[release-notes] MSBuild in .NET 11 Preview 4

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

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+# MSBuild in .NET 11 Preview 4 - Release Notes
+
+.NET 11 Preview 4 includes new MSBuild features & enhancements:
+
+- [TaskHost callbacks enabled by default](#taskhost-callbacks-enabled-by-default)
+- [SdkResolver environment variables take precedence over the ambient environment](#sdkresolver-environment-variables-take-precedence-over-the-ambient-environment)
+- [Multithreaded build: more in-box tasks enlightened](#multithreaded-build-more-in-box-tasks-enlightened)
+
+These features are mostly in service of the [Multithreaded execution](https://github.com/dotnet/msbuild/blob/main/documentation/specs/multithreading/multithreaded-msbuild.md) feature, which is still under active development.
+
+## TaskHost callbacks enabled by default
+
+Tasks that run in an out-of-process TaskHost — for example, when a project uses `-mt` (multithreaded) builds or when a task explicitly opts into `TaskHostFactory` — can now call back into the engine through `IBuildEngine` to build other projects. The `BuildProjectFile`, `BuildProjectFilesInParallel`, and related overloads are forwarded from the TaskHost to the owning worker node, so callbacks behave the same as in-proc builds ([dotnet/msbuild #13350](https://github.com/dotnet/msbuild/pull/13350), [dotnet/msbuild #13579](https://github.com/dotnet/msbuild/pull/13579)).
+
+The most visible impact is WPF: XAML compilation uses `GenerateTemporaryTargetAssembly`, which calls `BuildProjectFile` from a TaskHost. Before this change, building any WPF project with `-mt` failed with `MSB5022`. With Preview 4, this works without any opt-in.
+
+```shell
+dotnet build MyWpfApp.csproj -mt
+
+The previous opt-in environment variable (`MSBUILDENABLETASKHOSTCALLBACKS=1`) is no longer required; the feature is on by default.
+
+## SdkResolver environment variables take precedence over the ambient environment
+
+When an SDK resolver provides environment variables for a project, those values now win over variables of the same name in the ambient process environment ([dotnet/msbuild #12655](https://github.com/dotnet/msbuild/pull/12655)).
+
+Previously, an ambient environment variable would silently override the value supplied by the resolver, which made the resolver an unreliable source of truth for SDK-provided configuration. The new precedence order, from highest to lowest, is:
+
+1. Properties defined in XML
+2. SDK-resolved environment variables (first SDK to set a name wins)
+3. Ambient process environment variables
+
+When an SDK overrides an existing ambient value, MSBuild logs a diagnostic showing both the ambient and the SDK-provided value so the change is auditable. This fix also includes a serialization fix so the SDK-provided variables flow correctly to out-of-proc nodes.
+
+## Multithreaded build: more in-box tasks enlightened
+
+The MSBuild multithreaded build mode (`-mt`) requires built-in tasks to declare that they are safe to run on multiple threads in the same process. Preview 4 enlightens additional in-box tasks so they can participate in `-mt` builds without falling back to a TaskHost:
+
+- `RequiresFramework35SP1Assembly` ([dotnet/msbuild #13575](https://github.com/dotnet/msbuild/pull/13575))
+- `ToolTask` and `Al` ([dotnet/msbuild #13423](https://github.com/dotnet/msbuild/pull/13423))
+- `AspNetCompiler` ([dotnet/msbuild #13424](https://github.com/dotnet/msbuild/pull/13424))
+- `GetReferenceAssemblyPaths` ([dotnet/msbuild #13495](https://github.com/dotnet/msbuild/pull/13495))
+- `SGen` ([dotnet/msbuild #13457](https://github.com/dotnet/msbuild/pull/13457))
+
+No project changes are needed; these tasks now run directly on worker threads when a build uses `-mt`, reducing the number of out-of-process TaskHost handoffs during a multithreaded build.
+
+<!-- Filtered features (significant engineering work, but too niche for release notes):
+  - Migration of unit tests to RoslynCodeTaskFactory / xunit.v3 / various test stabilizations: test-only changes.
+  - Branding bumps, VMR merges, gh-aw / skill / reviewer-agent automation: repo infrastructure.
+  - GetCanonicalForm refactor (#13532), ValueTuple deploy removal (#13507), bootstrap re-sign on macOS (#13513): internal plumbing with no user-visible behavior change.
+  - Localized string check-in (#13437): localization automation.
+  - vs18.6 backport entries: covered by the corresponding main-branch entry already documented.
+-->
+
+## Bug fixes
+
+- **Build engine**
+  - `BuildManager.ShutdownAllNodes()` again cleans up idle `MSBuild.exe` processes — fixes a regression from 18.5 that caused Visual Studio to leave MSBuild processes running after shutdown ([dotnet/msbuild #13501](https://github.com/dotnet/msbuild/pull/13501)).
+  - Fixed a race condition in out-of-process TaskHost path resolution that caused `ArgumentNullException` in `Path.Combine` when multiple `RequestBuilder` threads resolved the task host executable name simultaneously under `-mt` ([dotnet/msbuild #13485](https://github.com/dotnet/msbuild/pull/13485)).
+  - Fixed a `~200 MB` allocation regression in Visual Studio caused by the `TaskItem` modifier cache being copied across AppDomains on .NET Framework ([dotnet/msbuild #13493](https://github.com/dotnet/msbuild/pull/13493)).
+  - Fixed `MSB1025` / `FormatException` when cancelling a build that uses the MSBuild server (`DOTNET_CLI_USE_MSBUILD_SERVER=1`) — caused by a stray `}` in a trace format string ([dotnet/msbuild #13535](https://github.com/dotnet/msbuild/pull/13535)).
+- **Static graph**
+  - Graph build errors for missing or invalid project references now name the referring project: `error MSB4025: The project file '...' could not be loaded when referenced by '<referrer>'.` ([dotnet/msbuild #12672](https://github.com/dotnet/msbuild/pull/12672)).
+- **Terminal Logger**
+  - Fixed a `Debug.Assert` failure for metaproj files (which have no evaluation phase) and a related issue where projects lost their evaluation ID after their `ProjectInstance` was cached or sent across nodes ([dotnet/msbuild #13480](https://github.com/dotnet/msbuild/pull/13480)).
+- **Tasks**
+  - `RoslynCodeTaskFactory` now logs the specific `MSB3753` diagnostic when an inline task class does not implement `ITask`, matching `CodeTaskFactory` behavior ([dotnet/msbuild #13517](https://github.com/dotnet/msbuild/pull/13517)).
+  - ASP.NET Web Site projects that depend on `netstandard` now copy the `netstandard.dll` facade into the site `Bin` folder so `aspnet_compiler` succeeds ([dotnet/msbuild #13058](https://github.com/dotnet/msbuild/pull/13058)).
+  - `GetCopyToOutputDirectoryItemsDependsOn` is appended to instead of overwritten, so projects can add custom targets at a specific build stage without losing the default dependencies ([dotnet/msbuild #13474](https://github.com/dotnet/msbuild/pull/13474)).
+- **Performance**
+  - Eliminated repeated `XmlChildNodes` allocations in `GetXmlNodeInnerContents`, a hot path during property and metadata evaluation on solution load ([dotnet/msbuild #13509](https://github.com/dotnet/msbuild/pull/13509)).
+  - Removed `ToLowerInvariant`/`ToUpperInvariant` allocations from `TargetPlatformSDK.GetHashCode` and `SdkReference.GetHashCode` ([dotnet/msbuild #13475](https://github.com/dotnet/msbuild/pull/13475)).
+
+## Community contributors
+
+Thank you contributors! ❤️
+
+- [@snechaev](https://github.com/dotnet/msbuild/pulls?q=is%3Apr+is%3Amerged+author%3Asnechaev)
+- [@OvesN](https://github.com/dotnet/msbuild/pulls?q=is%3Apr+is%3Amerged+author%3AOvesN)
+
+<!-- TODO: verify whether @nareshjo (#13509) is an external community contributor or a Microsoft employee submitting via the VS Perf Rel AI Agent; include in the list above if external. -->