How to Use C# 9 in Xamarin.Forms Projects Today

.NET 5 is around the corner. It brings a plethora of good stuff, but what makes me excited the most are the new C# features: records and more advanced pattern matching. The record keyword facilitates the usage of immutable state, and I really wanted to use it.

But no matter what I’ve tried, setting TFM to .NET 5 didn’t work for me on Mac. No worries, as it turned out C# 9 can be used today with .NET Standard projects!

Here’s how:

• Switch Visual Studio for Mac to Preview channel
• Create a new Xamarin.Forms project

• In the library project change the TFM to .NETStandard2.1, language version to 9:

  <TargetFramework>netstandard2.1</TargetFramework>
  <LangVersion>9</LangVersion>

• Add the following code:

  // Without this dummy class C#9's records won't compile
  namespace System.Runtime.CompilerServices
  {
  public static class IsExternalInit
  {
  }
  }

YMMV, but it works on my machine with the following configuration:

• macOS Catalina 10.15.6
• Xcode 12.0
• Visual Studio Community for Mac 8.8 Preview (8.8 build 2120)
• Mono 6.12.0.93
• Roslyn 3.8.0-2.20414.4
• .NET Core SDK 3.1.402
• Xamarin.iOS 13.22.1.23
• Xamarin.Android 11.1.0.3

Alas, Rider 2020.3 EAP 1 chokes on records. VSfM and VS Code are fine with it.

Enjoy.