How to enable C# 6.0 feature in Visual Studio 2013?

Question

Under VS2013 you can install the new compilers into the project as a nuget package. That way you don't need VS2015 or an updated build server.

https://www.nuget.org/packages/Microsoft.Net.Compilers/

Install-Package Microsoft.Net.Compilers

The package allows you to use/build C# 6.0 code/syntax. Because VS2013 doesn't natively recognize the new C# 6.0 syntax, it will show errors in the code editor window although it will build fine.

Using Resharper, you'll get squiggly lines on C# 6 features, but the bulb gives you the option to 'Enable C# 6.0 support for this project' (setting saved to .DotSettings).

As mentioned by @stimpy77: for support in MVC Razor views you'll need an extra package (for those that don't read the comments)

Install-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform

If you want full C# 6.0 support, you'll need to install VS2015.

Information for obsoleted prerelease software:

According to this it's just a install and go for Visual Studio 2013:

In fact, installing the C# 6.0 compiler from this release involves little more than installing a Visual Studio 2013 extension, which in turn updates the MSBuild target files.

So just get the files from https://github.com/dotnet/roslyn and you are ready to go.

You do have to know it is an outdated version of the specs implemented there, since they no longer update the package for Visual Studio 2013:

You can also try April's End User Preview, which installs on top of Visual Studio 2013. (note: this VS 2013 preview is quite out of date, and is no longer updated)

So if you do want to use the latest version, you have to download the Visual Studio 2015.

A lot of the answers here were written prior to Roslyn (the open-source .NET C# and VB compilers) moving to .NET 4.6. So they won't help you if your project targets, say, 4.5.2 as mine did (inherited and can't be changed).

But you can grab a previous version of Roslyn from https://www.nuget.org/packages/Microsoft.Net.Compilers and install that instead of the latest version. I used 1.3.2. (I tried 2.0.1 - which appears to be the last version that runs on .NET 4.5 - but I couldn't get it to compile*.) Run this from the Package Manager console in VS 2013:

PM> Install-Package Microsoft.Net.Compilers -Version 1.3.2

Then restart Visual Studio. I had a couple of problems initially; you need to set the C# version back to default (C#6.0 doesn't appear in the version list but seems to have been made the default), then clean, save, restart VS and recompile.

Interestingly, I didn't have any IntelliSense errors due to the C#6.0 features used in the code (which were the reason for wanting C#6.0 in the first place).

* version 2.0.1 threw error The "Microsoft.CodeAnalysis.BuildTasks.Csc task could not be loaded from the assembly Microsoft.Build.Tasks.CodeAnalysis.dll. Could not load file or assembly 'Microsoft.Build.Utilities.Core, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified. Confirm that the declaration is correct, that the assembly and all its dependencies are available, and that the task contains a public class that implements Microsoft.Build.Framework.ITask.

UPDATE One thing I've noticed since posting this answer is that if you change any code during debug ("Edit and Continue"), you'll like find that your C#6.0 code will suddenly show as errors in what seems to revert to a pre-C#6.0 environment. This requires a restart of your debug session. VERY annoying especially for web applications.

It worth mentioning that the build time will be increased for VS 2015 users after:

Install-Package Microsoft.Net.Compilers

Those who are using VS 2015 and have to keep this package in their projects can fix increased build time.

Edit file packages\Microsoft.Net.Compilers.1.2.2\build\Microsoft.Net.Compilers.props and clean it up. The file should look like:

<Project DefaultTargets="Build" 
         xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
</Project>

Doing so forces a project to be built as it was before adding Microsoft.Net.Compilers package

It is possible to use full C# 6.0 features in Visual Studio 2013 if you have Resharper.
You have to enable Resharper Build and voilá! In Resharper Options -> Build - enable Resharper Build and in "Use MSBuild.exe version" choose "Latest Installed"

This way Resharper is going to build your C# 6.0 Projects and will also not underline C# 6.0 code as invalid.

I am also using this although I have Visual Studio 2015 because:

1. Unit Tests in Resharper don't work for me with Visual Studio 2015 for some reason
2. VS 2015 uses a lot more memory than VS 2013.

I am putting this here, as I was looking for a solution for this problem for some time now and maybe it will help someone else.