ByValue, I Think He’s Got It

Recently I responded to an email from a student seeking clarification about the difference between ByValue and ByProperty when it comes to parameter binding. This is what makes pipelined expressions work in Windows PowerShell. When you look at cmdlet help, you'll see that some parameters accept pipeline binding, which is what you are looking for. Often this means you don't need to resort to a ForEach-Object construct. Here's an example.

ByValue means if I see something I'll use it. ByProperty means I'm looking for a specific object property. For example, Get-Service uses the Name property to find a service.

PS S:\> help get-service -Parameter Name

-Name <string[]>
    Specifies the service names of services to be retrieved. Wildcards are permitted. By default, Get-Service gets all
    of the services on the computer.

    Required?                    false
    Position?                    1
    Default value
    Accept pipeline input?       true (ByValue, ByPropertyName)
    Accept wildcard characters?  true

This parameter accepts both types of binding. This means you can do this:

PS S:\> "wuauserv" | get-service

Status   Name               DisplayName
------   ----               -----------
Running  wuauserv           Windows Update

This is an example of byValue. Get-Service sees something in the pipeline and assumes it is a service name. However, for Get-Service this would also work.

PS S:\> $all=get-service
PS S:\> $all[0] | get-service

Status   Name               DisplayName
------   ----               -----------
Stopped  AeLookupSvc        Application Experience

$all[0] is a service object with name property which when piped to Get-Service, finds the property and binds to it. Here's another example that shows binding by property name can be from any object, not just a service object.

PS S:\> new-object psobject -Property @{Name="wuauserv";Date=Get-date;computername=$env:computername} | get-service

Status   Name               DisplayName
------   ----               -----------
Running  wuauserv           Windows Update
>/pre>

This is a custom object with a name property that happens to be valid service name. If you try it with a non-valid name, you'll get an error which proves it is binding on the property name. The other properties in my New-Object example are just to have something. Let me wrap this discussion with one more example that leverages parameter binding:

PS C:\work> get-content servers.txt | where {$_ -AND (Test-Connection $_ -quiet -count 2) } | Select @{Name="Computername";Expression={$_.Trim()}} | get-service wuauserv | select Name,status,Machinename

Name                                             Status MachineName
----                                             ------ -----------
wuauserv                                        Running CHI-DC01
wuauserv                                        Running CHI-DC02
wuauserv                                        Stopped CHI-FP01
wuauserv                                        Running CHI-DB01
wuauserv                                        Running CHI-EX01

This is a one line expression that leverages the pipeline and uses some helpful (I hope) techniques. The first part using Get-Content retrieves the list of computer names from the text file. Because the text file might have blank lines and some computers might be offline, each name is piped to Where-Object which will only pass on names that exist (skipping blanks) and that can be pinged. To speed things up I'm only sending 2 pings. Now the fun part. I could use ForEach-Object and pass $_ as the value for -Computername. But according to help, this parameter accepts binding by property name.
PS S:\> help get-service -Parameter Computername

-ComputerName 
    Gets the services running on the specified computers. The default is the local computer.

    Type the NetBIOS name, an IP address, or a fully qualified domain name of a remote computer. To specify the local c
    omputer, type the computer name, a dot (.), or "localhost".

    This parameter does not rely on Windows PowerShell remoting. You can use the ComputerName parameter of Get-Service
    even if your computer is not configured to run remote commands.

    Required?                    false
    Position?                    named
    Default value                Localhost
    Accept pipeline input?       true (ByPropertyName)
    Accept wildcard characters?  false

So I'll take the computername value coming from Where-Object and use a hash table with Select-Object to define a new "property" name, called Computername. I also take the liberty of trimming off any leading or trailing spaces, just in case. Now I have an object with a property called Computername that is piped to Get-Service which binds on the computername property. The rest is merely formatting.
Look for opportunities to bind by parameter, which means reading cmdlet help which is a good habit to have regardless.





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