How to use the "-Property" parameter for PowerShell's "Measure-Object" cmdlet?

Question

Why does

$a = GPS AcroRd32 | Measure
$a.Count

work, when

GPS AcroRd32 | Measure -Property Count

doesn't?

The first example returns a value of 2, which is what I want, an integer.

The second example returns this:

Measure-Object : Property "Count" cannot be found in any object(s) input.
At line:1 char:23
+ GPS AcroRd32 | Measure <<<<  -Property Count
    + CategoryInfo          : InvalidArgument: (:) [Measure-Object], PSArgumentException
    + FullyQualifiedErrorId : GenericMeasurePropertyNotFound,Microsoft.PowerShell.Commands.MeasureObjectCommand

---

This Scripting Guy entry is where I learned how to use the "Count" Property in the first code sample.

The second code sample is really confusing. In this Script Center reference, the following statement works:

Import-Csv c:\scripts\test.txt | Measure-Object score -ave -max -min

It still works even if it's re-written like so:

Import-Csv c:\scripts\test.txt | Measure-Object -ave -max -min -property score

I don't have too many problems with accepting this until I consider the Measure-Object help page. The parameter definition for -Property <string[]> states:

The default is the Count (Length) property of the object.

If Count is the default, then shouldn't an explicit pass of Count work?

GPS AcroRd32 | Measure -Property Count # Fails

The following provides me the information I need, except it doesn't provide me with an integer to perform operations on, as you'll see:

PS C:\Users\Me> $a = GPS AcroRd32 | Measure
PS C:\Users\Me> $a

Count    : 2
Average  :
Sum      :
Maximum  :
Minimum  :
Property :

PS C:\Users\Me> $a -is [int]
False

---

So, why does Dot Notation ($a.count) work, but not an explicitly written statement (GPS | Measure -Property Count)?

If I'm supposed to use Dot Notation, then I will, but I'd like to take this opportunity to learn more about how and *why PowerShell works this way, rather than just building a perfunctory understanding of PowerShell's syntax. To put it another way, I want to avoid turning into a Cargo Cult Programmer/ Code Monkey.

Solution 1

One thing you need to know is that in PowerShell generally, and particulary in CmdLets you manipulate objects or collection of objects.

Example: if only one 'AcroRd32' is running Get-Process will return a [System.Diagnostics.Process], if more than one are running it will return a collection of [System.Diagnostics.Process].

In the second case you can write:

(GPS AcroRd32).count

Because a collection has a count property. The duality object collection is also valid in CmdLets parameters that most of the time supports objects or list of objects (collection built with the operator ,).

PS C:\> (gps AcroRd32) -is [object[]]
True

Just use the Get-Member cmdlet:

PS C:\> (gps AcroRd32) | Get-Member

   TypeName: System.Diagnostics.Process

Name                       MemberType     Definition
----                       ----------     ----------
Handles                    AliasProperty  Handles = Handlecount
...                        ...

And

PS C:\>  Get-Member -InputObject (gps AcroRd32)

   TypeName: System.Object[]

Name           MemberType    Definition
----           ----------    ----------
Count          AliasProperty Count = Length
...            ...

Solution 2

Because the COUNT property is a property of the OUTPUT object (i.e. results of Measure-Object), not the INPUT object.

The -property parameter specifies which property(ies) of the input objects are to be evaluated. None of these is COUNT unless you pass an array or arrays or something.

Solution 3

I think what you want is something like this:

gps AcroRd32 | measure-object | select -expand Count

Solution 4

If you're just looking for the count you can do the following:

$a = GPS AcroRd32
$a.Count = 2

$a = GPS AcroRd32 sets $a to an array of process objects. The array has a member call, Count, that will allow you to determine the number of elements already.

The Measure-Object commandlet (with alias measure) is used to measure the average, maximum, minimum, and sum values of a property. So you could do something like $a | measure -property Handles -sum and get a count of the total number of open handles.

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Comments

[Stisfa]

Get-Member -InputObject (gps AcroRd32) - this helped me to understand where the Count Property came from. Now I can see why my "complete statement" doesn't work, since it was the wrong syntax. Thanks for showing me how to use (GPS AcroRd32).Count!