[release-notes] C# in .NET 11 Preview 4

release-notes/11.0/preview/preview4/csharp.md

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+# C# in .NET 11 Preview 4 - Release Notes
+
+.NET 11 Preview 4 includes the following C# language and compiler updates:
+
+- [Unsafe Evolution adds support for unsafe property accessors](#unsafe-evolution-adds-support-for-unsafe-property-accessors)
+- [Clearer diagnostic for misplaced `#!` shebang directives](#clearer-diagnostic-for-misplaced--shebang-directives)
+- [Opt-in compilation cache for the VBCSCompiler build server](#opt-in-compilation-cache-for-the-vbcscompiler-build-server)
+
+C# updates:
+
+- [What's new in C# 14](https://learn.microsoft.com/dotnet/csharp/whats-new/csharp-14)
+
+## Unsafe Evolution adds support for unsafe property accessors
+
+> This is a preview feature for .NET 11.
+
+Unsafe Evolution is the ongoing work to let `unsafe` denote *requires-unsafe* members directly, rather than relying on a separate `[RequiresUnsafe]` attribute. Preview 4 extends that work to property accessors, so individual `get` and `set` accessors can now carry the `unsafe` modifier ([dotnet/roslyn #83115](https://github.com/dotnet/roslyn/pull/83115)).
+
+```csharp
+public class Buffer
+{
+    private byte[] _data = new byte[16];
+
+    public unsafe byte* Pointer
+    {
+        get { fixed (byte* p = _data) return p; }
+    }
+
+    public int First
+    {
+        get => _data[0];
+        unsafe set
+        {
+            fixed (byte* p = _data) { *p = (byte)value; }
+        }
+    }
+}
+```
+
+Tracked under the [Unsafe Evolution test plan](https://github.com/dotnet/roslyn/issues/81207); the corresponding language-spec change is in [dotnet/csharplang #10091](https://github.com/dotnet/csharplang/pull/10091). Unsafe Evolution remains a preview feature; the surface and rules can still change before it ships.
+
+## Clearer diagnostic for misplaced `#!` shebang directives
+
+When a `#!` shebang directive appeared anywhere except the start of the file, the compiler reported `CS1040` ("Preprocessor directives must appear as the first non-whitespace character on a line"). That message is misleading — the shebang *is* the first character on its line; the real problem is that it isn't on line 1.
+
+The compiler now reports a dedicated error, `CS9378`, with a message that points directly at the actual rule ([dotnet/roslyn #83112](https://github.com/dotnet/roslyn/pull/83112)).
+
+```csharp
+class Foo { }
+#!/usr/bin/env dotnet
+// error CS9378: '#!' must be the first characters on the first line of the file
+```
+
+`CS9378` also covers leading whitespace before `#!` and whitespace between `#` and `!`, which previously surfaced as the same misleading `CS1040`. This makes shebang errors easier to act on when authoring file-based C# apps.
+
+## Opt-in compilation cache for the VBCSCompiler build server
+
+The C#/VB build server (`VBCSCompiler`) now supports an opt-in file-system compilation cache. When a compilation has the same deterministic key as a previous one, the server restores the cached outputs instead of running the full compile and emit pipeline ([dotnet/roslyn #82881](https://github.com/dotnet/roslyn/pull/82881)).
+
+The cache is enabled by setting the `ROSLYN_CACHE_PATH` environment variable, or by passing the `use-global-cache` feature flag through MSBuild. Notes on the current shape:
+
+- The cache key is the existing deterministic compilation key, so cached outputs only replay for byte-identical inputs.
+- Cache eviction is not implemented yet — managing the cache directory is the user's responsibility.
+- Side effects such as compiler warnings and `/reportanalyzer` output are not cached and will not be replayed from a hit.
+
+This is most useful for CI scenarios that re-build the same projects with stable inputs. Treat it as experimental for this preview.
+
+<!-- Filtered features (significant engineering work, but not language/compiler product news for this preview):
+  - File-based apps automatic discovery (dotnet/roslyn #82863): Area-IDE / VSCode "Run File" tooling. Belongs in tooling notes.
+  - Preserve `#:` directives during formatting (dotnet/roslyn #82996): formatter/IDE behavior, not a language change.
+  - Fix with-element indentation in collection expressions on multiple lines (dotnet/roslyn #83030): formatter/IDE.
+  - Fixers: bounds checks and boundary \r/\n stripping (dotnet/roslyn #83098): IDE code fixer.
+  - LSP TypeHierarchy support, Call Hierarchy fixes, completion behavior, rename, hot reload, EnC, brokered services (numerous PRs): IDE/tooling, not C# language.
+  - fix: raising of CS8618 on static members (dotnet/roslyn #83146): Area-IDE — IDE-side analyzer behavior, not a compiler diagnostic change.
+  - Improve TextLine and line table performance (dotnet/roslyn #83000): internal compiler-host plumbing; no user-visible API or workflow change.
+  - Numerous test ports to Linux, arcade dependency updates, VMR source-code updates, build/CI/loc changes: infrastructure.
+  - VB-only items (e.g. minor cleanups related to attributes in VB, VB EnC ANE fix): not part of the C# notes for this preview.
+-->
+
+## Bug fixes
+
+- **C# compiler**
+  - Fixed ref safety for collection expressions whose target type is an `IEnumerable<T>` of a `ref struct`, so the compiler now correctly enforces ref-struct lifetime rules in this case ([dotnet/roslyn #82401](https://github.com/dotnet/roslyn/pull/82401)).
+
+## Community contributors
+
+Thank you contributors! ❤️
+
+- [@CyrusNajmabadi](https://github.com/dotnet/roslyn/pulls?q=is%3Apr+is%3Amerged+author%3ACyrusNajmabadi)
+- [@DoctorKrolic](https://github.com/dotnet/roslyn/pulls?q=is%3Apr+is%3Amerged+author%3ADoctorKrolic)
+- [@dusrdev](https://github.com/dotnet/roslyn/pulls?q=is%3Apr+is%3Amerged+author%3Adusrdev)
+- [@Thomas-Shephard](https://github.com/dotnet/roslyn/pulls?q=is%3Apr+is%3Amerged+author%3AThomas-Shephard)