How do I implement a timeout when using File.Copy?

Question

I'm using System.IO.File.Copy to copy a file from a remote share to my local system. How can I implement a timeout if the copy takes too long?

Answer 1

For example, it can be done this way using async-await pattern:

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();
}

You might encapsulate this to re-use it whenever you want:

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;
        }             
    }
}

Somewhere in your project you might call the above method this way:

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

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

Note I've used Thread.Abort() instead of using CancellationToken. In your use case you need to call a synchronous method for which you can't use the so-called cancellation pattern, and I believe this can be one of the few cases where Thread.Abort() could be a valid option.

At the end of the day, if there's a timeout, the code will abort the thread executing the File.Copy, thus, it should be enough to stop the I/O operation.

Answer 2

You can implement a simple method something like the following, built on Stream.CopyToAsync() which accepts a cancellation token:

static async Task Copy(string destFilePath, string sourceFilePath, int timeoutSecs)
{
    var cancellationSource = new CancellationTokenSource(TimeSpan.FromSeconds(timeoutSecs));

    using (var dest = File.Create(destFilePath))
    using (var src = File.OpenRead(sourceFilePath))
    {
        await src.CopyToAsync(dest, 81920, cancellationSource.Token);
    }
}

As you can see, it is possible to create a CancellationTokenSource() which automatically cancels itself after the specified time.

You can use the Copy method using async:

try
{
    await Copy(@"c:\temp\test2.bin", @"c:\temp\test.bin", 60);
    Console.WriteLine("finished..");
}
catch (OperationCanceledException ex)
{
    Console.WriteLine("cancelled..");
}
catch (Exception ex)
{
    Console.WriteLine("error..");
}

or the old way:

var copyInProgress = Copy(@"c:\temp\test2.bin", @"c:\temp\test.bin", 60);

copyInProgress.ContinueWith(
        _ => { Console.WriteLine("cancelled.."); },
        TaskContinuationOptions.OnlyOnCanceled
    );

copyInProgress.ContinueWith(
        _ => { Console.WriteLine("finished.."); },
        TaskContinuationOptions.OnlyOnRanToCompletion
    );

copyInProgress.ContinueWith(
        _ => { Console.WriteLine("failed.."); },
        TaskContinuationOptions.OnlyOnFaulted
    );

copyInProgress.Wait();

It is easy to improve the above code to use a second cancellation token which can be controlled by the user (via cancel button). All you need to use is CancellationTokenSource.CreateLinkedTokenSource