Bug: invoke-mstest-single-assembly-strictmode-count-throw #713

• Work Mode: minor-audit

Summary

scripts/vscode/Invoke-MSTest.ps1 throws before running a single test whenever its -SearchRoot matches exactly one *.Test.dll. Under Set-StrictMode -Version Latest the discovery pipeline yields a bare System.String for a single match, and the guard that follows reads .Count on it. The scoped, single-project form of the wrapper is therefore unusable, and only the repository-wide -SearchRoot . form works.

Environment

• OS/version: Windows 11 Pro 10.0.26200
• Python version: not applicable; the affected script is PowerShell, run under pwsh -NoProfile
• Command/flags used: pwsh -NoProfile -File scripts/vscode/Invoke-MSTest.ps1 -SearchRoot QuickFiler.Test -Configuration Debug
• Data source or fixture: any project directory whose bin/<Configuration> tree contains exactly one *.Test.dll

Steps to Reproduce

1. Build the solution so that QuickFiler.Test/bin/Debug/QuickFiler.Test.dll exists.
2. Run pwsh -NoProfile -File scripts/vscode/Invoke-MSTest.ps1 -SearchRoot QuickFiler.Test -Configuration Debug.
3. Observe that the script terminates before invoking vstest.console.exe.

The scalar half of the defect reproduces with no build at all:

pwsh -NoProfile -Command 'Set-StrictMode -Version Latest; $x = "one-string"; $x.Count'

Expected Behavior

A -SearchRoot naming a single test project discovers that project's one test assembly and runs it, exactly as -SearchRoot . discovers nine assemblies and runs them. Scoping the run to one project is the natural way to get fast feedback on a single project's tests.

Actual Behavior

The script throws PropertyNotFoundException: The property 'Count' cannot be found on this object. Verify that the property exists.

Set-StrictMode -Version Latest is set at scripts/vscode/Invoke-MSTest.ps1:77. The discovery pipeline at :107-113 ends in Select-Object -ExpandProperty FullName, which unrolls to a bare System.String rather than a one-element array when exactly one object flows out of the pipeline. The guard at :115 is:

if (-not $testAssemblies -or $testAssemblies.Count -eq 0) {

For a non-empty string the left operand is $false, so -or proceeds to evaluate the right operand and reads .Count on a scalar, which StrictMode rejects. :120 would fail identically at $($testAssemblies.Count).

With -SearchRoot . the pipeline emits nine assemblies, the variable binds an array, .Count resolves, and the script behaves correctly. That is why the defect has gone unnoticed: the repository-wide form is the one in habitual use.

Logs / Screenshots

• Attached minimal logs or screenshot
• Snippet:

type=String
PropertyNotFoundException: The property 'Count' cannot be found on this object. Verify that the property exists.

Impact / Severity

• Blocker
• High
• Medium
• Low

No production code is affected and no CI job is affected, because CI and the habitual local invocation both use the repository-wide search root. The cost is developer and agent feedback latency: a per-project test run is the fast path, and it is unavailable, so every scoped verification has to run all nine assemblies. It also silently pushes plans and acceptance criteria toward the repository-wide run, which carries known load-driven flakiness that a single-assembly run does not.

Source

From: docs/features/potential/2026-08-31-invoke-mstest-single-assembly-strictmode-count-throw.md