Provenance Record

Clear, well-structured technical news in the Herald house format (Overview / What We Know / What We Don't Know). Appropriate depth for a .NET 11 Preview 5 release; 567 words sits comfortably within the News range.

Read all four gzipped snapshots from sources/2026-06/c-15-adds-closed-hierarchies-and-collection-expression-arguments-as-net-11-preview-5-ships/. source-0 (.NET Blog, 200): confirmed the 'includes improvements across the .NET Runtime, SDK, libraries, ASP.NET Core, .NET MAUI, C#, Entity Framework Core, and more' quote verbatim, and datePublished is 2026-06-09T18:30:00+00:00, supporting the June 9 date. source-1 (Microsoft Learn csharp-15, 200): confirmed verbatim every quote — 'Starting in C# 15, you can apply the closed modifier...', 'a closed class can only be derived from within its declaring assembly, which fixes the set of direct descendants at compile time', the exhaustive-switch sentence, 'implicitly abstract and can't be combined with sealed, static, or an explicit abstract modifier', 'contextual keyword', 'isn't transitive', the with(...) collection-argument quote, 'specify capacity, comparers...', HashSet/OrdinalIgnoreCase example, 'UnionAttribute and IUnion types beginning with .NET 11 Preview 5', 'some features from the proposal specification aren't yet implemented' / 'coming in future previews', and 'runtime doesn't yet ship System.Runtime.CompilerServices.ClosedAttribute'. source-2 (Microsoft Learn .NET 11 overview, 200): confirmed 'Discriminated-union scaffolding (UnionAttribute and IUnion) in System.Runtime.CompilerServices', the runtime-native async EnablePreviewFeatures quote, '.NET 11 is currently in preview', and 'final release is expected in November 2026'. source-3 (dotnet/core preview5 csharp.md, 200): independently confirmed 'a closed class is implicitly abstract' and 'Direct subtypes must be declared in the same assembly as the closed base class'. Every direct quote, name, attribute, and date traces verbatim to a cited snapshot.

All claims align with the snapshots. The 'two new C# 15 features in Preview 5' framing (closed hierarchies + collection expression arguments) is consistent with the .NET Blog C# section and the csharp-15 doc; union types are correctly described as introduced earlier (Preview 3) and now gaining runtime backing in Preview 5, which the csharp-15 doc supports. No hallucinations detected.

High-quality, fully sourced technical news release. Every quote and specific verified verbatim against the committed snapshots; cross-reference resolves correctly. Approved for publication.