async use with FileSystemWatcher in Net 5

Question

I'm trying to create an application in Net 5 that watches a folder and any time files are being dropped in the folder, it should run a certain set of tasks (getting some info from the files, copy them to new location, among others). I thought I could implement both Net's FileSystemWatcher and concurrent collections (from the Collections.Concurrent namespace here, but I run into the problem that I cannot run it with being async.

I initialize the watcher like this (following the docs of MS):

public BlockingCollection<string> fileNames = new BlockingCollection<string>();
public void InitiateWatcher()
{
    using FileSystemWatcher watcher = new FileSystemWatcher(@"C:\temp"); //test dir

    watcher.NotifyFilter = NotifyFilters.Attributes
             | NotifyFilters.CreationTime
             | NotifyFilters.DirectoryName
             | NotifyFilters.FileName

    watcher.Created += OnCreated;

    watcher.Filter = "*.*";
    watcher.IncludeSubdirectories = true;
    watcher.EnableRaisingEvents = true;

    Console.WriteLine("Press e to exit.");
    Console.ReadLine();
}

private void OnCreated(object sender, FileSystemEventArgs e)
{
    string value = $"Created: {e.FullPath}";
    // this of course creates problems, since it is a async task running in sync mode.
    await PipelineStages.ReadFilenamesAsync(_sourcePath, fileNames);
    // do other work with the result from ReadFilenameAsync
    Console.WriteLine(value);
}

My PipelineStages class, which does most of the real work with the files, looks like this:

public static class PipelineStages
{
    public static Task ReadFilenamesAsync(string path, BlockingCollection<string> output)
    {
        return Task.Factory.StartNew(() =>
        {
            foreach (string fileName in Directory.EnumerateFiles(path, "*.*", SearchOption.AllDirectories))
            {
                output.Add(fileName);
            }
            output.CompleteAdding();
        }, TaskCreationOptions.LongRunning);
    }
}

If I turn the OnCreated method async it throws error that the return type is not valid. That kinda makes sense to me, although I don't know the way forward on this.

I see two errors: One error is when the code hits the output.add(fileName) line, it throws an System.InvalidOperationException: 'The collection has been marked as complete with regards to additions.'

The other one is that I notice in the onCreated method that filenames get written to the console that are in very different folders, seemingly randomly.

So, basically, I have two questions:

Is this the right approach? (If not, what are other alternatives)
If this is the right approach, how can I fix these problems?

`FileSystemWatcher` handlers should do as little as possible and return as quickly as possible. A better approach IMO is to dump off the notification to another thread. I personally use an instance of the `System.Threading.Channels.Channel` class. Each one of my `FileSystemWatcher` event-handlers takes the event data, writes it to Channel, and returns. Meanwhile my main thread has a one-time await-function that continuously monitors the channel (with `await foreach`) and prossesses each notification. — Joe, Mar 08 '22 at 20:38
Does this answer your question? [Should I avoid 'async void' event handlers?](https://stackoverflow.com/questions/19415646/should-i-avoid-async-void-event-handlers) — tymtam, Mar 09 '22 at 00:44
Have you tried making the event handler asynchronous, by adding the `async` keyword? `private async void OnCreated(object sender, FileSystemEventArgs e)` What exactly is the compile error that you got? — Theodor Zoulias, Mar 09 '22 at 01:53
@TheodorZoulias there is no compile error. The error is when running the code: When trying to add the filename to the BlockingCollection output it throws an error System.InvalidOperationException: 'The collection has been marked as complete with regards to additions.' Updated my post. — CMorgan, Mar 09 '22 at 17:04
Have you tried removing the `output.CompleteAdding();` line? — Theodor Zoulias, Mar 09 '22 at 21:12

async use with FileSystemWatcher in Net 5

0 Answers0