Enabling C# 9 in Xamarin & .NET Standard Projects

Whoa! .NET 5 is out and C# 9 is out! This means you are ready to use records, new pattern matching, and more! But wait a second... your apps haven't been upgraded to .NET 5 yet or are a project type that don't support .NET  so you don't have access to the new C# 9 features :( Or do you!?!?!?! Yes, of course you do! Most new features in C# are compiler based and not necessarily runtime based.  When you create a new project the compiler determines a default C# language based on these rules:

So, if you are using .NET Standard 2.0 (the default for Xamarin.Forms) you only have access to C# 7.3 by default, which is sad. However, you can override the default inside of the .csproj. Simply open it up and add the property group:

<PropertyGroup>
   <LangVersion>preview</LangVersion>
</PropertyGroup>

You can also pick the specific version of C# that you would like:

<PropertyGroup>
   <LangVersion>9.0</LangVersion>
</PropertyGroup>

Now you are ready to start using C# 9 and convert all those classes over to records... almost! I say almost because there is one quirk with enabling C# 9 in non-.NET 5 project and when you use a record you will be granted with a System.Runtime.CompilerServices.IsExternalInit is no defined or imported error.

This can easily be fixed by adding the following code into any file. I like to add it into the AssemblyInfo.cs file:

namespace System.Runtime.CompilerServices
{
    public class IsExternalInit { }
}

There you go! now you can start using all those new fancy features!! Checkout these docs:

• What's new in C# 9
• What's new in .NET 5
• Versioning C# in projects