Available C# and .NET features

This document is to supplement BizHawk development, and will be deleted once the project is on .NET Core. Yoshi will maintain a copy on GitLab for the benefit of other projects.

BizHawk-specific notes:

• All projects in /src use C# 12 currently, as do all the .NET projects in /External*Projects but new features have only been adopted in the main solution.
• In the added "convention" column, I've used required/disallowed/encouraged/discouraged/allowed like RFC 2119's MUST / MUST NOT / SHOULD / SHOULD NOT / MAY, respectively. Unsupported is also MUST NOT (because attempting to use the feature will result in an error).
• Each project's target (Framework vs. Standard) is at the top of its project file, or you can check the project graph.

Legend:
✔️ Available
⭕ Available with polyfill
❌ Not available
? Unknown
Some features are marked ✔️ despite ostensibly needing a polyfill because they are enhancements to existing features and work anywhere the base feature is available—for example, switching on a Span<char> requires no additional polyfill.
The official BCL type reference now has a "netstandard2.0 w/ polyfills" filter, though it obviously won't have third-party polyfills.

🔵 Availability in .NET Framework 4.8 (net48)
🟢 Availability in .NET Standard 2.0 (netstandard2.0)
I have not considered Framework 4.7.2 and below as there is little reason not to upgrade to Framework 4.8. I have not considered Framework 4.8.1 because it matches Framework 4.8 in terms of language features and is generally not useful. I use Standard 2.0 and not Standard 2.1 as the latter is not subsumed by Framework 4.8, rendering it useless.

Note: .NET calls destructuring "deconstructing", not to be confused with destructing which .NET calls "finalising".

Feature	🔵 net48	🟢 ns2.0	convention for main BizHawk solution
prop backing field as field	✔️	✔️	waiting for .NET 10 LTS
^ C# 14 ^	🔵 net48	🟢 ns2.0	---
partial props/indexers	✔️	✔️️	waiting for .NET 10 LTS
allows ref struct constraint	❌	❌	unsupported
interfaces for ref structs	✔️	✔️️	discouraged (very useless)
ref structs and unsafe in state-machine methods	?	?	presumably unsupported
escape sequence for ␛ char	✔️	✔️️	waiting for .NET 10 LTS
sugar for lock with Lock	⭕	⭕	should be able to use polyfill w/ C# 12
enhanced params	✔️	✔️	waiting for .NET 10 LTS
^ C# 13 ^	🔵 net48	🟢 ns2.0	---
[UnsafeAccessor]	❌	❌	unsupported
[InlineArray]	❌	❌	unsupported
using aliases for tuples, etc.	✔️	✔️	disallowed (use a struct instead of a tuple alias and we'll consider replacing them with <Using/> later)
default arguments for lambdas	✔️	✔️	discouraged (you should be using local methods)
ref readonly parameters	✔️	✔️	allowed
unified Span/Array/List init syntax	✔️	✔️	encouraged
primary constructors on non-records	✔️	✔️	encouraged
^ C# 12 ^	🔵 net48	🟢 ns2.0	---
file access modifier	✔️	✔️	allowed
simple ref fields	❌	❌	unsupported
required props	⭕	⭕	disallowed
Encoding.UTF8.GetBytes shorthand	✔️	✔️	encouraged
enhanced nameof	✔️	✔️	encouraged
pattern matching for Span<char>	✔️	✔️	encouraged
Kotlin-like raw string literals	✔️	✔️	allowed
list pattern matching	✔️	✔️	encouraged
multi-line expressions in interpolated strings	✔️	✔️	discouraged
generic maths using static abstract members	❌	❌	unsupported
genericised attributes	✔️	✔️	allowed
^ C# 11 ^	🔵 net48	🟢 ns2.0	---
per-method AsyncMethodBuilder	?	?	unsupported
enhanced null analysis	✔️	✔️	allowed
enhanced destructuring	✔️	✔️	encouraged
sealed ToString in records	✔️	✔️	encouraged when records are used
limited string interpolation in consts	✔️	✔️	encouraged
attributes for lambdas	✔️	✔️	allowed
type inference for lambdas	✔️	✔️	allowed
pattern matching IV	✔️	✔️	encouraged
namespace A;	✔️	✔️	disallowed (will mass-migrate)
global using A;/<Using>	✔️	✔️	disallowed (will mass-migrate)
custom interpolated string handlers	⭕	⭕	allowed
with for structs	✔️	✔️	allowed
enhanced struct field init	✔️	✔️	discouraged
record struct	✔️	✔️	discouraged
^ C# 10 ^	🔵 net48	🟢 ns2.0	---
enhanced partial methods	✔️	✔️	allowed
[ModuleInitializer] method	⭕	⭕	discouraged
attributes on local methods	✔️	✔️	allowed
discarding lambda parameters	✔️	✔️	encouraged
foreach picks up extension GetEnumerators	✔️	✔️	discouraged (surely this can only be used for stupid)
covariant return type when overriding	❌	❌	unsupported
enhanced type inference	✔️	✔️	omit explicit type cast where possible, otherwise place the cast on default branch of switch and first branch of ternary
static lambdas	✔️	✔️	encouraged
target-typed new()	✔️	✔️	encouraged
basic function pointers	✔️	✔️	encouraged
nint/nuint keywords	✔️	✔️	allowed
pattern matching III	✔️	✔️	encouraged
unindented Main	✔️	N/A	N/A (neither executable uses it)
with for records	✔️	✔️	encouraged when records are used
init	⭕	⭕	discouraged
record class	✔️	✔️	discouraged
^ C# 9 ^	🔵 net48	🟢 ns2.0	---
@$"" (instead of $@"")	✔️	✔️	disallowed
stackalloc as arg for Span param	✔️	✔️	allowed
??=	✔️	✔️	encouraged
Index and Range (^ and .. operators)	⭕	⭕	discouraged (we also have a generic Range<T: unmanaged, IComparable<T>> but it has some problems, like not working with ..)
async streams	⭕	⭕	unknown
NRTs (attribute-based analysis)	⭕	⭕	encouraged
NRTs (syntax and basic analysis)	✔️	✔️	encouraged for new files, see project graph for when #nullable enable is needed
static local methods	✔️	✔️	encouraged
using statement without block	✔️	✔️	encouraged
pattern matching II	✔️	✔️	encouraged
switch expression	✔️	✔️	encouraged
default interface methods	❌	❌	unsupported
readonly methods/getters/setters	✔️	✔️	encouraged
^ C# 8 ^	🔵 net48	🟢 ns2.0	---
field attribute target for auto-prop backing field	✔️	✔️	discouraged (surely this can only be used for stupid)
stackalloc with array intialiser	✔️	✔️	encouraged
^ C# 7.3 ^	🔵 net48	🟢 ns2.0	---
Span and co.	⭕	⭕	encouraged
ref struct (stack-bound)	✔️	✔️	allowed
readonly struct and in parameters	✔️	✔️	encouraged
^ C# 7.2 ^	🔵 net48	🟢 ns2.0	---
inferred tuple field names	✔️	✔️	discouraged
default without type	✔️	✔️	encouraged for non-nullable value types, or as default! to appease compiler when definitely assigned, discouraged otherwise
async Main	?	?	N/A (neither executable uses it)
^ C# 7.1 ^	🔵 net48	🟢 ns2.0	---
throw expression	✔️	✔️	encouraged
enhanced int literals	✔️	✔️	encouraged
discards	✔️	✔️	encouraged
ref returns/locals	✔️	✔️	allowed
expression-bodied constructors	✔️	✔️	encouraged
local methods	✔️	✔️	preferred over lambdas/delegates if used multiple times or to unsub from event
basic pattern matching	✔️	✔️	encouraged
KeyValuePair<K, V>.Deconstruct	⭕	⭕	allowed
basic tuples and destructuring	✔️	✔️	encouraged
out var	✔️	✔️	encouraged
^ C# 7.0 ^	🔵 net48	🟢 ns2.0	---
nameof	✔️	✔️	encouraged
interpolated string literals	✔️	✔️	preferred over string.Format or concatenation
null-conditional member access	✔️	✔️	required
expression-bodied methods/props	✔️	✔️	encouraged
inline initialisation of auto-props	✔️	✔️	encouraged
catch (Exception e) when (predicate(e))	✔️	✔️	encouraged
using static A;	✔️	✔️	allowed only for the file's own namespace
^ C# 6 ^	🔵 net48	🟢 ns2.0	---
[Caller*]	✔️	✔️	allowed
async	✔️	✔️	allowed
^ C# 5 ^	🔵 net48	🟢 ns2.0	---
covariant and contravariant generics	✔️	✔️	encouraged
default arguments	✔️	✔️	preferred over overloads
named arguments	✔️	✔️	encouraged for primitive-typed parameters or when 2+ parameters are of the same type
dynamic type	✔️	⭕	discouraged
^ C# 4 ^	🔵 net48	🟢 ns2.0	---
object initialisation syntax	✔️	✔️	encouraged
simple partial methods	✔️	✔️	allowed
var	✔️	✔️	preferred except when target-typed new() can be used
extension methods	✔️	✔️	allowed
Expression trees	✔️	✔️	discouraged (what are these even for)
lambdas	✔️	✔️	preferred over delegates
LINQ's query expression syntax	✔️	✔️	disallowed
anonymous classes	✔️	✔️	disallowed (use tuples)
auto-props	✔️	✔️	encouraged
^ C# 3 ^	🔵 net48	🟢 ns2.0	---
anonymous delegates	✔️	✔️	disallowed
^ C# 2 ^	🔵 net48	🟢 ns2.0	---
using alias	✔️	✔️	discouraged unless there's a conflict
delegate constructors	✔️	✔️	disallowed
unsafe (pointers etc.)	✔️	✔️	discouraged

The official C# version history is here, with this separate list for vNext.