René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Array subexpression @($list) throws \"Argument types do not match\" for List[object] created by New-Object","articleBody":"## Summary\n\nThe array-subexpression operator itself throws `System.ArgumentException: Argument types do not match` when it wraps a `System.Collections.Generic.List[object]` created by `New-Object`.\n\nUI Automation, `ConvertTo-Json`, and the original `$pair[1]` access are not required. They only made the issue visible in the original application.\n\n## Minimal reproduction\n\n```powershell\n$list = New-Object System.Collections.Generic.List[object]\n@($list)\n```\n\nThe list can be empty.\n\n### Actual output\n\n```text\nOperationStopped: Argument types do not match\n```\n\n### Expected output\n\n`@($list)` should not throw and should behave consistently with the same public .NET type created with its constructor. In the tested environment, the constructor-created empty list produces an empty `object[]`:\n\n```powershell\n$list = [System.Collections.Generic.List[object]]::new()\n@($list) # succeeds\n```\n\n## Comparison\n\n```powershell\nfunction Test-Case($name, [scriptblock]$body) {\n    try {\n        \u0026 $body | Out-Null\n        $name + ': OK'\n    }\n    catch {\n        $name + ': FAIL - ' + $_.Exception.Message\n    }\n}\n\n$newObjectList = New-Object System.Collections.Generic.List[object]\n$constructorList = [System.Collections.Generic.List[object]]::new()\n\nTest-Case 'New-Object + @()' { @($newObjectList) }\nTest-Case 'constructor + @()' { @($constructorList) }\nTest-Case 'New-Object + [array]' { [array]$newObjectList }\nTest-Case 'New-Object + ToArray' { $newObjectList.ToArray() }\n```\n\nObserved:\n\n```text\nNew-Object + @(): FAIL - Argument types do not match\nconstructor + @(): OK\nNew-Object + [array]: OK\nNew-Object + ToArray: OK\n```\n\nBoth values report the same public types:\n\n```text\nGetType():                         System.Collections.Generic.List[System.Object]\nPSObject.BaseObject type:          System.Collections.Generic.List[System.Object]\nPSObject.ImmediateBaseObject type: System.Collections.Generic.List[System.Object]\n```\n\nThe failure is specific in the tested matrix:\n\n| Construction/type | `@(...)` |\n|---|---|\n| `New-Object List[object]` | FAIL |\n| `[List[object]]::new()` | OK |\n| `New-Object List[string]` | OK |\n| `New-Object List[int]` | OK |\n| `New-Object ArrayList` | OK |\n| `New-Object Queue[object]` | OK |\n\n## Stack trace\n\nThe exception occurs while evaluating `@($list)`, before any JSON serialization. The top stack frames are:\n\n```text\nat System.Linq.Expressions.Expression.Condition(...)\nat System.Management.Automation.Language.PSEnumerableBinder.MaybeDebase(...)\nat System.Management.Automation.Language.PSToObjectArrayBinder.Bind(...)\n```\n\n## Workarounds\n\n```powershell\n$list = [System.Collections.Generic.List[object]]::new()\n[array]$list\n$list.ToArray()\n```\n\nIn the original application, removing the unnecessary array-subexpression also avoids the failure:\n\n```diff\n- controls = @($pair[1])\n+ controls = $pair[1]\n```\n\n## Environment\n\nReproduced on:\n\n- PowerShell 7.6.3 on Windows 11 10.0.26200\n- Windows PowerShell 5.1.26100.8655 on the same machine\n\nThe original report attributed the failure to UI Automation-derived hashtables and `ConvertTo-Json`. Further reduction after reviewer feedback shows that neither is required.\n\n## Related issues\n\n- [PowerShell #5579](https://github.com/PowerShell/PowerShell/issues/5579): implicit `PSObject` wrapping and serialization behavior\n- [PowerShell #4280](https://github.com/PowerShell/PowerShell/issues/4280) and [PR #4296](https://github.com/PowerShell/PowerShell/pull/4296): earlier work on array-subexpression behavior and `List[object]`\n","author":{"url":"https://github.com/shuangxue520","@type":"Person","name":"shuangxue520"},"datePublished":"2026-06-03T15:07:17.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/27558/PowerShell/issues/27558"}