title: Using a try block to test if a file is locked in powershell
body: <p>I want to know if its bad form to use try blocks to test if a file is locked.  Here's the background.
  I need to send text output of an application to two serial printers simultaneously.  My solution was to use MportMon, and a Powershell script. The way it's supposed to work is the application default prints to the MportMon virtual printer port, which actually makes a uniquely named file in a "dropbox" folder.  The powershell script uses a filesystemwatcher to monitor the folder and when a new file is created, it takes the textual content and pushes it out two serial printers, then deletes the file, so as not to fill up the folder.  I was having a problem when trying to read the text from the file that the virtual printer created.  I found that I was getting errors becasue the file was still locked.  To fixed the problem, I used a FSM to impliment the logic and instead of checking for a lock everytime before attempting to get the content from the file, I used a try block that attempts to read content from the file, if it fails, the catch block just reaffirms the state that the FSM is in, and the process is repeated until successful.  It seems to work fine, but I've read somewhere that its bad practice.  Is there any danger in this method, or is it safe and reliable?  Below is my code. </p>
  
  <pre><code>$fsw = New-Object system.io.filesystemwatcher
  $q = New-Object system.collections.queue
  $path = "c:\DropBox"
  $fsw.path = $path
  $state = "waitforQ"
  [string]$tempPath = $null
  
      Register-ObjectEvent -InputObject $fsw -EventName created -Action {
      $q.enqueue( $event.sourceeventargs.fullpath )
      }
  
      while($true) {
      switch($state)
      {
          "waitforQ" {
                      echo "waitforQ"
                      if ($q.count -gt 0 ) {$state = "retrievefromQ"}
                      }
          "retrievefromQ"  {
                      echo "retrievefromQ"
                      $tempPath = $q.dequeue()
                      $state = "servicefile"
                      }
          "servicefile"  {
                          echo " in servicefile "
  
                      try
                          {
                          $text = Get-Content -ErrorAction stop $tempPath 
                          #echo "in try"
                          $text | out-printer db1
                          $text | out-printer db2
                           echo " $text "
                          $state = "waitforQ"
                          rm $tempPath
                          }
                      catch
                          {
                          #echo "in catch" 
                          $state = "servicefile"
                          }
                      }
  
             Default {    $state = "waitforQ"  }
      }
      }
  </code></pre>