概念
异步执行计算限制的操作，可以使用线、线程池、Task在多个内核上调度任务，使多个线程并发的工作，从而高效的使用系统资源，同时提升应用程序的吞吐能力。
异步I/O操作，允许将任务交由硬件设备处理，期间完全不占用线程和CPU资源，这样系统资源可以高效的使用。I/O操作的结果是由线程池来处理的。
Windows中使用I/O完成端口的形式完成异步I/O，<Windows核心编程>有详细描述。
执行异步操作是构建高性能、可伸缩应用程序的关键，它允许用非常少的线程执行许多操作，和线程池配合，异步操作允许利用机器的所有CPU。
为此，CLR团队提供了一种模式：异步编程模型(Asynchronous Programming Modelm APM)
如Sytem.IO.Stream的类型都提供了BeginReadBeginWrite的类别的异步操作
通过使用Wintellect Power Threading库可以显著减少异步开发的复杂性，如下是个例子：
private static void ImplementedViaAsyncEnumerator()
{
// Start 1 server per CPU
for (Int32 n = 0; n < Environment.ProcessorCount; n++)
{
var ae = new AsyncEnumerator();
ae.BeginExecute(PipeServerAsyncEnumerator(ae), ae.EndExecute);
}
// Now make a 100 client requests against the server
for (Int32 n = 0; n < 100; n++)
{
var ae = new AsyncEnumerator();
ae.BeginExecute(PipeClientAsyncEnumerator(ae, "localhost", "Request #" + n), ae.EndExecute);
}
}
// This field records the timestamp of the most recent client's request
private static DateTime s_lastClientRequestTimestamp = DateTime.MinValue;
// The SyncGate enforces thread-safe access to the s_lastClientRequestTimestamp field
private static readonly SyncGate s_gate = new SyncGate();
private static IEnumerator<Int32> PipeServerAsyncEnumerator(AsyncEnumerator ae)
{
// Each server object performs asynchronous operations on this pipe
using (var pipe = new NamedPipeServerStream(
"Echo", PipeDirection.InOut, -1, PipeTransmissionMode.Message,
PipeOptions.Asynchronous | PipeOptions.WriteThrough))
{
// Asynchronously accept a client connection
pipe.BeginWaitForConnection(ae.End(), null);
/*ae.End 这个方法返回一个委托，它引用了AsyncEnumerator内部的方法
操作完成时，线程池通知AsyncEnumerator对象继续执行yield return 1语句后面的迭代器方法
*/
yield return 1; //异步操作的地方，使用这个允许线程返回它原来的地方，便于它做更多的工作
// A client connected, let's accept another client
// 客户连接后再建立一个新的对象提供服务
var aeNewClient = new AsyncEnumerator();
aeNewClient.BeginExecute(PipeServerAsyncEnumerator(aeNewClient), aeNewClient.EndExecute);
// Accept the client connection
//DequeueAsyncResult返回当异步操作完成时，由线程池线程传给AsyncEnumerator对象的IAsyncResult对象
pipe.EndWaitForConnection(ae.DequeueAsyncResult());
// Asynchronously read a request from the client
Byte[] data = new Byte[1000];
pipe.BeginRead(data, 0, data.Length, ae.End(), null);
yield return 1;
// The client sent us a request, process it.
Int32 bytesRead = pipe.EndRead(ae.DequeueAsyncResult());
// Get the timestamp of this client's request
DateTime now = DateTime.Now;
// We want to save the timestamp of the most-recent client request. Since multiple
// clients are running concurrently, this has to be done in a thread-safe way
s_gate.BeginRegion(SyncGateMode.Exclusive, ae.End()); // Request exclusive access
yield return 1; // The iterator resumes when exclusive access is granted
if (s_lastClientRequestTimestamp < now)
s_lastClientRequestTimestamp = now;
s_gate.EndRegion(ae.DequeueAsyncResult()); // Relinquish exclusive access
// My sample server just changes all the characters to uppercase
// But, you can replace this code with any compute-bound operation
data = Encoding.UTF8.GetBytes(
Encoding.UTF8.GetString(data, 0, bytesRead).ToUpper().ToCharArray());
// Asynchronously send the response back to the client
pipe.BeginWrite(data, 0, data.Length, ae.End(), null);
yield return 1;
// The response was sent to the client, close our side of the connection
pipe.EndWrite(ae.DequeueAsyncResult());
} // Close the pipe NamedPipeServerStream对象销毁
}
private static IEnumerator<Int32> PipeClientAsyncEnumerator(AsyncEnumerator ae, String serverName, String message)
{
// Each client object performs asynchronous operations on this pipe
using (var pipe = new NamedPipeClientStream(serverName, "Echo",
PipeDirection.InOut, PipeOptions.Asynchronous | PipeOptions.WriteThrough))
{
pipe.Connect(); // Must Connect before setting ReadMode
pipe.ReadMode = PipeTransmissionMode.Message;
// Asynchronously send data to the server
Byte[] output = Encoding.UTF8.GetBytes(message);
pipe.BeginWrite(output, 0, output.Length, ae.End(), null);
yield return 1;
// The data was sent to the server
pipe.EndWrite(ae.DequeueAsyncResult());
// Asynchronously read the server's response
Byte[] data = new Byte[1000];
pipe.BeginRead(data, 0, data.Length, ae.End(), data);
yield return 1;
// The server responded, display the response and close out connection
Int32 bytesRead = pipe.EndRead(ae.DequeueAsyncResult());
Console.WriteLine("Server response: " + Encoding.UTF8.GetString(data, 0, bytesRead));
} // Close();
}
IAsyncResult转为Task
异步的通用接口IAsyncResult，使用Task的形式可以加入其它的控制（返回值处理等），如：
普通的异步处理方法
// Instead of this:
WebRequest webRequest = WebRequest.Create("http://Wintellect.com/");
webRequest.BeginGetResponse(result =>
{
WebResponse webResponse = null;
try
{
webResponse = webRequest.EndGetResponse(result);
Console.WriteLine("Content length: " + webResponse.ContentLength);
}
catch (WebException we)
{
Console.WriteLine("Failed: " + we.GetBaseException().Message);
}
finally
{
if (webResponse != null) webResponse.Close();
}
}, null);
Console.ReadLine(); // for testing purposes
使用Task的形式进行请求
// Make a Task from an async operation that FromAsync starts
/*WebRequest*/
webRequest = WebRequest.Create("http://Wintellect.com/");
var t1 = Task.Factory.FromAsync<WebResponse>(webRequest.BeginGetResponse, webRequest.EndGetResponse, null, TaskCreationOptions.None);
var t2 = t1.ContinueWith(task =>
{
WebResponse webResponse = null;
try
{
webResponse = task.Result;
Console.WriteLine("Content length: " + webResponse.ContentLength);
}
catch (AggregateException ae)
{
if (ae.GetBaseException() is WebException)
Console.WriteLine("Failed: " + ae.GetBaseException().Message);
else throw;
}
finally { if (webResponse != null) webResponse.Close(); }
});
try
{
t2.Wait(); // for testing purposes only
}
catch (AggregateException) { }
应用程序及其线程处理模型
每个应用程序都可能引入自己的线程处理模型
控制台、Windows服务、Asp.net、Web Service：没有引入任何种类的线程处理模型，任何线程都可以在任何时候做它爱做的任何事情
GUI程序（Windows Form WPF Silverlight）：引入了一个线程处理模型，在这个模型中，创建窗口的线程是唯一能对那个窗口进行更新的线程
Private static AsyncCallback SyncContextCallback(AsyncCallback callback)
{
// Capture the calling thread's SynchronizationContext-derived object
SynchronizationContextsc = SynchronizationContext.Current;
// If there is no SC, just return what was passed in
if (sc == null) return callback;
// Return a delegate that, when invoked, posts to the captured SC a method that
// calls the original AsyncCallback passing it the IAsyncResult argument
returnasyncResult =>sc.Post(result => callback((IAsyncResult)result), asyncResult);
}
如下的形式在UI程序中使用以上的处理就不用再处理手动的线程切换问题了
var webRequest = WebRequest.Create("http://Wintellect.com/");
webRequest.BeginGetResponse(SyncContextCallback(ProcessWebResponse), webRequest);
base.OnMouseClick(e);
private void ProcessWebResponse(IAsyncResult result)
{
// If we get here, this must be the GUI thread, it's OK to update the UI
var webRequest = (WebRequest)result.AsyncState;
using (var webResponse = webRequest.EndGetResponse(result))
{
Text = "Content length: " + webResponse.ContentLength;
}
}
详细参考：
Clr Via C#