How to enable C# 9.0-preview

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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>

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