Why does "Get-ChildItem -File | Get-FileHash" work?

System.IO.FileInfo / System.IO.DirectoryInfo instances output by PowerShell cmdlets have a .PSPath property^[*] that contains the instances' fully qualified path, which is the full file-system path prefixed by the PS provider name (e.g., Microsoft.PowerShell.Core\FileSystem::C:\windows).

File-processing cmdlets such as Get-FileHash have a -LiteralPath parameter which has an alias name of -PSPath.

Because a -LiteralPath parameter (typically) accepts input from the pipeline by property name, input objects that have a .PSPath property automatically bind to it, by virtue of the PSPath parameter alias name.

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How to discover this behavior:

• Via the online help topic:

• Programmatically:

  • Note: Get-Help Get-FileHash -Parameter LiteralPath | Select-Object name, aliases, pipelineinput works too in this case, but this approach is generally restricted to target commands that come with MAML-based help files, and even those that do can have help files that are out of sync with the actual command definition.

PS> & {
        Get-Command $args[0] | % Parameters | % $args[1] |
        Select-Object Name, Aliases, @{
            n = 'Accepts pipeline input';
            e = { $(if ($_.Attributes.ValueFromPipeline) { 'by value' }), $(if ($_.Attributes.ValueFromPipelineByPropertyName) { 'by property name' }) -join ', ' -replace '^, ' }
        }
    } Get-FileHash LiteralPath


Name        Aliases      Accepts pipeline input
----        -------      ----------------------
LiteralPath {PSPath, LP} by property name

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^{[*] It is PowerShell's file-system provider that adds this property, among others. The underlying .NET types do not have it. See this answer for more information.}

From the About Functions Advanced Parameters documentation, ValueFromPipelineByPropertyName argument section:

The ValueFromPipelineByPropertyName argument indicates that the parameter accepts input from a property of a pipeline object. The object property must have the same name or alias as the parameter.

For example, if the function has a ComputerName parameter, and the piped object has a ComputerName property, the value of the ComputerName property is assigned to the function's ComputerName parameter.

Update: Originally linked the wrong source code file. The correct Get-FileHash source code is here. As @mklement0 correctly answered the Get-ChildItem cmdlet outputs an object with a PSPath property, which makes this work.