Question

我有一个观察者方法，我从fomr1的构造函数调用：

FileSystemWatcher watcher;
        private void WatchDirectory()
        {
            watcher = new FileSystemWatcher();
            watcher.Path = userVideosDirectory;
            watcher.NotifyFilter = NotifyFilters.LastWrite | NotifyFilters.Size;
            watcher.Filter = "*.mp4";
            watcher.Changed += new FileSystemEventHandler(OnChanged);
            watcher.EnableRaisingEvents = true;
        }

然后是Onchanged事件：

private void OnChanged(object source, FileSystemEventArgs e)
        {

                var info = new FileInfo(e.FullPath);
                fileforupload = info.FullName;
                if (IsFileLocked(info) == false)
                {
                    sy.SetResult(true);
                    watcher.EnableRaisingEvents = false;
                    watcher.Dispose();
                }
        }

这是IsFileLocked方法：

protected virtual bool IsFileLocked(FileInfo file)
        {
            FileStream stream = null;
            try
            {
                stream = file.Open(FileMode.Open, FileAccess.ReadWrite, FileShare.None);
            }
            catch (IOException)
            {
                return true;
            }
            finally
            {
                if (stream != null)
                    stream.Close();
            }

我在这种方法中使用它：

public string SendResponse(HttpListenerRequest request)
        {            
                    sy = new TaskCompletionSource<bool>();
                    WatchDirectory();
                    sy.Task.Wait();
                    Youtube_Uploader youtubeupload = new Youtube_Uploader(fileforupload);
                    return false;
        }

例外是OnChanged事件：

sy.SetResult(true);

sy是：

TaskCompletionSource<bool> sy;

在OnChanged方法中我做了：

watcher.EnableRaisingEvents = false;
watcher.Dispose();

但有时仍会遇到异常。例外是：

An attempt was made to transition a task to a final state when it had already completed

System.InvalidOperationException was unhandled
  _HResult=-2146233079
  _message=An attempt was made to transition a task to a final state when it had already completed.
  HResult=-2146233079
  IsTransient=false
  Message=An attempt was made to transition a task to a final state when it had already completed.
  Source=mscorlib
  StackTrace:
       at System.Threading.Tasks.TaskCompletionSource`1.SetResult(TResult result)
       at Automatic_Record.Form1.OnChanged(Object source, FileSystemEventArgs e) in d:\C-Sharp\Automatic_Record\Automatic_Record\Automatic_Record\Form1.cs:line 197
       at System.IO.FileSystemWatcher.OnChanged(FileSystemEventArgs e)
       at System.IO.FileSystemWatcher.NotifyFileSystemEventArgs(Int32 action, String name)
       at System.IO.FileSystemWatcher.CompletionStatusChanged(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* overlappedPointer)
       at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)
  InnerException:

Answer 1

这种情况正在发生，因为FileSystemWatcher会针对任何单个更改触发多个onChanged事件。因此，在禁用EnableRaisingEvent之前，观察者可能已经触发了多个onChanged事件。您需要在onChanged方法中放置锁定，或者使用TaskCompletionSource.SetResult而不是使用TaskCompletionSource.TrySetResult。

有关TrySetResult的详细信息，请参阅此处：https://msdn.microsoft.com/en-us/library/dd449176(v=vs.110).aspx

我有时会在System.Threading.Tasks.TaskCompletionSource <bool>上一直遇到异常我该如何解决？

1 个答案: