About This Article
This article was created using an automated generation workflow leveraging generative AI. It reviews the Microsoft Learn FileSystemWatcher specifications and organizes them into a trial that separately observes duplicate events, debouncing, and missed events.Verification Status: 📘 Official .NET specifications verified, PowerShell physical device verification pending
Success Criteria for This Attempt: Success is achieved if multiple events arrive for the same file in a short time, and subsequent processing candidates are successfully consolidated into a single execution.
FileSystemWatcher does not guarantee that saving a file once will always trigger exactly one Changed event. Microsoft also explains that multiple events can occur during typical file system operations.
Smoke Test: Observing Raw Events First
$dir = Join-Path $env:TEMP 'papanda-watch'
New-Item -ItemType Directory -Force $dir | Out-Null
$w = [IO.FileSystemWatcher]::new($dir)
$w.EnableRaisingEvents = $true
$sub = Register-ObjectEvent $w Changed -Action {
"[EVENT] $([DateTime]::Now.ToString('HH:mm:ss.fff')) $($Event.SourceEventArgs.Name)"
}
"A" | Set-Content (Join-Path $dir 'demo.txt')
"B" | Add-Content (Join-Path $dir 'demo.txt')
Start-Sleep 1
Unregister-Event -SourceIdentifier $sub.Name -ErrorAction SilentlyContinue
$w.Dispose()
Check Here
[EVENT]Observe how many lines are output. Because the number of events can vary depending on the environment, we do not fix expectations to a rigid count like "it should appear twice".Treating the Potential for Duplication as a Design RequirementThis is a crucial perspective.
Understanding Debouncing
flowchart LR E1[event] --> T[タイマーを更新] E2[event] --> T E3[event] --> T T -->|静かな時間が経過| J[後続処理を1回]
Debouncing is a method of taking rapid consecutive events and "waiting until things settle down to consolidate them into a single event." However, this isnot a countermeasure against missed events. It is not a persistent queue that guarantees protection against internal FileSystemWatcher buffer overflows or changes occurring while the process is stopped.
Trying a Configuration Change
Comparing wait times of 100ms and 1000ms reveals the trade-off between consolidation ease and response speed. In practical business applications, to "avoid starting processing before the file copy completes," separate readiness checks are necessary, such as verifying whether the file size has stabilized or whether the file can be successfully opened.
Applying to Production Work
This can be applied to automatic ingestion of CSV files arriving in a shared folder, post-processing of scanned PDFs, and log monitoring. For administrative tasks, rather than "processing immediately when a file is placed," building a flow that includesevent reception -> debouncing -> file readiness check -> recording processed IDswill reduce incidents.
Failures and Edge Cases
Debouncing cannot recover lost events.
We will test overwriting files with the same name, renaming, and bulk file insertion in separate scenarios.
Subsequent processing should be made idempotent, designed so that it does not break even if the same file is received twice.
Clean up watchers and event subscriptions properly using constructs like finally blocks.
