.NET的異步調用和IAsyncResult接口

1.關于委托的BeginInvoke和EndInvoke方法

.NET Framework 允許您異步調用任何方法。為此，應定義與您要調用的方法具有相同簽名的委托；公共語言運作庫會自動使用适當的簽名為該委托定義 BeginInvoke 和 EndInvoke 方法。

例如：

namespace carydemo2

{

    public class CaryAsync

    {       

        public string LongtimeOperation(int data, out int threadId)

        {

            Console.WriteLine("Long time operation begin");

            data = data + 123;

            Thread.Sleep(500);

            threadId = Thread.CurrentThread.ManagedThreadId;

            return String.Format("Data:{0}.", data.ToString());

        }

    }

    public delegate string CaryAsyncCaller(int data, out int threadId);

    public class CaryTest

    {

        private static int threadId;

        public static void Main()

        {           

            CaryAsync ca = new CaryAsync();

            CaryAsyncCaller caller = new CaryAsyncCaller(ca.LongtimeOperation);

            IAsyncResult result = caller.BeginInvoke(3000, out threadId, null, null);

            Thread.Sleep(0);

            Console.WriteLine("Main thread :{0}", Thread.CurrentThread.ManagedThreadId);

            result.AsyncWaitHandle.WaitOne();

            while (result.IsCompleted == false)

            {

                Thread.Sleep(10);

            }

            string returnValue = caller.EndInvoke(out threadId, result);

            Console.WriteLine("The caller thread: {0}, Data :\"{1}\".", threadId, returnValue);

        }     

}

調用你可以使用 BeginInvoke 傳回的 IAsyncResult 的 AsyncWaitHandle 屬性來擷取 WaitHandle。異步調用完成時會發出 WaitHandle 信号，而您可以通過調用 WaitOne 方法等待它。這樣你就可以在異步調用完成之前或之後，在通過調用 EndInvoke 檢索結果之前，還可以執行其他處理。 

可以使用由 BeginInvoke 傳回的 IAsyncResult 的 IsCompleted 屬性來發現異步調用何時完成。 

還可以在調用完成時執行回調方法，代碼如下：

        {           

            IAsyncResult result = caller.BeginInvoke(3000, out threadId, new AsyncCallback(CaryCallBack), caller);

            Console.ReadLine();

        static void CaryCallBack(IAsyncResult ar)

            CaryAsyncCaller caller = (CaryAsyncCaller)ar.AsyncState;

            string returnValue = caller.EndInvoke(out threadId, ar);

            Console.WriteLine("The caller thread:{0},return value\"{1}\"", threadId, returnValue);

2.IAsyncResult接口：它表示異步操作的狀态.該接口定義了4個公用屬性，如下：

   AsyncState  擷取使用者定義的對象，它限定或包含關于異步操作的資訊。

   AsyncWaitHandle  擷取用于等待異步操作完成的 WaitHandle。

   CompletedSynchronously  擷取異步操作是否同步完成的訓示。

   IsCompleted  擷取異步操作是否已完成的訓示。 

IAsyncResult 接口由包含可異步操作的方法的類實作。它是啟動異步操作的方法的傳回類型，如 FileStream.BeginRead，也是結束異步操作的方法的第三個參數的類型，如 FileStream.EndRead。當異步操作完成時，IAsyncResult 對象也将傳遞給由 AsyncCallback 委托調用的方法。 

支援 IAsyncResult 接口的對象存儲異步操作的狀态資訊，并提供同步對象以允許線程在操作完成時終止。 

3. AsyncCallback 委托      

引用在異步操作完成時調用的回調方法。AsyncCallback 引用的事件處理程式包含完成用戶端異步任務的程式邏輯。AsyncCallback 使用 IAsyncResult 接口擷取異步操作的狀态。

下面為例子：

namespace carydemo1

    public class AsyncResult : System.IAsyncResult

        public object AsyncState

            get; set;

        System.Threading.WaitHandle asyncWaitHandle = new AutoResetEvent(false);

        public System.Threading.WaitHandle AsyncWaitHandle

            get { return asyncWaitHandle; }

        public bool CompletedSynchronously

        public bool IsCompleted

            get;

            set;

        public int Data

            set;get;

    class TestClass

        AsyncCallback asyncCallback;

        AsyncResult asyncResult;

        public TestClass()

            asyncCallback = new AsyncCallback(callback);

            asyncResult = new AsyncResult();

        void callback(IAsyncResult asyncResult)

            AsyncResult temp = asyncResult as AsyncResult;

            ((AutoResetEvent)temp.AsyncWaitHandle).Set();

        public AsyncResult AsyncMethod(int value, object asyncState)

            Console.WriteLine("Start...");

            this.asyncResult.AsyncState = asyncState;

            this.asyncResult.Data = value;

            Thread t = new Thread(new ThreadStart(myThread));

            t.Start();

            return this.asyncResult;

        }

        void myThread()

            Console.WriteLine("begin ");

            for (int i = 0; i < 3; i++)

                Console.WriteLine(i);

                asyncResult.Data = asyncResult.Data + i;

                Thread.Sleep(500);

            Console.WriteLine("end...");

            asyncCallback(this.asyncResult);

    class Program

        static void Main(string[] args)

            TestClass obj = new TestClass();

            AsyncResult r = obj.AsyncMethod(50, null);

            r.AsyncWaitHandle.WaitOne();

            System.Console.WriteLine(r.Data);

            System.Console.WriteLine("完成");

            System.Console.Read();