Variable scoping in PowerShell

Question

A sad thing about PowerShell is that function and scriptblocks are dynamically scoped.

But there is another thing that surprised me is that variables behave as a copy-on-write within an inner scope.

$array=@("g") function foo() {     $array += "h"     Write-Host $array }  & {     $array +="s"     Write-Host $array } foo  Write-Host $array 

The output is:

g s g h g 

Which makes dynamic scoping a little bit less painful. But how do I avoid the copy-on-write?

Answer 1

The PowerShell scopes article (about_Scopes) is nice, but too verbose, so this is quotation from my article:

In general, PowerShell scopes are like .NET scopes. They are:

• Global is public
• Script is internal
• Private is private
• Local is current stack level
• Numbered scopes are from 0..N where each step is up to stack level (and 0 is Local)

Here is simple example, which describes usage and effects of scopes:

$test = 'Global Scope' Function Foo {     $test = 'Function Scope'     Write-Host $Global:test                                  # Global Scope     Write-Host $Local:test                                   # Function Scope     Write-Host $test                                         # Function Scope     Write-Host (Get-Variable -Name test -ValueOnly -Scope 0) # Function Scope     Write-Host (Get-Variable -Name test -ValueOnly -Scope 1) # Global Scope } Foo 

As you can see, you can use $Global:test like syntax only with named scopes, $0:test will be always $null.

Answer 2

You can use scope modifiers or the *-Variable cmdlets.

The scope modifiers are:

• global used to access/modify at the outermost scope (eg. the interactive shell)
• script used on access/modify at the scope of the running script (.ps1 file). If not running a script then operates as global.

(For the -Scope parameter of the *-Variable cmdlets see the help.)

Eg. in your second example, to directly modify the global $array:

& {   $global:array +="s"   Write-Host $array } 

For more details see the help topic about_scopes.