Prof. Powershell

PowerShell Tip: Utilizing the –As Operator

Here's how to get PowerShell to treat an object or command as a different object or command.

In the previous article we looked at some shortcuts for defining objects of a specific type. Those techniques work just fine at the time you are defining or creating the object. But another way to work with object types is to instruct PowerShell to treat something as something else. In these situations we can use the –As operator.

For example, you might have a string:

PS C:\> $s = "7/4/1976"

To treat this as a datetime object is as simple as, well, using –AS:

PS C:\> $s -as [datetime]

Sunday, July 4, 1976 12:00:00 AM

You might use something like this with Read-Host.

PS C:\> $c = Read-host "Enter the name of a wmiclass"
Enter the name of a wmiclass: win32_service
PS C:\> $c -as [wmiclass]

 

NameSpace: ROOT\cimv2

Name                  Methods              Properties
----                  -------              ----------
Win32_Service         {StartService, St... {AcceptPause, AcceptStop, Captio...

You can use –AS with type accelerators or just about any class name.

PS C:\> $svc = "bits" PS C:\> $svc -as [serviceprocess.servicecontroller]

Status   Name               DisplayName
------   ----               -----------
Running  bits               Background Intelligent Transfer Ser...
PS C:\> 1 -as [bool]
True
PS C:\> 0 -as [bool]
False
PS C:\> "172.16.45.123" -as [ipaddress]

 

Address           : 2066550956
AddressFamily     : InterNetwork
ScopeId           :
IsIPv6Multicast   : False
IsIPv6LinkLocal   : False
IsIPv6SiteLocal   : False
IPAddressToString : 172.16.45.123

There's no guarantee this will always work, or that you will get the results you expect, so the best thing to do is just try it.

In addition to treating something as something else, you can also ask PowerShell to test if something is of a specific type. Can you guess the operator?

PS C:\> $t -is [int]
False
PS C:\> $t -is [string]
True

Use this operator in your scripts to help validate input and data to make sure it is of the expected type. The –IS operator will return True or False which makes it easy to use in an IF statement.

PS C:\> $a = get-process powershell
PS C:\> if ($a -is [array]) {"found multiple powershell processes"}
found multiple powershell processes
PS C:\> $a

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessNam
-------  ------    -----      ----- -----   ------     -- ----------
472      30   123080      95504   638     2.01   1240 powershell
522      35   173292      28152   822   359.16   4512 powershell

If you want to test the opposite there is a –IsNot operator.

PS C:\> 4 -is [int]
True
PS C:\> 4 -isnot [int]
False

Although I would image the use case for this might be limited. Most of the time I'm more interested to know if something is a certain type.

Finally, let's wrap up this lesson with some PowerShell commands that will show you all of the available type accelerators on your computer. In PowerShell 2.0 you can run these commands:

$acceleratorsType = [type]::gettype("System.Management.Automation.TypeAccelerators")
$acceleratorsType::Add("accelerators", $acceleratorsType)
[accelerators]::Get

You will get a hashtable of accelerators and their values.

Key                                Value
---                                 -----
Alias                               System.Management.Automation.AliasAttribute
AllowEmptyCollection                System.Management.Automation.AllowEmptyCollect...
AllowEmptyString                    System.Management.Automation.AllowEmptyStringA...
AllowNull                           System.Management.Automation.AllowNullAttribute
array                               System.Array
bool                                System.Boolean

In PowerShell 3.0 you can do this in a single command.

[psobject].assembly.gettype("System.Management.Automation.TypeAccelerators")::Get

Once you know the accelerator you can use it to quickly define a new object, test if an object is that type or cast an object to that type. Just remember that these are accelerators or shortcuts and not an exhaustive list of all possible object types.

About the Author

Jeffery Hicks is a multi-year Microsoft MVP in Windows PowerShell, Microsoft Certified Professional and an IT veteran with almost 25 years of experience, much of it spent as an IT infrastructure consultant specializing in Microsoft server technologies with an emphasis in automation and efficiency. He works today as an independent author, trainer and consultant. Jeff is a regular contributor to a variety on online sites, as well as frequent speaker at technology conferences and user groups. Keep up with Jeff and his projects at http://jdhitsolutions.com/blog.

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