powershell - Why is a variable defined locally successfully accessible from within another function?

Question

See code below. Put both files in the same directory and run Form1.ps1 from the PS ISE

As you can see, the (local) variable $localVar is defined in the event handler $button2_Click. As such, I assumed $localVar would not/could not exist outside the scope of $button2_Click with scope defined by the braces that define the event handler.

However, as you can see, I use the contents of $localVar to load $textbox2.Text in the function fA. When you click the Test button, both textboxes display the contents of $localVar

What's going on? Why is $button2_Click's $localVar accessible from within fA?

Form1.ps1

function fA
{
    $textbox2.Text = $localVar
}

$button2_Click = 
{
    $localVar = "set in `$button2_Click"
    $textbox1.Text = $localVar
    fA
}

. (Join-Path $PSScriptRoot 'Form1.designer.ps1')

$textbox1.Text = ""
$Form1.ShowDialog()

Form1.designer.ps1

[void][System.Reflection.Assembly]::Load('System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a')
[void][System.Reflection.Assembly]::Load('System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089')
$Form1 = New-Object -TypeName System.Windows.Forms.Form
[System.Windows.Forms.Button]$button2 = $null
[System.Windows.Forms.TextBox]$textBox1 = $null
[System.Windows.Forms.TextBox]$textBox2 = $null
[System.Windows.Forms.Label]$label1 = $null
[System.Windows.Forms.Label]$label2 = $null
[System.Windows.Forms.Button]$button1 = $null
function InitializeComponent
{
$button2 = (New-Object -TypeName System.Windows.Forms.Button)
$textBox1 = (New-Object -TypeName System.Windows.Forms.TextBox)
$textBox2 = (New-Object -TypeName System.Windows.Forms.TextBox)
$label1 = (New-Object -TypeName System.Windows.Forms.Label)
$label2 = (New-Object -TypeName System.Windows.Forms.Label)
$Form1.SuspendLayout()
#
#button2
#
$button2.Location = (New-Object -TypeName System.Drawing.Point -ArgumentList @([System.Int32]148,[System.Int32]12))
$button2.Name = [System.String]'button2'
$button2.Size = (New-Object -TypeName System.Drawing.Size -ArgumentList @([System.Int32]77,[System.Int32]36))
$button2.TabIndex = [System.Int32]0
$button2.Text = [System.String]'Test'
$button2.UseVisualStyleBackColor = $true
$button2.add_Click($button2_Click)
#
#textBox1
#
$textBox1.Location = (New-Object -TypeName System.Drawing.Point -ArgumentList @([System.Int32]67,[System.Int32]69))
$textBox1.Name = [System.String]'textBox1'
$textBox1.Size = (New-Object -TypeName System.Drawing.Size -ArgumentList @([System.Int32]106,[System.Int32]20))
$textBox1.TabIndex = [System.Int32]1
#
#textBox2
#
$textBox2.Location = (New-Object -TypeName System.Drawing.Point -ArgumentList @([System.Int32]247,[System.Int32]69))
$textBox2.Name = [System.String]'textBox2'
$textBox2.Size = (New-Object -TypeName System.Drawing.Size -ArgumentList @([System.Int32]106,[System.Int32]20))
$textBox2.TabIndex = [System.Int32]2
#
#label1
#
$label1.AutoSize = $true
$label1.Location = (New-Object -TypeName System.Drawing.Point -ArgumentList @([System.Int32]12,[System.Int32]72))
$label1.Name = [System.String]'label1'
$label1.Size = (New-Object -TypeName System.Drawing.Size -ArgumentList @([System.Int32]47,[System.Int32]13))
$label1.TabIndex = [System.Int32]3
$label1.Text = [System.String]'textbox1'
#
#label2
#
$label2.AutoSize = $true
$label2.Location = (New-Object -TypeName System.Drawing.Point -ArgumentList @([System.Int32]194,[System.Int32]72))
$label2.Name = [System.String]'label2'
$label2.Size = (New-Object -TypeName System.Drawing.Size -ArgumentList @([System.Int32]47,[System.Int32]13))
$label2.TabIndex = [System.Int32]4
$label2.Text = [System.String]'textbox2'
#
#Form1
#
$Form1.ClientSize = (New-Object -TypeName System.Drawing.Size -ArgumentList @([System.Int32]374,[System.Int32]110))
$Form1.Controls.Add($label2)
$Form1.Controls.Add($label1)
$Form1.Controls.Add($textBox2)
$Form1.Controls.Add($textBox1)
$Form1.Controls.Add($button2)
$Form1.Name = [System.String]'Form1'
$Form1.ResumeLayout($false)
$Form1.PerformLayout()
Add-Member -InputObject $Form1 -Name base -Value $base -MemberType NoteProperty
Add-Member -InputObject $Form1 -Name button2 -Value $button2 -MemberType NoteProperty
Add-Member -InputObject $Form1 -Name textBox1 -Value $textBox1 -MemberType NoteProperty
Add-Member -InputObject $Form1 -Name textBox2 -Value $textBox2 -MemberType NoteProperty
Add-Member -InputObject $Form1 -Name label1 -Value $label1 -MemberType NoteProperty
Add-Member -InputObject $Form1 -Name label2 -Value $label2 -MemberType NoteProperty
Add-Member -InputObject $Form1 -Name button1 -Value $button1 -MemberType NoteProperty
}
. InitializeComponent

Answer 1

Function fA sees your variable, because it runs in a child scope of the script block in which $localVar was created - this is general PowerShell behavior, and not specific to the ISE.

When you create a variable with $localVar = ..., it is local in the following sense:

• visible and directly modifiable in the same scope, but not in any parent scopes.

• visible in all child scopes, but not directly modifiable there.

  • Caveat: Functions imported from modules run in a separate scope domain (a.k.a. session (sub)state) that only shares the global scope as an ancestor with code running outside of modules and in other modules. Therefore, a module imported from a function does not see its caller's variables (and functions and aliases), if the caller is in non-module code (other than the global scope) or from a different module.
    Another way of putting it: a function imported from a module does not run in a child scope of a non-module caller (outside the global scope), and therefore doesn't see that caller's definitions.

  • You can use the $private: scope modifier to prevent child scopes from seeing a variable.

  • If you assign to a variable (by name only) that was originally created in a parent scope, (e.g., $localVar = ...), you'll instead create a new, local variable, in the current scope, which shadows the original variable.

    • It is possible to modify a parent scope's variables, but you need to use either Set-Variable -Scope or a scope modifier such as $script: (see links below).

For more information, see:

• Get-Help about_Scopes

• the last section of this answer, which provides a concise summary.