CODEWITHSUNDEEP
powershell
Vimes
Vimes

Reputation: 11957

Powershell Select-Object's -Property parameter

I'm stumped by some of the behavior of the Select-Object cmdlet. Here is an example.

PS C:\> $object = get-item C:\windows\Temp
PS C:\> $time = $object.CreationTime
PS C:\> $time.GetType().FullName
System.DateTime
PS C:\> $result = Select-Object -InputObject $object -Property "CreationTime"
PS C:\> $result.GetType().FullName
System.Management.Automation.PSCustomObject
PS C:\>
PS C:\> $result.CreationTime.GetType().FullName
System.DateTime

Notice that CreationTime's type is System.DateTime but when I select it using Select-Object the returned object is of type System.Management.Automation.PSCustomObject. Its some new object that has CreationTime as its property.

Let's look at the help for Select-Object to explain this.

SYNOPSIS
Selects specified properties of an object or set of objects...

That's what I wanted, the property itself... not some object with the property. But if we read further...

DESCRIPTION
...If you use Select-Object to select specified properties, it copies the values of those properties from the input objects and creates new objects that have the specified properties and copied values.

I have no idea why that's useful, but it explains this object returned.

Using -ExpandProperty instead of -Property seems to give the property itself

PS C:\> $result2 = Select-Object -InputObject $object -ExpandProperty "CreationTime"
PS C:\> $result2.GetType().FullName
System.DateTime

By why does -ExpandProperty do this? Let's look at its help:

-ExpandProperty
Specifies a property to select... If the property contains an object, the properties of that object are displayed in the output.

In this case, the property is an object and we didn't get "the properties of that object" we only got the object itself.

Can someone tell me:

  1. Why does -ExpandProperty get the property-object itself when the help mentions expanding arrays and getting properties of the property-object (not the property-object itself)? This is where I am pretty confused. Am I reading this wrong? I feel like this needs another iteration of technical editing.
  2. The object that gets returned with "-Property", is it useful for something?

Upvotes: 2

Views: 10521

Answers (2)

mjolinor
mjolinor

Reputation: 68341

To expand on the answer to #1: The most common use of -expandproperty is to expand a property that is an array into the array elements, particularly when doing an export-csv. 

PS C:\> get-process lsass | select threads | convertto-csv -notype
"Threads"
"System.Diagnostics.ProcessThreadCollection"

Not very useful. Now, use -expandproperty on that thread collection:

PS C:\> get-process lsass | select -expand threads | convertto-csv -notype

 "BasePriority","CurrentPriority","Id","IdealProcessor","PriorityBoostEnabled","PriorityLevel","PrivilegedProcessorTime"
 ,"StartAddress","StartTime","ThreadState","TotalProcessorTime","UserProcessorTime","WaitReason","ProcessorAffinity","Si
 te","Container"
 "9","10","572",,,,,"2000143616",,"Wait",,,"LpcReceive",,,
 "9","10","588",,,,,"2000143616",,"Wait",,,"UserRequest",,,
 "9","9","592",,,,,"2000143616",,"Wait",,,"UserRequest",,,
 "9","10","596",,,,,"0",,"Wait",,,"EventPairLow",,,
 "9","9","1404",,,,,"0",,"Wait",,,"UserRequest",,,
 "9","9","3896",,,,,"0",,"Wait",,,"EventPairLow",,,
 "9","9","848",,,,,"0",,"Wait",,,"EventPairLow",,,
 "9","11","6216",,,,,"2000143616",,"Wait",,,"UserRequest",,,
 "9","9","7924",,,,,"2000143616",,"Wait",,,"EventPairLow",,,

Upvotes: 4

Mike Shepard
Mike Shepard

Reputation: 18176

Select-object (without -expandproperty) returns a PSCustomObject with the set of properties you select. That's because you might want to select more than one property (and couldn't expect to get, for instance, a datetime object for 2 properies).

The answer to #2 is that sometimes it's better to have an object with fewer properties and lose the methods. Also, you can use select-object with expressions to return "calculated" properties.

Upvotes: 3

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