Using FileSystemWatcher Correctly (Part 1)

by Dec 17, 2018

A FileSystemWatcher can monitor a file or folder for changes, so your PowerShell code can immediately be notified when new files are copied to a folder, or when files are deleted or changed.

Often, you find example code for synchronous monitoring like this:

# make sure you adjust this
$PathToMonitor = "c:\test"

$FileSystemWatcher = New-Object System.IO.FileSystemWatcher
$FileSystemWatcher.Path  = $PathToMonitor
$FileSystemWatcher.IncludeSubdirectories = $true

Write-Host "Monitoring content of $PathToMonitor"
explorer $PathToMonitor
while ($true) {
  $Change = $FileSystemWatcher.WaitForChanged('All', 1000)
  if ($Change.TimedOut -eq $false)
      # get information about the changes detected
      Write-Host "Change detected:"
      $Change | Out-Default

      # uncomment this to see the issue
      #Start-Sleep -Seconds 5
      Write-Host "." -NoNewline

This example works just fine. When you add files to the folder being monitored, or make changes, the type of change is detected. You could easily take that information and invoke actions. For example, in your IT, people could drop files with instructions into a drop folder, and your script could respond to these files.

However, this approach has a back side: when a change is detected, control is returned to your script so it can process the change. If another file change occurs while your script is no longer waiting for events, it gets lost. You can easily check for yourself:

Add a lengthy statement such as “Start-Sleep -Seconds 5” to the code that executes when a change was detected, then apply multiple changes to your folder. As you’ll see, the first change will be detected, but during the five second interval in which PowerShell is kept busy, all other change events get lost.

In tomorrows’ tip we’ll make sure your FileSystemWatcher isn’t skipping any changes!

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