Why doesn't $hash.key syntax work inside the ExpandString method?

Question

The following Powershell script demonstrates the issue:

$hash = @{\'a\' = 1; \'b\' = 2}
Write-Host $hash[\'a\']        # => 1
Write-Host $hash.a                   


        

Answer 1

I was trying to store text that prompts the user in a text file. I wanted to be able to have variables in the text file that are expanded from my script.

My settings are stored in a PSCustomObject called $profile and so in my text I was trying to do something like:

Hello $($profile.First) $($profile.Last)!!!

and then from my script I was trying to do:

$profile=GetProfile #Function returns PSCustomObject 
$temp=Get-Content -Path "myFile.txt"
$myText=Join-String $temp
$myText=$ExecutionContext.InvokeCommand.ExpandString($myText) 

which of course left me with the error

Exception calling "ExpandString" with "1" argument(s): "Object reference not set to an instance of an object."

Finally I figured out I only needed to to store the PSCustomObject values I want in regular old variables, change the text file to use those instead of the object.property version and everything worked nicely:

$profile=GetProfile #Function returns PSCustomObject 
$First=$profile.First
$Last=$profile.Last
$temp=Get-Content -Path "myFile.txt"
$myText=Join-String $temp
$myText=$ExecutionContext.InvokeCommand.ExpandString($myText) 

And in the text I changed to

Hello $First $Last!!!

Answer 2

I use this method, since this bug exists in v4 (not in v5)

function render() {
    [CmdletBinding()]
    param ( [parameter(ValueFromPipeline = $true)] [string] $str)

    #buggy
    #$ExecutionContext.InvokeCommand.ExpandString($str)

    "@`"`n$str`n`"@" | iex
}

Usage for your example:

  '$($hash.a)' | render

Answer 3

The ExpandString api is not exactly meant for use from PowerShell scripts, it was added more for C# code. It's still a bug that your example doesn't work (and I think it's been fixed in V4), but it does mean there is a workaround - one that I recommend for general use.

Double quoted strings effectively (but not literally) call ExpandString. So the following should be equivalent:

$ExecutionContext.InvokeCommand.ExpandString('$($hash.a)')
"$($hash.a)"