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The Wire · Showcase

UNIONS LAND IN ROSLYN, RUNTIME TIGHTENS CDAC APIS

By RepoJournal · Filed · About .NET

C# unions just shipped to main in Roslyn [ref:3], while the runtime team hardened debugger contracts and cut WebAssembly build times.

The second wave of union implementation merged into Roslyn main [1] with full test coverage and championed proposal backing. This is the feature teams have been waiting for. In parallel, Roslyn squashed a critical LSP progress reporting bug [2] where project initialization would hang indefinitely because the API didn't auto-close work-done progress on dispose. Razor script tag formatting got fixed [3], cleaning up a long-standing indentation issue. Over in runtime, the team implemented four new DacDbi cDAC APIs [4] to simplify generic type context resolution, then fixed a gnarly bug where early debugger attachment would miss GC contract descriptors entirely [5]. WebAssembly builds got faster: incremental MSBuild targets now skip expensive Webcil conversion and BootJson generation on no-op rebuilds [6]. MAUI cleared house by removing the dormant ci-doctor workflow and pushing all agentic workflows to v0.72.1 [7], tightening automation across the board.

Action items

References

  1. [1] Merge Unions to main ↗ dotnet/roslyn
  2. [2] Fix progress reporting to report done on dispose ↗ dotnet/roslyn
  3. [3] Fix single line script tag formatting ↗ dotnet/roslyn
  4. [4] Implement DacDbi cDAC APIs and simplify generic type context APIs ↗ dotnet/runtime
  5. [5] [cDAC] Reparse pending sub-descriptors on Flush ↗ dotnet/runtime
  6. [6] [browser] WebAssembly SDK targets more incremental ↗ dotnet/runtime
  7. [7] Remove ci-doctor, upgrade all gh-aw workflows to v0.72.1 ↗ dotnet/maui

FAQ

What changed in .NET on May 16, 2026?
C# unions just shipped to main in Roslyn , while the runtime team hardened debugger contracts and cut WebAssembly build times.
What should .NET teams do about it?
Test union syntax in Roslyn main - this is ready for production validation • Pull WebAssembly incremental build targets upgrade to cut your rebuild times • Monitor cDAC sub-descriptor fix if you're using early debugger attachment
Which .NET repositories shipped on May 16, 2026?
dotnet/roslyn, dotnet/runtime, dotnet/maui

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