"mocking" environment variables in a ScriptBlock

Invoke-WithMockedEnv-Demo.ps1

function Invoke-WithMockedEnv {
  param(
    [Parameter(ValueFromPipeline)]
    [scriptblock]$ScriptBlock,

    [Hashtable]$mockValues
  )

  $originalValues = @{ }
  foreach ($v in $mockValues.Keys) {
    $sKey = "env:$v"
    if (Test-Path $sKey) {
      Write-Verbose "backing up '$sKey'"
      $originalValues[$v] = (Get-Item -Path $sKey).Value
    }
    else {
      $originalValues[$v] = $null
    }
    Set-Item -Path $sKey -Value $mockValues[$v]
  }
  try {
    & $ScriptBlock
  }
  finally {
    foreach ($v in $originalValues.Keys) {
      $sKey = "env:$v"
      Set-Item -Path $sKey -Value $originalValues[$v]
    }
  }
}

function Print-EnvVar($varName) {
  $envVarName = "env:$varName"
  if (Test-Path $envVarName) {
    $v = Get-Item $envVarName
    Write-Host " (+) $envVarName = $($v.Value)" -ForegroundColor Green
  }
  else {
    Write-Host " (!) $envVarName does not exist" -ForegroundColor Red
  }
}

function Print-SomeEnvValues {
  Print-EnvVar "COMPUTERNAME"
  Print-EnvVar "weridval"
}

Write-Host " -- before mock -- "
Print-SomeEnvValues

Write-Host " -- mocked -- "
$mockedVals = @{
  COMPUTERNAME = "flynns-box"
  weridval     = "whatever"
}
Invoke-WithMockedEnv -mockValues $mockedVals {
  Print-SomeEnvValues
}

Write-Host " -- after mock -- "
Print-SomeEnvValues

if you execute this piece of code, it outputs

 -- before mock -- 
 (+) env:COMPUTERNAME = MWINBOX2
 (!) env:weridval does not exist
 -- mocked --
 (+) env:COMPUTERNAME = flynns-box
 (+) env:weridval = whatever
 -- after mock --
 (+) env:COMPUTERNAME = MWINBOX2
 (!) env:weridval does not exist