#PSTip PowerShell 4 – Filtering a collection by using a method syntax

Note: This tip requires PowerShell 3.0 or above.

One of the new features introduced in PowerShell 4.0 is the collection filtering by using a method syntax. You can now filter a collection of objects using a simplified Where-Object like syntax specified as a method call.

PS> (Get-Process).where("name -like p*")
Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
465      30    86396     100996   624     2.56   9524 powershell
893      91   346044     382620  1050   515.21   9736 powershell_ise

Awesome! Now, if you open PowerShell and try this out you’ll probably get the following error:

PS > (Get-Process).where("name -like p*")
Method invocation failed because [System.Diagnostics.Process] does not contain a method named 'where'.
At line:1 char:1
+ (gps).where("name -like p*")
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : MethodNotFound

What happened? Collection filtering is a part of the PSDesiredStateConfiguration module and to have it available you first need to import the module.

PS> Import-Module PSDesiredStateConfiguration
PS> (Get-Process).where("name -like p*")

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
709      66   138168     163976   744     5.24   7120 powershell
893      91   346044     382620  1050   515.21   9736 powershell_ise

# now let's examine the method signature:
PS>  (Get-Process).where
Script              :
                               $prop, $psop, $val = [string] $args[0] -split '(
                      -eq|-ne|-gt|-ge|-lt|-le|-like|-notlike|-match|-notmatch)'
                               $operation = @{ Prop = $prop.Trim(); Value =
                      $val.Trim(); $psop = $true }
                               $this | where @operation

OverloadDefinitions : {System.Object where();}
MemberType          : ScriptMethod
TypeNameOfValue     : System.Object
Value               : System.Object where();
Name                : where
IsInstance          : False

As you can see the where method is actually a ScriptMethod, and if you open the PSDesiredStateConfiguration psm1 file you’ll find the command that extends the System.Array type using the Update-TypeData cmdlet (line 849).

Update-TypeData -Force -MemberType ScriptMethod -MemberName where -TypeName System.Array -Value {
         $prop, $psop, $val = [string] $args[0] -split '(-eq|-ne|-gt|-ge|-lt|-le|-like|-notlike|-match|-notmatch)'
         $operation = @{ Prop = $prop.Trim(); Value = $val.Trim(); $psop = $true }
         $this | where @operation
    }

Having to load the module each time you want to use the method is not that convenient so why not adding it to every PowerShell session you open. Add the above command to your profile and you’re good to go.

where() method is not limited to PowerShell 4.0 only. Simplified syntax was introduced in PowerShell 3.0 so you can enable this on systems with that version as well.