c# - 使用 File.Copy 时如何实现超时？

Question

我正在使用 System.IO.File.Copy 将文件从远程共享复制到本地系统。如果复制时间太长，如何实现超时？

Answer 1

例如，可以使用async-await模式来完成:

Task timeoutTask = Task.Delay(TimeSpan.FromSeconds(10));

// I use a completion source to set File.Copy thread from its own
// thread, and use it later to abort it if needed
TaskCompletionSource<Thread> copyThreadCompletionSource = new TaskCompletionSource<Thread>();

// This will await while any of both given tasks end.
await Task.WhenAny
(
    timeoutTask,
    Task.Factory.StartNew
    (
        () =>
        {
            // This will let main thread access this thread and force a Thread.Abort
            // if the operation must be canceled due to a timeout
            copyThreadCompletionSource.SetResult(Thread.CurrentThread);
            File.Copy(@"C:\x.txt", @"C:\y.txt");
        }
    )
);


// Since timeoutTask was completed before wrapped File.Copy task you can 
// consider that the operation timed out
if (timeoutTask.Status == TaskStatus.RanToCompletion)
{
    // Timed out!
    Thread copyThread = await copyThreadCompletionSource.Task;
    copyThread.Abort();
}

您可以封装它以便在需要时重新使用它:

public static class Timeout
{
    public static async Task<bool> ForAsync(Action operationWithTimeout, TimeSpan maxTime)
    {
        Contract.Requires(operationWithTimeout != null);

        Task timeoutTask = Task.Delay(maxTime);
        TaskCompletionSource<Thread> copyThreadCompletionSource = new TaskCompletionSource<Thread>();

        // This will await while any of both given tasks end.
        await Task.WhenAny
        (
            timeoutTask,
            Task.Factory.StartNew
            (
                () =>
                {
                    // This will let main thread access this thread and force a Thread.Abort
                    // if the operation must be canceled due to a timeout
                    copyThreadCompletionSource.SetResult(Thread.CurrentThread);
                    operationWithTimeout();
                }
            )
        );


        // Since timeoutTask was completed before wrapped File.Copy task you can 
        // consider that the operation timed out
        if (timeoutTask.Status == TaskStatus.RanToCompletion)
        {
            // Timed out!
            Thread copyThread = await copyThreadCompletionSource.Task;
            copyThread.Abort();

            return false;
        }
        else
        {
            return true;
        }             
    }
}

在项目中的某个地方，您可以这样调用上述方法:

bool success = await Timeout.ForAsync(() => File.Copy(...), TimeSpan.FromSeconds(10));

if(success)
{
   // Do stuff if File.Copy didn't time out!
}

请注意，我使用了 Thread.Abort() 而不是使用 CancellationToken。在您的用例中，您需要调用一个无法使用所谓的取消模式的同步方法，我相信这可能是 Thread.Abort() 可以使用的少数情况之一是一个有效的选项。

最终，如果超时，代码将中止执行File.Copy的线程，因此，它应该足以停止 I/O 操作。