NullableAttribute Missing

[TaffarelJr]

What would cause CSharpScript.Create(...) to not be able to find/generate the NullableAttribute class?

I have a C# script that references an external DLL (.NET 7) containing an extension method. The script compiles just fine, and I can confirm it successfully calls the extension method.

• If I change the return type of the extension method from string to string? (nullable), the script still compiles and runs just fine--which I assume means it can handle nullable reference types.
• BUT if I change the type of one of the extension method's parameters from string to string? (nullable), then it bombs with the following errors:

error CS0234: The type or namespace name 'NullableAttribute' does not exist in the namespace 'System.Runtime.CompilerServices' (are you missing an assembly reference?)
error CS0234: The type or namespace name 'Nullable' does not exist in the namespace 'System.Runtime.CompilerServices' (are you missing an assembly reference?)

I really don't know of any way to debug this issue. My understanding is, the attribute should be generated automatically by the compiler; so what could possibly be preventing it?
https://github.com/dotnet/roslyn/blob/main/docs/features/nullable-metadata.md