the original application, and reference assemblies used by the application).

NuGet Package References

.NET Core is a platform of packages. You can see how these packages are referenced in a set of simple .NET Core samples that we have published. There are two metapackages that you will typically reference in your project file, one or the other. One is for apps and libraries that will only run on .NET Core and the other for libraries that you intend to run on all .NET platforms.

Microsoft .NET Core

Most of the time, you will reference the Microsoft.NETCore.App package. This package represents the same set of libraries that are shipped with the various .NET Core installers. The .NET Core tools understand this reference and can use the locally installed copy of .NET Core instead of relying on the versions from NuGet. The NuGet packages are used for compilation, however.

You will also add references to other NuGet packages, for example to ASP.NET Core packages.

.NET Standard Library

The .NET Standard Library is a new concept that represents the APIs available in all .NET implementations, at least those that support it. The .NET Framework, .NET Core and Mono/Xamarin will or already do support the .NET Standard Library. The .NET Standard Library can be thought of as the next version of Portable Class Libraries, but the set of APIs available and the way you create the libraries are much different.

The set of APIs exposed by the .NET Standard Library is currently smaller than we intend. In the next few releases, we intend to expand this set of libraries considerably to provide much more compatibility with the .NET Framework. We’ll publish more on this plan soon.

The .NET Standard versions. Each new .NET Standard version includes more APIs and provides support for later .NET platform versions, such as .NET Framework 4.6. Earlier .NET Standard versions include fewer APIs and support more .NET platform versions. As a library implementer, you target a particular .NET Standard version, to get the right combination of APIs and platform support.

You can create .NET Standard libraries of your own by referencing the NETStandard.Library package. It references all of the packages that are part of the .NET Standard Library. You also specify a .NET Standard target framework version so that you compile your library against the right .NET Standard version. You can package up your library with dotnet pack and deploy the NuGet package to or another NuGet server.

.NET Core Tools Telemetry

The .NET Core tools include a new telemetry feature so that we can collect usage information about the .NET Core Tools. It’s important that we understand how the tools are being used so that we can improve them. Part of the reason the tools are in Preview is that we don’t have enough information on the way that they will be used. The telemetry is only in the tools and does not affect your app.


The telemetry feature is on by default. The data collected will be anonymous in nature and published in an aggregated form for use by both Microsoft and community engineers under a Creative Commons license.

You can opt-out of the telemetry feature by setting an environment variable DOTNET_CLI_TELEMETRY_OPTOUT (e.g. export on OS X/Linux, set on Windows) to true (e.g. “true”, 1). Doing this will stop the collection process from running.

Data Points

The feature collects the following pieces of data:

  • The command being used (e.g. “build”, “restore”)
  • The ExitCode of the command
  • For test projects, the test runner being used
  • The timestamp of invocation
  • The framework used
  • Whether runtime IDs are present in the “runtimes” node
  • The CLI version being used

The feature will not collect any personal data, such as usernames or emails. It will not scan your code and not extract any project-level data that can be considered sensitive, such as name, repo or author (if you set those in your project.json). We want to know how the tools are used, not what you are using the tools to build. If you find sensitive data being collected, that’s a bug. Please file an issue and it will be fixed.


We use the MICROSOFT .NET LIBRARY EULA for the .NET Core Tools, which we also use for all .NET NuGet packages. We recently added a “DATA” section re-printed below, to enable telemetry from the tools. We want to stay with one EULA for .NET Core and only intend to collect data from the tools, not the runtime or libraries.

DATA. The software may collect information about you and your use of the software, and send that to Microsoft. Microsoft may use this information to improve our products and services. You can learn more about data collection and use in the help documentation and the privacy statement at Your use of the software operates as your consent to these practices.


We’re excited to be releasing .NET Core 1.0 RC2, .NET Core SDK 1.0 Preview 1 and ASP.NET Core 1.0 RC2. We hope that you enjoy using these releases.

Please give us feedback on the releases. We’ll be watching for feedback through GitHub issues and other feedback forums. The best place to start is the Core repo. You can file an issue there. Do file an issue on one of the more specific .NET Core repos if you are familiar with them: coreclr, corefx and cli.

Thanks for your trying out .NET Core RC2!


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