Support for CSharp 9

[danielmarbach]

• Record types
• Init accessor (?)

[stakx]

I'll add another to the list:

• Map System.IntPtr and System.UIntPtr to the new type keywords nint and nuint (?)

This is debatable. System.IntPtr has traditionally been used to represent an unmanaged pointer in interop scenarios. nint doesn't transport those semantics quite as well, it looks more like any regular integer type. I am not sure what we should do about these keywords.

Init accessor

This would be reasonably straightforward as the setter's return type gets annotated with a System.Runtime.CompilerServices.IsExternalInit modreq.

This would be a prerequisite for...

Record types

Sensible support for record is currently very difficult for several reasons:

• There is no officially sanctioned way of distinguishing records from classes (see Records: Metadata determination of whether a type is a record dotnet/csharplang#4121). We can infer record from the presence of a method named <Clone>$, but that is likely implementation-specific to the Roslyn compiler.

• The compiler provides default implementations for a whole bunch of methods if user code doesn't provide them. Ideally, we would filter those out in the generated output. Unfortunately, that isn't feasible, as it is currently impossible to distinguish compiler-generated from user-provided methods; see record Equals/GetHashCode - use CompilerGenerated attribute dotnet/roslyn#47613 (comment).

I think we should start by focusing on init-only setters.

[Shane32]

As of 5/17/2022, Visual Studio 17.2.1 and/or SDK 6.0.300 uses the CompilerGenerated attribute for more methods of record types which changes the output of PublicApiGenerator. See:

• record Equals/GetHashCode - use CompilerGenerated attribute dotnet/roslyn#47613
• Add CompilerGeneratedAttribute to record members dotnet/roslyn#58542

In my application, these methods of struct record types are not listed anymore in the output:

• Deconstruct
• Equals (both typed and object overload)
• GetHashCode
• ToString
• operator !=
• operator ==

Additionally, for class record types, these methods are not listed in the output:

• PrintMembers
• protected constructor for cloning
• public clone method (named <Clone>$)

[sungam3r]

Also see #247

[stakx]

As with IntPtr vs. nint (see above), I am also wondering whether record types should actually be rendered using the record keyword or simply as the class-es that they really are (see also the discussion at dotnet/csharplang#4121 (comment)). Any opinions on this?

[sungam3r]

I think that no one wants to see all that internal default stuff for records : <Clone>$, PrintMembers, etc. I would like to see only overridden methods and record keyword instead of class.

[sungam3r]

Actually all that stuff is filtered out now by CompilerGenerated attribute. OK. Another argument - what if one has switched from record to class or vice versa? It can be a breaking change for consumers that override some method so I would like to see when it happens.

[sungam3r]

  // in core library
        public record User(string login, string password); // switch to User1

        public class User1 : System.IEquatable<User1>
        {
            public User1(string login, string password) { }

            public string login { get; init; }

            public string password { get; init; }
        }

        // in some external library
        public record Admin(string login, string password) : User(login, password)
        {
            protected override bool PrintMembers(StringBuilder builder) // BOOM after swithing from record to class for User
            {
                builder.Append("BlaBlaBla");
                return base.PrintMembers(builder);
            }
        }

User and User1 APIs are equal.

[sungam3r]

Well, I found a bug. See PublicApiGeneratorTests.Record. This test has wrong expectedOutput if we treat User as class (for now we do). It should contain public bool Equals(User other) but this method is filtered out by CompilerGenerated attribute. The fix for that situation - switch class keyword to record!