Make classes sealed by default

[mostmand]

In C# each class in inheritable unless it is defined as sealed. Is it possible to make classes sealed as default unless defined as inheritable? Maybe a feature flag in csproj file to opt in could do the trick.

Additional context

Jon Skeet's response to a related question
Sealed classes are more performant than normal classes

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[KalleOlaviNiemitalo]

Apart from C# language changes: There is already an analyzer that shows which classes could be safely sealed. The project templates in .NET SDK could be changed to seal classes.

[KalleOlaviNiemitalo]

C# language change was shot down in dotnet/csharplang#6506 (comment).

[KalleOlaviNiemitalo]

The analyzer was added in dotnet/roslyn-analyzers#5594 to fix dotnet/runtime#49944. The code analysis warning CA1852 is not yet documented at https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/.

[martincostello]

Looks like this analyzer landed in 7.0.101, which causes false positives for top-level programs.

See #29164 (comment).

[baronfel]

Closing this as the language change was rejected and an analyzer is available for users that want it.