Feature Nullable Reference Types not available

Describe the bug When compiling a template I get the following errors

RazorLight.Compilation.TemplateCompilationException: Failed to compile generated Razor template:
- (11:1) Das Feature "Nullable-Verweistypen" ist in C# 7.3 nicht verfügbar. Verwenden Sie Sprachversion 8.0 oder höher.
- (17:1) Das Feature "Nullable-Verweistypen" ist in C# 7.3 nicht verfügbar. Verwenden Sie Sprachversion 8.0 oder höher.
- (24:1) Das Feature "Nullable-Verweistypen" ist in C# 7.3 nicht verfügbar. Verwenden Sie Sprachversion 8.0 oder höher.
- (30:1) Das Feature "Nullable-Verweistypen" ist in C# 7.3 nicht verfügbar. Verwenden Sie Sprachversion 8.0 oder höher.
- (32:1) Das Feature "Nullable-Verweistypen" ist in C# 7.3 nicht verfügbar. Verwenden Sie Sprachversion 8.0 oder höher.
- (38:1) Das Feature "Nullable-Verweistypen" ist in C# 7.3 nicht verfügbar. Verwenden Sie Sprachversion 8.0 oder höher.

Which translates to The feature "Nullable reference type" is not available in C# 7.3. Use Version 8.0 or higher. I’m not using nullable reference types anywhere in my model or template. My template isn’t even 38 lines long.

To Reproduce

• Create a classlib that targets netcoreapp3.1.
• Create a ASP.NET Core App that targets netcoreapp3.1.
• Install RazorLight into the classlib and reference it from the main project.
• Add IRazorLightEngine to the DI via an extension method that is placed in the classlib

serviceCollection.AddSingleton<IRazorLightEngine>(_ =>
                new RazorLightEngineBuilder()
                    .UseFileSystemProject(Path.Combine(Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location), "MailTemplates"))
                    .UseMemoryCachingProvider()
                    .Build()
            );

• Call this extension method in your ConfigureServices
• Try to compile a template

Expected behavior The template compiles without errors.

Information (please complete the following information):

• OS: Windows 10 1909
• Platform .NET Core 3.1
• RazorLight version 2.0.0-beta4
• Visual Studio version Visual Studio Professional 2019

[hwinkelmann]

What helped in my case was to explicitly switch to C# 8 by adding <LangVersion>8.0</LangVersion> to some PropertyGroup in the .csproj files. Hope this helps!

[dariel312]

@jzabroski Thanks for this solution, I was also having this problem and spent like 3 hours trying to fix it and was literally about to submit an issue before GitHub referred me to this issue.