Prof. Powershell

Constant Companion

There's power in defining constants, even in PowerShell. Check out this week's tip.

• By Jeffery Hicks
• 11/07/2011

In Windows PowerShell, we use variables as a place holder for a value of collection of values. Most of the time we can create and define a variable in one step:

PS C:\> $x=100

If we need to change the value of $x all we need to do is assign it a new value:

PS C:\> $x=200

But what if you were relying on the value of $x and didn't want it to be either intentionally or accidently modified? PowerShell offers a few options when defining your variable. You can make a variable read-only or a constant. In either situation, you need to use the New-Variable cmdlet and the -Option parameter:

PS C:\> New-Variable -Name M -Value "Jeff" -Option ReadOnly
PS C:\> $m
Jeff
PS C:\> $m="Don"
Cannot overwrite variable M because it is read-only or constant.
At line:1 char:3
+ $m <<<< ="Don"
   + CategoryInfo : WriteError: (M:String) [], SessionStateUnauthorizedAccessException
   + FullyQualifiedErrorId : VariableNotWritable

When a variable is read-only, you cannot modify its value. As you've seen, PowerShell will complain. However, you are allowed to delete the variable and re-create it:

PS C:\> remove-variable m -force
PS C:\> $m="Don"
PS C:\> $m
Don

You must use -Force to delete the variable. To create a truly protected variable with New-Variable, use a value of Constant with -Option:

PS C:\> $wshell=new-object -com "wscript.shell"
PS C:\> new-variable vbExclaim 48 -Option Constant
PS C:\> $wshell.popup("That didn't go well.",15,"Oops",$vbExclaim)

It's unlikely that I'll overwrite a variable with a name like vbExclaim. But this value will never, ever change so I might as well define it as a constant.

Be aware that if you already have a variable defined with the same name, you need to delete it before you can declare a read-only or constant variable with the New-Variable cmdlet. Since VBScript is full of constants, I find it helpful to define them as such when using VBScript COM objects in Windows PowerShell.

By the way, you can also do the same thing with aliases:

PS C:\> Set-Alias np $env:WinDir\notepad.exe -option ReadOnly
PS C:\> New-Alias slo Select-Object -Option ReadOnly

If you find the need to use constants throughout the day in your PowerShell work, then defining them in your PowerShell profile is a good idea. As to when to make a variable ReadOnly and when a Constant depends on your environment, your script, who will be running it and the consequences if variables don't have the values you are expecting.