为什么我的异步回调在同一线程中运行？

Question

I'm trying to use the FtpWebRequest async calls ( BeginGetResponse / EndGetResponse ).

However, it appears that the callback from BeginGetResponse is running in the same thread as my application, when I was under the impression it would use a different (and thread pool) thread. This means my application blocks inside the callback before continuing.

I've setup a LINQPad proof-of-concept as follows:

"Starting".Dump();
Thread.CurrentThread.GetHashCode().Dump(); 
Thread.CurrentThread.IsThreadPoolThread.Dump();

IAsyncResult result = request.BeginGetResponse((ar) => 
{
     "Inside Callback".Dump();
     Thread.CurrentThread.GetHashCode().Dump();
     Thread.CurrentThread.IsThreadPoolThread.Dump();
     var resp = request.EndGetResponse(ar);
     "Callback Complete".Dump();
}, null);

"After Callback".Dump();

This prints output as follows:

Starting
33
False
Inside Callback
33
False
Callback Complete
After Callback

What I would expect is something like this (assuming the callback took long enough to run):

Starting
33
False
Inside Callback
44
True
After Callback // App continues despite callback running
Callback Complete

With the callback running on the same application thread it means if something inside the callback takes a long time (eg introducing a Thread.Sleep for argument's sake), my application blocks there. This means I cannot setup a timeout on the request (eg with a ThreadPool.RegisterWaitForSingleObject ).

Am I missing something here?

Answer 1

当可以通过BeginXXX方法立即完成异步操作时，它不会在线程池线程上运行回调，而是将IAsyncResult.CompletedSynchronously属性设置为true并同步执行回调。

Answer 2

我假设BeginGetResponse方法在调用者线程的上下文中调用回调