How do I write the value of a single property of a object?

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qbanet359’s helpful answer uses direct property access (.LoadPercentage) on the result object, which is the simplest and most efficient solution in this case.

In PowerShell v3 or higher this even works with extracting property values from a collection of objects, via a feature called member-access enumeration.

E.g., ((Get-Date), (Get-Date).AddYears(-1)).Year returns 2019 and 2018 when run in 2019, which are the .Year property values from each [datetime] instance in the array.

In cases where you do want to use Select-Object (or its built-in alias, select), such as when processing a large input collection item by item:

To use Select-Object to extract a single property value, you must use -ExpandProperty:

Get-WmiObject win32_processor | Select-Object -ExpandProperty LoadPercentage

Background:

Select-Object by default creates custom objects ([pscustomobject] instances[1]
) that have the properties you specify via the -Property parameter (optionally implicitly, as the 1st argument).

This applies even when specifying a single property[2], so that select LoadPercentage (short for: Select-Object -Property LoadPercentage) creates something like the following object:

$obj = [pscustomobject] @{ LoadPercentage = 4 } # $obj.LoadPercentage yields 4

Because you use Add-Content to write to your log file, it is the .ToString() string representation of that custom object that is written, as you would get if you used the object in an expandable string (try "$([pscustomobject] @{ LoadPercentage = 4 })").

By contrast, parameter -ExpandProperty, which can be applied to a single property only, does not create a custom object and instead returns the value of that property from the input object.

  • Note: If the value of that property happens to be an array (collection), its elements are output individually; that is, you’ll get multiple outputs per input object.

[1] Strictly speaking, they’re [System.Management.Automation.PSCustomObject] instances, whereas type accelerator [pscustomobject], confusingly, refers to type [System.Management.Automation.PSObject], for historical reasons; see this GitHub issue.

[2] There’s a hotly debated request on GitHub to change Select-Object‘s default behavior with only a single property; while the discussion is interesting, the current behavior is unlikely to change.

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