Records with sealed base ToString override

• Proposed
• Prototype: Done
• Implementation: Done
• Specification: Not Started

Summary

This proposal would lift the restriction on declaring sealedToString overrides in non-sealed records and lift the restriction on a record inheriting from a base type that has a sealedToString override. The semantics ofsealed override are clear, they are appropriate for closed hierarchies whereToString is controlled by the base, and the current errors seem to deny its otherwise consistent functioning out of an overabundance of caution.

Motivation

There's currently no way to specify aToString implementation once for all derived records. Because of this, the need to overrideToString can have a jarring result. Consider this hand-rolled discriminated union based on records:

publicabstractrecordSomeDiscriminatedUnion(stringName){publicsealedrecordFirstKind(stringName,intA):SomeDiscriminatedUnion(Name);publicsealedrecordSecondKind(stringName,intB):SomeDiscriminatedUnion(Name);publicsealedrecordThirdKind(stringName,boolC):SomeDiscriminatedUnion(Name);publicsealedrecordFourthKind(stringName):SomeDiscriminatedUnion(Name);}

OverridingToString has no effect on derived records unless each derived record is given a manual override that returnsbase.ToString(). Sealing the override would perfectly describe the intended result, but this is currently blocked by the compiler with errors CS0239 and CS8870.

The simplest option currently available is much less pleasing. One factor is its repetition. Another factor is that the base record no longer contains a readable list of single-line members.

publicabstractrecordSomeDiscriminatedUnion(stringName){publicsealedrecordFirstKind(stringName,intA):SomeDiscriminatedUnion(Name){publicoverridestringToString()=>Name;}publicsealedrecordSecondKind(stringName,intB):SomeDiscriminatedUnion(Name){publicoverridestringToString()=>Name;}publicsealedrecordThirdKind(stringName,boolC):SomeDiscriminatedUnion(Name){publicoverridestringToString()=>Name;}publicsealedrecordFourthKind(stringName):SomeDiscriminatedUnion(Name){publicoverridestringToString()=>Name;}}

Detailed design

Non-sealed records would support sealed overrides ofToString the same way classes do rather than producing error "CS8870 'BaseRecord.ToString()' cannot be sealed because containing record is not sealed."

Derived records would support inheriting when the baseToString is sealed by skipping the synthesizedToString override rather than producing error "CS0239 'DerivedRecord.ToString()': cannot override inherited member 'BaseRecord.ToString()' because it is sealed."

Drawbacks

Alternatives

A community source generator could be developed that would look for an attribute on the base record or the base record'sToString override and addpublic override string ToString() => base.ToString(); to each derived record that doesn't already declare aToString member. This feels like a workaround at best. The semantics ofsealed override are clear, they are appropriate for closed hierarchies whereToString is controlled by the base, and the current errors seem to deny its otherwise consistent functioning out of an overabundance of caution.

Unresolved questions

Design meetings

https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-02-10.md#sealed-record-ToString