c# - 释放文件锁时收到通知

Question

[使用C#和Windows作为平台]

我有一台相机，可以将 JPG 文件写入我 PC 的本地文件夹中。我想加载相机丢弃的每个文件，所以我有一个 FileSystemWatcher，它会在创建新图片时通知我，但相机在写入文件时锁定了文件，所以如果我尝试加载它在收到创建文件的通知后，我收到一个异常消息，提示文件已锁定。

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到锁被释放，这样我就可以加载文件并确保它不再被使用？

Answer 1

您将无法绕过试错法，即尝试打开文件，捕获 IOException，然后重试。但是，您可以像这样在单独的类中隐藏这种丑陋之处:

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“watcher”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 并适本地处理 FileSystemWatcher。该代码仅显示了如何实现所需功能的示例。