PowerShell: What is the point of ForEach-Object with InputObject?

Question

The documentation for ForEach-object says "When you use the InputObject parameter with ForEach-Object, instead of piping command results to ForEach-Object, the InputObject value is treated as a single object." This behavior can easily be observed directly:

PS C:\WINDOWS\system32> ForEach-Object -InputObject @(1, 2, 3) {write-host $_}
1 2 3

This seems weird. What is the point of a "ForEach" if there is no "each" to do "for" on? Is there really no way to get ForEach-object to act directly on the individual elements of an array without piping? if not, it seems that ForEach-Object with InputObject is completely useless. Is there something I don't understand about that?

Answer 1

In the case of ForEach-Object, or any cmdlet designed to operate on a collection, using the
-InputObject as a direct parameter doesn't make sense because the cmdlet is designed to operate on a collection, which needs to be unrolled and processed one element at a time. However, I would also not call the parameter "useless" because it still needs to be defined so it can be set to allow input via the pipeline.

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Why is it this way?

-InputObject is, by convention, a generic parameter name for what should be considered to be pipeline input. It's a parameter with [Parameter(ValueFromPipeline = $true)] set to it, and as such is better suited to take input from the pipeline rather passed as a direct argument. The main drawback of passing it in as a direct argument is that the collection is not guaranteed to be unwrapped, and may exhibit some other behavior that may not be intended. From the about_pipelines page linked to above:

When you pipe multiple objects to a command, PowerShell sends the objects to the command one at a time. When you use a command parameter, the objects are sent as a single array object. This minor difference has significant consequences.

To explain the above quote in different words, passing in a collection (e.g. an array or a list) through the pipeline will automatically unroll the collection and pass it to the next command in the pipeline one at a time. The cmdlet does not unroll -InputObject itself, the data is delivered one element at a time. This is why you might see problems when passing a collection to the -InputObject parameter directly - because the cmdlet is probably not designed to unroll a collection itself, it expects each collection element to be handed to it in a piecemeal fashion.

Consider the following example:

# Array of hashes with a common key
$myHash = @{name = 'Alex'}, @{name='Bob'}, @{name = 'Sarah'}

# This works as intended
$myHash | Where-Object { $_.name -match 'alex' }

The above code outputs the following as expected:

Name                           Value
----                           -----
name                           Alex

But if you pass the hash as InputArgument directly like this:

Where-Object -InputObject $myHash { $_.name -match 'alex' }

It returns the whole collection, because -InputObject was never unrolled as it is when passed in via the pipeline, but in this context $_.name -match 'alex' still returns true. In other words, when providing a collection as a direct parameter to -InputObject, it's treated as a single object rather than executing each time against each element in the collection. This can also give the appearance of working as expected when checking for a false condition against that data set:

Where-Object -InputObject $myHash { $_.name -match 'frodo' }

which ends up returning nothing, because even in this context frodo is not the value of any of the name keys in the collection of hashes.

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In short, if something expects the input to be passed in as pipeline input, it's usually, if not always, a safer bet to do it that way, especially when passing in a collection. However, if you are working with a non-collection, then there is likely no issue if you opt to use the -InputObject parameter directly.