Getting notified when a file lock is released

问题

[Using C# and Windows as platform]

I have a camera that writes JPG files to a local folder in my PC. I want to load each file the camera drops, so I have a FileSystemWatcher that notifies me whenever a new picture is created, but the camera locks the file while it's being written, so if I try to load it just after being notified of its creation, I get an exception saying that the file is locked.

Currently, I have a while loop (with a Thread.Sleep) that retries to load the image every 0.2 seconds, but it feels a bit dirty.

Is there a more elegant way to wait until the lock has been released, so I can load the file being sure that it's no longer used??

回答1:

You will not be able to get around trial and error approach, i.e., try to open the file, catch IOException, try again. However, you can hide this ugliness in a separate class like so:

public class CustomWatcher
{
    private readonly FileSystemWatcher watcher;
    public event EventHandler<FileSystemEventArgs> CreatedAndReleased;

    public CustomWatcher(string path)
    {
        watcher = new FileSystemWatcher(path, "*.jpg");
        watcher.Created += OnFileCreated;
        watcher.EnableRaisingEvents = true;
    }

    private void OnFileCreated(object sender, FileSystemEventArgs e)
    {
        // Running the loop on another thread. That means the event
        // callback will be on the new thread. This can be omitted
        // if it does not matter if you are blocking the current thread.
        Task.Run(() =>
        {
            // Obviously some sort of timeout could be useful here.
            // Test until you can open the file, then trigger the CreeatedAndReleased event.
            while (!CanOpen(e.FullPath))
            {
                Thread.Sleep(200);
            }
            OnCreatedAndReleased(e);
        });
    }

    private void OnCreatedAndReleased(FileSystemEventArgs e)
    {
        CreatedAndReleased?.Invoke(this, e);
    }

    private static bool CanOpen(string file)
    {
        FileStream stream = null;
        try
        {
            stream = File.Open(file, FileMode.Open, FileAccess.Read, FileShare.None);
        }
        catch (IOException)
        {
            return false;
        }
        finally
        {
            stream?.Close();
        }
        return true;
    }
}

This "watcher" can be used like this:

var watcher = new CustomWatcher("path");
watcher.CreatedAndReleased += (o,e) => 
{
    // Now, your watcher has managed to open and close the file,
    // so the camera is done with it. Obviously, any other application
    // is able to lock it before this code manages to open the file.
    var stream = File.OpenRead(e.FullPath);
}

Disclaimer: CustomWatcher probably needs to be IDisposable and dispose of the FileSystemWatcher appropriately. The code merely shows an example of how to achieve the desired functionality.

来源：https://stackoverflow.com/questions/36205450/getting-notified-when-a-file-lock-is-released