Nullable VS IsExternalInit

https://github.com/manuelroemer/Nullable (backport nullable attributes so you can use things like [NotNullWhen]). Alternatively, https://github.com/tunnelvisionlabs/ReferenceAssemblyAnnotator/ is even niftier, as it gives you .NET 5's annotations as well. So the compiler tells you whether a Dictionary.TryGetValue result is nullable or not based on annotations that didn't exist in .NET Standard 2.0.

If you need the attributes, reference https://github.com/manuelroemer/Nullable or https://github.com/tunnelvisionlabs/ReferenceAssemblyAnnotator. The latter is bigger, but retroactively adds nullability annotations to runtime references (such as Dictionary.TryGetValue).

You can also just reference the https://github.com/manuelroemer/IsExternalInit NuGet package.

init only properties (likely to come up if you use records) - you can either use the IsExternalInit nuget package or manually define the attributes - either way, the feature works perfectly fine

…and a NuGet reference to https://www.nuget.org/packages/IsExternalInit, I can then do public record Foo(string s); and have it build.

You only need https://github.com/manuelroemer/IsExternalInit for that (and to set LangVersion).

Even for .NET Framework code, I tend to use recent C# versions. I reference https://github.com/jnm2/ReferenceAssemblyAnnotator to get nullable support and https://github.com/manuelroemer/IsExternalInit for records, and that takes care of most of the features I care about.

features that require BCL additions. Since you're on the older BCL, those will need to be backported. For example, to use init; and record, you can use https://github.com/manuelroemer/IsExternalInit.