// 有返回值
public static async Task<TResult> TimeoutAfter<TResult>(this Task<TResult> task,TimeSpan timeout)
{
    using (var timeoutCancellationTokenSource = new CancellationTokenSource())
    {
        var completedTask = await Task.WhenAny(task,Task.Delay(timeout,timeoutCancellationTokenSource.Token));
        if (completedTask == task)
        {
            timeoutCancellationTokenSource.Cancel();
            return await task;  // Very important in order to propagate exceptions
        }
        else
        {
            throw new TimeoutException("The operation has timed out.");
        }
    }
}

// 无返回值
public static async Task TimeoutAfter(this Task task,TimeSpan timeout) {
    using (var timeoutCancellationTokenSource = new CancellationTokenSource())
    {
        var completedTask = await Task.WhenAny(task,timeoutCancellationTokenSource.Token));
        if (completedTask == task)
        {
            timeoutCancellationTokenSource.Cancel();
            await task;  // Very important in order to propagate exceptions
        }
        else
        {
            throw new TimeoutException("The operation has timed out.");
        }
    }
}