异步关键字和的TaskScheduler的选择 [英] async keyword and choice of the TaskScheduler
问题描述
我想知道用async关键字编译时，编译器选择的TaskScheduler的方式背后的原因。
I would like to know the reasoning behind the way the compiler choose the TaskScheduler when compiling using the async keyword.
我的测试方法由SignalR（ASP.NET主机，IIS8，WebSocket的运输）的OnConnectedAsync方法。
My test method is called by SignalR (ASP.NET host, IIS8, websocket transport) on the OnConnectedAsync method.
protected override async Task OnConnectedAsync(IRequest request, string connectionId)
{
SendUpdates();
}
启动对当前同步上下文任务将导致在System.Web.AspNetSynchronizationContext.OperationStarted（）一个InvalidOperationException
Starting a task on the Current synchronization context will result to an InvalidOperationException in System.Web.AspNetSynchronizationContext.OperationStarted()
这是异步操作不能在这个时间开始。异步操作只能在异步处理程序或模块内或在在页面生命周期的某些事件开始。如果在执行一个页面出现了这种异常，保证页面标记＆LT;％@页面异步=真正的％方式＆gt;
An asynchronous operation cannot be started at this time. Asynchronous operations may only be started within an asynchronous handler or module or during certain events in the Page lifecycle. If this exception occurred while executing a Page, ensure that the Page is marked
<%@ Page Async="true" %>.
精细。有了这个SendUpdates定义，我得到了上面的异常：
Fine. With this SendUpdates definition, I get the above exception:
private async void SendUpdates()
{
Task.Run(async () =>
{
while (true)
{
await Task.Delay(1000);
await Connection.Broadcast("blabla");
}
});
}
不过，更有趣的是，当我不明白的例外。以下工作：
But even more interesting is when I don't get the exception. The following works:
private void SendUpdates()
和下面的作品太
private async Task SendUpdates()
此最后一个工作过，但它的基本相同的上述实施例。
this last one works too, but it's essentially the same as the above example.
private Task SendUpdates()
{
return Task.Run(async () =>
{
while (true)
{
await Task.Delay(1000);
await Connection.Broadcast("blabla");
}
});
}
你知道编译器如何选择在这里使用的调度？
Do you know how the compiler choose which scheduler to use here?
推荐答案
其中一个主要的指导方针以书面形式异步 code是避异步无效 - 即，使用异步任务而不是的异步无效除非你'重新实施一个异步事件处理程序。
One of the primary guidelines in writing async code is "avoid async void" - that is, use async Task instead of async void unless you're implementing an async event handler.
异步无效方法使用的 OperationStarted 的SynchronizationContext 和 OperationCompleted ;看到我的MSDN文章这是所有关于SynchronizationContext的了解更多详情。
async void methods use SynchronizationContext's OperationStarted and OperationCompleted; see my MSDN article It's All about the SynchronizationContext for more details.
ASP.NET检测调用 OperationStarted 和（正确地）拒绝它，因为它是非法的把一个异步事件处理程序存在。当你纠正code使用异步任务，然后ASP.NET不再看到一个异步事件处理程序
ASP.NET detects the call to OperationStarted and (correctly) rejects it because it's illegal to put an async event handler there. When you correct the code to use async Task, then ASP.NET no longer sees an async event handler.
您可能会发现我的介绍到异步 / 等待帖子有帮助的。
You may find my intro to async / await post helpful.
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