PowerShell : retrieve JSON object by variable value

Question:

$json = ConvertFrom-Json "{key:true}"
$key = "key"
Write-Host $json[$key]

$json = ConvertFrom-Json "{key:true}"

$key = "key"

Write-Host $json[$key]

I’d like that to print true but it doesn’t. I know $json.key would work. Can this be done?

If you know that $json.key would work, then why you’re switching from dot to square brackets?
All these will work:

$json = ConvertFrom-Json "{key:true}"
$key = "key"

Write-Host $json.$key
Write-Host $json.$($key)
Write-Host $json."$key"

$json = ConvertFrom-Json "{key:true}"

$key = "key"

Write-Host $json.$key

Write-Host $json.$($key)

Write-Host $json."$key"