Functions/Set-DefaultValue.ps1
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<#
.SYNOPSIS This function sets a variable to a specified default value in the global scope if it is null. .EXAMPLE $var = $null $var | Set-DefaultValue 42 # $var is now 42 .EXAMPLE $var = 5 $var | Set-DefaultValue 42 # $var is still 5 #> function Set-DefaultValue { [CmdletBinding(PositionalBinding=$true)] param ( # The default value for the variable. [Parameter(Mandatory=$true)] [ValidateNotNull()] $defaultValue, # The variable to be assigned a default value if it is null. [Parameter(ValueFromPipeline=$true)] [AllowNull()] $variable ) # Retrieve the line containing the invocation of this function $invocationLine = ((Get-Variable -Scope 0) | Where-Object { $_.Name -eq "MyInvocation" }).Value.Line # This function should be passed the external variable using the pipeline # If not, we cannot extract the name of the external variable if ($invocationLine.IndexOf("|") -eq -1) { throw "Set-DefaultValue should be given the variable through the pipeline - `$variable | Set-DefaultValue `$defaultValue." } # Extract the name of the external variable $externalVariableName = $invocationLine.Split("|")[0].Trim().TrimStart("$") # Set the external variable to its default value if ($null -eq $variable) { # Remove the variable from all scopes to ensure that only the global one will exist Remove-Variable -Name $externalVariableName -Scope Local -Force -Confirm:$false -ErrorAction SilentlyContinue Remove-Variable -Name $externalVariableName -Scope Script -Force -Confirm:$false -ErrorAction SilentlyContinue Remove-Variable -Name $externalVariableName -Scope 0 -Force -Confirm:$false -ErrorAction SilentlyContinue Remove-Variable -Name $externalVariableName -Scope 1 -Force -Confirm:$false -ErrorAction SilentlyContinue # Set the global variable to the default value Write-Verbose "Setting global variable '$($externalVariableName)' to the default value '$($defaultValue)'." Invoke-Expression "`$Global:$($externalVariableName) = `$defaultValue" } } |