Who watches the FileSystemWatcher?

One of the ways we sometimes implement communication with legacy systems is via the file-system. The legacy system will write files about some events into a predefined directory, and the other system watches this directory for changes. In C#/.NET, the handy FileSystemWatcher class is a great tool for that.

We had a working solution using that in production for the last two years. Then it suddenly stopped working. There was no apparent change in the related code, so I suspect our upgrade from .NET Core 2.1 to .NET 5.0 triggered the change in behavior. The code looked something like this:

public void BeginWatching()
{
    var filter = NotifyFilters.LastAccess | NotifyFilters.LastWrite
        | NotifyFilters.FileName | NotifyFilters.DirectoryName;

    var watcher = new FileSystemWatcher
    {
        Path = directory,
        NotifyFilter = filter,
        Filter = "*.txt",
        EnableRaisingEvents = true,
    };
    /* Hook up some events here... */
}

And after some debugging, it turned out none of the attached event handlers were firing anymore. The solution was to not let go of the watcher instance, and keep it around in the enclosing class.

this.watcher = new FileSystemWatcher

This makes sense, of course. Before, the watcher was only a local variable, and could be collected by the garbage collector at any moment. That, in turn, disabled the process, causing no more events to be emitted.

Interestingly, a few days later, my student asked about his FileSystemWatcher also no longer working. I immediatly suspected the same problem, but when we looked at the code, he had already moved the watcher into a property of the enclosing class. Turns out, for him the problem was just one level up: the enclosing class was only created as a local variable, and the contained watcher stopped after that went out of scope.

Now the only question is: why did we never observe this before? Either something in the GC changed, or something in the implementation of the watcher changed. Can anyone enlighten the situation?

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