Fix TaskParameterTaskItem.ToString() returning type name instead of item-spec

[huulinhnguyen-dev]

Fix TaskParameterTaskItem.ToString() returning type name instead of item-spec

Fixes #13896

Context

When building xunit with a freshly built MSBuild using the /mt (multithreaded)
flag, the build fails with:

Could not find input reference assembly 'Microsoft.Build.BackEnd.TaskParameter+TaskParameterTaskItem'

The same build succeeds without /mt.

TaskParameterTaskItem is the internal wrapper MSBuild uses to marshal ITaskItem instances across node/thread boundaries. It did not override ToString(), so it fell back to object.ToString() and returned its full .NET type name
(Microsoft.Build.BackEnd.TaskParameter+TaskParameterTaskItem) instead of the item-spec. Every other ITaskItem implementation (ProjectItemInstance.TaskItem, Utilities.TaskItem, TaskItemData) overrides ToString() to return the item-spec.

This only surfaces under /mt: non-thread-safe tasks are routed to a TaskHost for isolation (TaskRouter.NeedsTaskHostInMultiThreadedMode → AssemblyTaskFactory → TaskHostTask), so task outputs come back wrapped as TaskParameterTaskItem (TaskHostTask.cs:641). When a downstream task builds a path via item.ToString() — here the third-party AnnotateReferenceAssemblies task from TunnelVisionLabs.ReferenceAssemblyAnnotator, used by xunit — it gets the type name instead of the path and fails. Without /mt, the same tasks run in-process via TaskExecutionHost and receive native ProjectItemInstance.TaskItem objects (correct ToString()), so the build succeeds.

This is a long-standing latent bug; it only became visible now because /mt marshals task parameters broadly. Most tasks use item.ItemSpec, which is always correct, which is why only repos that use a task calling item.ToString() (like xunit) are affected.

Changes Made

• Added a ToString() override to TaskParameterTaskItem in src/Shared/TaskParameter.cs, returning the escaped item-spec to match ProjectItemInstance.TaskItem.ToString().

The reported /mt (sidecar TaskHost) scenario deserializes items on the engine side using the src/Shared/TaskParameter.cs copy, so the fix there fully resolves the issue. The separate src/MSBuildTaskHost/BackEnd/TaskParameter.cs copy (the legacy net35/net472 out-of-proc task host) is intentionally left unchanged to keep the scope minimal, per review.

Testing

Notes

[Copilot]

Pull request overview

Fixes MSBuild’s marshaled ITaskItem wrapper (TaskParameterTaskItem) so ToString() returns the item-spec (instead of the wrapper’s type name), addressing failures that only surface when task parameters are marshaled (notably under /mt and TaskHost boundaries).

Changes:

• Override TaskParameterTaskItem.ToString() to return the (escaped) item-spec in both the engine and TaskHost copies of TaskParameterTaskItem.
• Add a regression test covering ToString() behavior and round-trip serialization for ITaskItem task parameters.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/Shared/TaskParameter.cs	Adds TaskParameterTaskItem.ToString() override for the shared (engine) marshaled task-item wrapper.
src/MSBuildTaskHost/BackEnd/TaskParameter.cs	Adds the same ToString() override for the TaskHost-side marshaled task-item wrapper.
src/Shared/UnitTests/TaskParameter_Tests.cs	Adds a regression test validating ToString() and serialization round-trip behavior.

[JanProvaznik]

thanks for the find, few things to adjust

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.