[release-notes] C# in .NET 11 RC 1

release-notes/11.0/preview/rc1/csharp.md

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+# C# in .NET 11 RC 1 - Release Notes
+
+.NET 11 RC 1 establishes C# 15 as the default language version and includes the
+following C# updates:
+
+- [C# 15 stabilizes unions and other language features](#c-15-stabilizes-unions-and-other-language-features)
+- [Unsafe Evolution refinements](#unsafe-evolution-refinements)
+
+C# updates:
+
+- [What's new in C# 15](https://learn.microsoft.com/dotnet/csharp/whats-new/csharp-15)
+
+## C# 15 stabilizes unions and other language features
+
+C# 15 is now selected by default for projects targeting .NET 11
+([dotnet/roslyn #84799](https://github.com/dotnet/roslyn/pull/84799)). The
+language version stabilizes the features introduced during the .NET 11 preview
+cycle: collection expression arguments, union types, non-virtual static
+interface members, closed class hierarchies, labeled `break` and `continue`,
+and extension indexers.
+
+For example, union types no longer require `<LangVersion>preview</LangVersion>`:
+
+```csharp
+public record Success(string Message);
+public record Failure(int ErrorCode);
+
+public union Result(Success, Failure);
+
+static string Describe(Result result) => result switch
+{
+    Success(var message) => message,
+    Failure(var errorCode) => $"Error {errorCode}"
+};
+```
+
+The compiler and Roslyn APIs also use the final `CSharp15` language-version
+names, and `ITypeSymbol.UnionCaseTypes` exposes the cases of a union to analyzers
+and other compiler-based tools
+([dotnet/roslyn #84707](https://github.com/dotnet/roslyn/pull/84707)).
+
+## Unsafe Evolution refinements
+
+> This is a preview feature for .NET 11.
+
+Unsafe Evolution remains independent of C# 15 and still requires
+`<LangVersion>preview</LangVersion>`. RC 1 completes several user-facing rules:
+
+- `await` is allowed inside an `unsafe` context
+  ([dotnet/roslyn #84616](https://github.com/dotnet/roslyn/pull/84616)).
+- `safe` can be applied anywhere that `unsafe` could mark a declaration as
+  *requires-unsafe*. This supports generated declarations such as
+  `LibraryImport` methods, even when `safe` has no other effect
+  ([dotnet/roslyn #84602](https://github.com/dotnet/roslyn/pull/84602)).
+- `unsafe` on delegates, static constructors, destructors, and type
+  declarations is now an error because it no longer establishes a meaningful
+  unsafe context
+  ([dotnet/roslyn #84378](https://github.com/dotnet/roslyn/pull/84378)).
+- Unsafe Evolution diagnostics are now reported consistently in the IDE
+  ([dotnet/roslyn #84603](https://github.com/dotnet/roslyn/pull/84603)).
+
+```csharp
+unsafe async Task<int> ReadAsync()
+{
+    int* value = stackalloc int[1];
+    *value = 42;
+    int result = *value;
+    await Task.Yield();
+    return result;
+}
+
+[LibraryImport("native")]
+internal static safe partial int GetValue();
+```
+
+Roslyn also exposes the final approved symbol API for determining whether a
+declaration requires an unsafe context
+([dotnet/roslyn #84674](https://github.com/dotnet/roslyn/pull/84674),
+[dotnet/roslyn #84784](https://github.com/dotnet/roslyn/pull/84784)).
+
+<!-- Filtered features (significant engineering work, but not shipped in RC 1):
+  - Sonic declaration/implementation split: the merge and dependent work were
+    reverted by dotnet/roslyn #84831, so no Sonic feature is documented here.
+-->