文件锁释放时收到通知

[使用C#和Windows作为平台]

我有一台相机，可以将 JPG 文件写入电脑的本地文件夹中。我想加载相机丢弃的每个文件，因此我有一个 FileSystemWatcher，它会在创建新图片时通知我，但相机在写入文件时锁定文件，所以如果我在收到创建通知后尝试加载它，我会收到一个异常：文件被锁定.

目前，我有一个 while 循环（带有 Thread.Sleep），每 0.2 秒重试一次加载图像，但感觉有点脏。

有没有更优雅的方法来等待锁被释放，这样我就可以加载文件并确保它不再使用？

您将无法绕过试错方法，即尝试打开文件，捕获 IOException，然后重试。但是，您可以将这种丑陋隐藏在单独的类中，如下所示：

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;
    }
}

这个“观察者”可以这样使用：

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);
}

免责声明：CustomWatcher 可能需要 IDisposable 并适当地处置 FileSystemWatcher。该代码仅显示如何实现所需功能的示例。