PowerShell Pitfalls (Non-Fatal Exceptions, Test-Path, Drives, Jobs)

So if you are trying to use PowerShell for some significant real stuff you will run into many pitfalls just like me. So much so I could dedicate a few blog posts just to get some functions out, but in trying to mount a TrueCrypt drive and checking its existence afterwards I ran into many annoying problems. So in C# I would just use Directory.Exists() and be done with it. But the PowerShell way is Test-Path. You can still use the C# version [System.IO.Directory]::Exists() but since using Test-Path and wanting to be more PowerShelly I stuck to it and forgot about Directory.Exists() I mean its the same thing anyway, right? No way! It seems like there are some other checks which I'm not sure what perhaps permissions etc the function tests for both existence and accessibility (permissions). All i really wanted to test was if the drive exists. So one other quick way is to useGet-ChildItem "C:\" and wrap this in a try except and check for exception of type System.Management.Automation.DriveNotFoundException

So much like:

function DriveExists($path)
{ 
 try
 {

  Get-ChildItem $path | Out-Null
 }
 catch
 {
  if ($_.Exception.gettype() -eq [System.Type]::GetType("System.Management.Automation.DriveNotFoundException"))
  {
   Write-Host "FAIL!"
   Write-Host $Error[0].exception
   return $false   
  }  
 }
 return $true
}

But this doesn't work either because the catch block is never executed, WTF? So it turns out that in powershell only fatal exceptions will invoke the catch block: To get around this you have to check the return value from the last command as suggested and then check the error:

   function DriveExists($path)
{ 
 Get-ChildItem $path -ErrorAction SilentlyContinue | Out-Null
 if (-not $?) # http://www.neolisk.com/techblog/powershell-specialcharactersandtokens
 {
  if ($Error[0].exception.gettype() -eq [System.Type]::GetType("System.Management.Automation.DriveNotFoundException"))
  {
   return $false   
  }    
 }  

 return $true
}

Now that I have this handy function again i found that for some reason a new drive mounted while executing the script cannot be seen, again WTF? It's as if it is cached in the session and can't be accessed. Then I think hey let me just test the existence by using Directory.Exists() and what do you know it works. In order to get Test-Path to work I had to run this out of process in a new session using a job, so something like this:

function DriveExistUsingJob($path)
{
 $ScriptBlock = {  Write-Output (Test-Path $args[0]) }
 $Job = Start-Job -ScriptBlock $ScriptBlock -ArgumentList @($path)
 Wait-Job $job | Out-Null
 $jobResult = [string](Receive-Job -Job $Job )
 while ($job.HasMoreData)
 {
  $jobResult = [string](Receive-Job -Job $Job)
 }
 
 return $jobResult
}

So why did I use PowerShell's Test-Path function one in the first place? I don't know I think I'll stick to the C# functions where I can, at least I know what that does. Remember: All I really wanted to do was to say Directory.Exists(), but no... It really makes me feel like Windows Powers Hell or at least mine.