csharplang/meetings/2020/LDM-2020-07-13.md

C# Language Design Meeting for July 13th, 2020

Agenda

1. Generics and generic type parameters in aliases
2. "closed" enum types
3. Allow ref assignment for switch expressions
4. Null suppressions nested under propagation operators
5. Relax rules for trailing whitespaces in format specifier
6. Private field consideration in structs during definite assignment analysis
7. List patterns
8. Property-scoped fields and the field keyword
9. File-scoped namespaces
10. Allowing ref/out on lambda parameters without explicit type
11. Using declarations with a discard
12. Allow null-conditional operator on the left hand side of an assignment
13. Top level statements and functions
14. Implicit usings

Quote of the day

"It does actually seem like this pattern is the 98% case..." "I just want to disagree with something [redacted] said... [they] said [they] thought it was the 98% case that this would apply to, and I think it's 99..." "Not fair, I was going to say that."

Discussion

This meeting was issue triage. There will not be much detail on any particular issue, just a general summary of the LDM's feeling and the milestone that we triaged to.

Generics and generic type parameters in aliases

https://github.com/dotnet/csharplang/issues/1239

In general, we want to make improvements in this area. We should also be open to making other enhancements in this space, such as allowing tuple syntax on the right side, or allowing C# keywords like int. There's also a set of features that we should investigate in parallel, such as globally-visible aliases or publicly- exported aliases.

Conclusion

Triaged for C# 10 for now so that we can look at the space hollistically in the near term.

"closed" enum types

https://github.com/dotnet/csharplang/issues/3179

This is a solution to the very common complaints around enum usage, particularly in switch expressions and other exhaustiveness scenarios. It will also work well with DUs, and we should make sure that whatever syntax we use there works well with closed or sealed enum types as well.

Conclusion

Triaged for C# 10 to look at in coordination with DUs.

Allow ref assignment for switch expressions

https://github.com/dotnet/csharplang/issues/3326

We like this proposal. You can already do this with ternaries, and it's odd that you can't do this with switch expressions as well. It's not high priority, but we'd be happy to see it in the language. Like ternaries, the ref version would not be target-typed.

Conclusion

Triaged for Any Time.

Null suppressions nested under propagation operators

https://github.com/dotnet/csharplang/issues/3393

This will be a breaking change, so we need to get it in as soon as possible.

Conclusion

C# 9 timeframe. We need to make a syntax proposal and approve it.

Relax rules for trailing whitespaces in format specifier

https://github.com/dotnet/csharplang/issues/3414

In reading this proposal, we're unsure whether the request was for ignoring trailing spaces in format specifiers, or allowing them and including them in the format specifier. We think that either way this is interpreted, it will be confusing: trailing spaces are allowed in front of an interpolated expression and mean nothing, but for things like DateTime, spaces are valid parts of a format specifier and will be respected in the output. We think that either behavior here would be confusing, with no clear indication on which the user wants.

Conclusion

Rejected.

Private field consideration in structs during definite assignment analysis

https://github.com/dotnet/csharplang/issues/3431

This is a bug from the native compiler that had to be reimplemented in Roslyn for compat, but it's always been one of the prime candidates for a warning wave (and is already supported as an error in the compiler via the strict feature flag). There is some contention whether it should be a warning or an error in this wave. There is also a concern that this will combine with the SkipLocalsInit feature in C# 9 to expose an uninitialized memory hole in C# with no use of unsafe or System.Unsafe required.

Conclusion

We need to look at this ASAP to make sure that we don't unintentionally expose unsafety in the language without user intention. We'll schedule this for next week.

Remove restriction that optional parameters must be trailing

https://github.com/dotnet/csharplang/issues/3434

It's possible to define such methods in metadata with the correct attribute usage and call them from C# today. This would just be about removing the restriction from defining them in C#.

Conclusion

Triaged for Any Time.

List patterns

https://github.com/dotnet/csharplang/issues/3435

We have an open proposal that with a prototype. We like the general direction it takes, but we need to have more detailed design review of it and possibly make a few different decisions. This is a direction we want to take pattern matching in future releases, so we'll continue iterating on this.

Conclusion

Triaged into C# 10 for design work.

Property-scoped fields and the field keyword

• https://github.com/dotnet/csharplang/issues/133
• https://github.com/dotnet/csharplang/issues/140

We've started doing design work around both of these issues. We should continue doing that.

Conclusion

Triaged into C# 10.

File-scoped namespaces

https://github.com/dotnet/csharplang/issues/137

This has been a large request for a long time that reflects the default of almost every single C# file written. There's still some design work to do, particularly in how that will affect using statements, but it's work that we should take on.

Conclusion

Triaged into C# 10.

Allowing ref/out on lambda parameters without explicit type

https://github.com/dotnet/csharplang/issues/338

We've heard this request a few times, and while we don't see any particular issue with this, it's not a priority for the team at this point. If a spec/implementation was provided we'd likely accept it but won't go out of our way to add it unless something else changes here.

Conclusion

Triaged to Any Time.

Using declarations with a discard

https://github.com/dotnet/csharplang/issues/2235

There's still some open design questions here, particularly around possibly ambiguous syntaxes. We need to keep iterating on this to find a design that we like.

Conclusion

Triaged for X.X, pending a new proposal.

Allow null-conditional operator on the left hand side of an assignment

https://github.com/dotnet/csharplang/issues/2883

Initial discussion shows a big LDT split on whether ?. should be able to effectively a conditional ref. We would need to discuss further, but aren't ready to outright approve or reject the feature.

Conclusion

Triage to X.X for future discussion.

Top level statements and functions

https://github.com/dotnet/csharplang/issues/3117

This is part 2 of the work we started in C# 9 with top-level statements: designing free-floating functions that can sit on the top level without any containing type. We need to continue examining this context of scripting unification, which we plan to continue doing on an ongoing basis.

Conclusion

Triage into C# 10 for ongoing discussion.

Implicit usings

https://github.com/dotnet/csharplang/issues/3428

This is another issue arising of discussions on top-level statements and using C# as a scripting language: attempting to make the language more brief by specifying common imports on the command line or as part of the project file is certainly one way to remove boilerplate. However, it's a controversial feature that often draws visceral reactions. We need to discuss this more and figure out if it's a feature we want to have in C#, taking into account our increased focus on scripting-like scenarios. It's worth noting that CSX has a limited form of this support already, and implementing this in C# proper would bring us one step closer to unifying the two dialects.

Conclusion

Triage to C# 10 for discussion.