Using C# 9 outside .NET 5

[johnkors]

Hi,

How is C# 9 tied to .NET 5? Will it be possible to use C# 9 outside .NET 5 if using Microsoft.Net.Compilers.Toolset ?

[PathogenDavid]

As with C#8 and .NET Core 3.0, C# 9 is only officially supported in combination with a .NET 5 SDK and a .NET 5 runtime. (Edit: Here's a newer source from documentation: C# language versioning: "C# 9.0 is supported only on .NET 5 and newer versions.")

If you're going to go the unsupported route, you're best off using the .NET 5 SDK and targeting an older framework (IE: <TargetFramework>netcoreapp3.1</TargetFramework> ) and explicitly setting your language version with <LangVersion>9</LangVersion>.

Microsoft.Net.Compilers.Toolset is explicitly not intended to support using newer compilers with older toolchains:

Using it as a long term solution for providing newer compilers on older MSBuild installations is explicitly not supported. That can and will break on a regular basis.

So I wouldn't recommend using it to accomplish this since you'd now be in a double-unsupported scenario.

[johnkors]

Thanks!

[johnkors]

Follow up: are there any well known unsupported features when using the .NET 5 SDK, using netcoreapp3.1 & C# 9 combo?

[PathogenDavid]

This is mostly off the top of my head, but here you go:

(Take "Works" with a grain of salt. This is based on my knowledge of the features and in a few cases my limited experience testing this scenario or quick tests. These are not guarantees.)

Feature	Notes
Target-typed new	Works
Skip locals init	Works (Requires manually defining SkipLocalsInitAttribute)
Lambda discard parameters	Works
Native ints	Works (Based on IntPtr so newer stuff there will be missing.)
Attributes on local functions	Works
Function pointers	Partially works (Managed function pointers work, unmanaged do not and are gated behind CS8889.)
Pattern matching improvements	Works
Static lambdas	Works
Records	Works (Init-only properties require defining IsExternalInit)
Target-typed conditional	Works
Covariant Returns	NO Runtime support required, gated behind CS8830
Extension GetEnumerator	Works
Module initializers	Works (Requires manually defining ModuleInitializerAttribute)
Extending Partial	Works
Top-level statements	Works

[PathogenDavid]

Since people found my previous comment helpful and I still find myself linking to it, here's an update for C# 10 features!

(As before, take "Works" with a grain of salt. This is based on my knowledge of the features and brief testing with the .NET 6 RC 1 compiler and .NET Framework 4.8. These are not guarantees, also remember this isn't supported by Microsoft.)

Feature	Notes
Record structs	Works
Global Using Directive	Works
Improved Definite Assignment	Works (This is actually tied to the compiler version, not the language version. As such it already works in C# 8 and 9)
Constant Interpolated Strings	Works
Extended property patterns	Works
Sealed record ToString	Works
Source Generator V2 APIs	Works (This is tied to the compiler version, not the language version)
Mix declarations and variables in deconstruction	Works
Async method builder override	Works (Requires manually defining AsyncMethodBuilderAttribute and any builders you wish to use)
Enhanced #line directive	Works
Lambda improvements	Works
Interpolated string improvements	Works (Requires manually defining InterpolatedStringHandlerAttribute, InterpolatedStringHandlerArgumentAttribute, and any interpolated string handlers you wish to use*)
File-scoped namespace	Works
Parameterless struct constructors	Works (Old runtimes have bug with Activator.CreateInstance(Type) and new() constraint! See dotnet/runtime#6536) -- Expression.Default/Expression.New also have problems, see dotnet/runtime#45397
Caller expression attribute	Works (Requires manually defining CallerArgumentExpressionAttribute)

* Without the framework providing implementations of ISpanFormattable for System.Int32 and such, it will be very tedious to implement a no/low-allocation handler.

---

Additionally, here's features which are still in preview for C# 10:

Feature	Notes
Static Abstract Members In Interfaces	NO Runtime support required, gated behind CS8919/CS8929
Generic attributes	INADVISABLE Older runtime support for generic attributes is VERY spotty, runtime behaviors might change for .NET 7, details below...

Generic attributes can be dangerous to use on older runtimes. To give a few examples:

• Prior to .NET Core 3.0, GetCustomAttributes will throw NotSupportedException when attempting to reflect generic attributes. (GetCustomAttributesData seems to work.)
  • (According to Test plan for "generic attributes" #36285, not even .NET Core 3.0 is safe, sounds like 3.1 is the first "safe" version.)
• Mono does not support querying for and open generic type when searching for Attributes runtime#56887
  • Additionally, depending on how this is resolved the behavior for .NET might change in .NET 7
• Cannot reflect on generic attributes whose constructors original definition uses type parameters runtime#56492 (Fixed in .NET 6)
• C++/CLI will crash when assemblies which contain generic attributes (even if the type isn't used.)
• System.Reflection.Metadata doesn't handle generic attributes runtime#58073 (Scheduled for .NET 7)
• This feature may still get gated behind a runtime check.

[jrmoreno1]

You wouldn't happen to have a list of features for C#8 would you?