具有超时的WriteAsync [英] WriteAsync with timeout

问题描述

我尝试编写一个具有超时的简单异步写入，如下所示，并且在缓冲区很大且等待时间很小的情况下，我希望该函数抛出TaskCanceledException。但是，这不会发生。 WriteAsync将阻塞许多秒钟，直到写入完成。我想念什么？

I try to code a simple async write with timeout as below and expect the function to throw a TaskCanceledException given a very large buffer and small waitTime. However, this does not happen. WriteAsync will block for many seconds until the write completes. What am I missing?

public async void WriteWithTimeout(Stream os, byte[] buf, int waitMs)
{
    CancellationTokenSource tokenSource = new CancellationTokenSource(waitMs); // cancel after waitMs milliseconds.
    await os.WriteAsync(buf, 0, buf.Length, tokenSource.Token);

    return;
}

从GUI线程调用：

try
{
    WriteWithTimeout(response.OutputStream, buf100M, w1ms);
}
catch(OperationCanceledException e)
{
    ConsoleWriteLine("Failed with exception: {0}", e.Message);
}        

推荐答案

异步无效。它必须返回一个任务，您必须等待它。

You can't catch a async void. It must return a task and you have to await it.

public async Task WriteWithTimeout(Stream os, byte[] buf, int waitMs)
{
    CancellationTokenSource tokenSource = new CancellationTokenSource(waitMs); // cancel after waitMs milliseconds.
    await os.WriteAsync(buf, 0, buf.Length, tokenSource.Token);

    return;
}

gui代码

try
{
    await WriteWithTimeout(response.OutputStream, buf100M, w1ms);
}
catch(OperationCanceledException e)
{
    ConsoleWriteLine("Failed with exception: {0}", e.Message);
}

取消仍会发生，但您只是看不到它们。

The cancellations still happen but you just don't observe them.

更新：

os.WriteAsync（只是一个同步完成，由 Task.Run（在幕后 ¹ 支持。取消令牌不会取消已经运行的 Task.Run（。在这种情况下，最好的方法是将取消内容包装在 Task.WhenAny（令牌也通过无限长的 Task.Delay（。

It is possible that os.WriteAsync( is just a synchronous completion just backed by a Task.Run( behind the scenes¹. The cancellation token won't cancel a already running Task.Run(. In that case the best way is to wrap the cancellation up in a Task.WhenAny( and pass in the token to there too via a infinitely long Task.Delay(.

public async Task WriteWithTimeout(Stream os, byte[] buf, int waitMs)
{
    CancellationTokenSource tokenSource = new CancellationTokenSource(waitMs); // cancel after waitMs milliseconds.
    var task = os.WriteAsync(buf, 0, buf.Length, tokenSource.Token);
    var waitedTask = await Task.WhenAny(task, Task.Delay(-1, token));
    await waitedTask; //Wait on the returned task to observe any exceptions.
}

---

^{1。例如，如果不将 FileOptions.Asynchronous 传递给构造函数，则这是 FileStream 的默认行为}

---

^{1. For example that is the default behavior of a FileStream if you don't pass inFileOptions.Asynchronous to the constructor}

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