问题
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.
回答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
... ...
回答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.
回答3:
I think what you want is something like this:
gps AcroRd32 | measure-object | select -expand Count
回答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.
来源：https://stackoverflow.com/questions/7614695/how-to-use-the-property-parameter-for-powershells-measure-object-cmdlet