csharp-15

Write, review and modernize C# 15 (.NET 11) code correctly. Covers every C# 15 feature - collection expression arguments ([with(...), ...]), union types (union, [Union], IUnion, custom unions), closed class hierarchies (closed), extension indexers, labeled break/continue, non-virtual static interface members on runtimes without default interface methods, and the preview memory-safety model (unsafe(expr), safe, requires-unsafe members). Use when code targets net11.0 or LangVersion 15/preview, when the user mentions any of these features, when replacing marker interfaces, OneOf-style results, goto/flag loop exits or capacity/comparer boilerplate, or when the build reports CS8121, CS8509, CS8655, CS8510, CS0121/CS0411 on a with(...) element, CS9370, CS9380 or CS9363.