Powershell and System.IO.FileSystemWatcher

Question

I am new to PowerShell and I am trying to use the System.IO.FileSystemWatcher to monitor the presence of a file in a specified folder. However, as soon as the file is detected I want to stop monitoring this folder immediately and stop the FileSystemWatcher. The plan is to incorporate the PowerShell script into a SQL Agent to enable users to restore their own databases. Basically I need to know the command to stop FileSystemWatcher from monitoring as soon as one file is found. Here is the script so far.

### SET FOLDER TO WATCH + FILES TO WATCH + SUBFOLDERS YES/NO
    $watcher = New-Object System.IO.FileSystemWatcher
    $watcher.Path = "C:\TriggerBatch"
    $watcher.Filter = "*.*"
    $watcher.IncludeSubdirectories = $true
    $watcher.EnableRaisingEvents = $true  

### DEFINE ACTIONS AFTER A EVENT IS DETECTED
    $action = { $path = $Event.SourceEventArgs.FullPath
                $changeType = $Event.SourceEventArgs.ChangeType
                $logline = "$(Get-Date), $changeType, $path"
                Add-content "C:\log2.txt" -value $logline              
              }    

### DECIDE WHICH EVENTS SHOULD BE WATCHED + SET CHECK FREQUENCY  
    $created = Register-ObjectEvent $watcher Created -Action $action

while ($true) {sleep 1} 

## Unregister-Event Created ??
##Stop-ScheduledTask ??

Answer 1

Unregister-Event $created.Id

This will unregister the event. You will probably want to add this to the $action.

Do note that if there are events in the queue they will still be fired.

This might help too.

Answer 2

Scriptblocks that are run as an action on a subscribed event have access to the $Args, $Event, $EventArgs and $EventSubscriber automatic variables.

Just add the Unregister-Event command to the end of your scriptblock, like so:

### DEFINE ACTIONS AFTER A EVENT IS DETECTED
$action = { $path = $Event.SourceEventArgs.FullPath
            $changeType = $Event.SourceEventArgs.ChangeType
            $logline = "$(Get-Date), $changeType, $path"
            Add-content "C:\log2.txt" -value $logline  
            Unregister-Event -SubscriptionId $EventSubscriber.SubscriptionId            
          }    

This is the pattern for an event that only performs an action once and then cleans itself up.

It's difficult to effectively explore these automatic variables since they are within the scope of a Job, but you can futz with them by assigning them to global variables while you are sketching out your code. You may also get some joy with Wait-Debugger and Debug-Runspace. In the case of the $EventSubscriber variable, it returns the exact object you get if you run Get-EventSubscriber (having created a single subscription already). That's how I found the SubscriptionId property.