WriteAsync 超时

我尝试编写一个带有超时的简单异步写入代码，如下所示，并期望该函数在给定非常大的缓冲区和较小的 waitTime 的情况下抛出 TaskCanceledException。然而，这并没有发生。 WriteAsync 将阻塞很多秒，直到写入完成。我缺少什么？

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);
}        

你无法捕获 async void。它必须返回一个任务，而你必须等待它。

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 code

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

取消仍然会发生，但你只是没有观察到它们。

Update:

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