How to enable C# 9.0-preview

As of October 2020:

Please see @Pac0's answer here: https://stackoverflow.com/a/64386529/159145

As of June 2020:

According to this page in the documentation you need to edit your *.csproj to set the <LangVersion> to preview.

Also mentioned in the blog-post about the preview-release, but not the above documentation page, is that you need to update your project's targetFramework property too to net5.0 _{(this is because the C# design team decided to restrict entire C# language versions to minimum BCL versions, unlike previously where you could use C# 7 with even .NET Framework 2.0 provided you reimplemented your own missing BCL types like ValueTuple and ExtensionAttribute).}

So your *.csproj file should look like this:

<Project>
 <PropertyGroup>
   <LangVersion>preview</LangVersion>
   <TargetFramework>net5.0</TargetFramework>
 </PropertyGroup>
</Project>

As per October 2020,

1. you can explicitly use the 9.0 language version in .csproj
2. Using target framework as .net 5 implicitly uses C# 9 by default .

The .csproj should be as such:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
  </PropertyGroup>
</Project>

You can add <LangVersion>9.0</LangVersion> if you wish, but it should be optional.

OutputType can be adapted, of course, and you'll need the .NET 5 SDK.

See for instance this blog for more information.

Firstly Download .NET 5 and then install Visual Studio Preview Edition. You will now have access to the latest features of C# 9. Also make sure that you project file includes the following.

 <PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>net5.0</TargetFramework>
<LangVersion>preview</LangVersion>