C#语言入门_异步编程

// 从文件流中读取字节块并将该数据写入给定的字节数组中
// array代表把读取的字节块写入的缓存区
// offset代表array的字节偏量，将在此处读取字节
// count 代表最多读取的字节数
public override int Read(byte[] array, int offset, int count )

// 开始异步读操作
// 前面的3个参数和同步方法代表的意思一样，这里就不说了，可以看到这里多出了2个参数
// userCallback代表当异步IO操作完成时，你希望由一个线程池线程执行的方法,该方法必须匹配AsyncCallback委托
// stateObject代表你希望转发给回调方法的一个对象的引用，在回调方法中，可以查询IAsyncResult接口的AsyncState属性来访问该对象
public override IAsyncResult BeginRead(byte[] array, int offset, int numBytes, AsyncCallback userCallback, Object stateObject
)

  #region use APM to download file asynchronously

        private static void DownloadFileAsync(string url)
        {
            try
            {
                // Initialize an HttpWebRequest object
                HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(url);

                // Create an instance of the RequestState and assign HttpWebRequest instance to its request field.
                RequestState requestState = new RequestState();
                requestState.request = myHttpWebRequest;
                myHttpWebRequest.BeginGetResponse(new AsyncCallback(ResponseCallback), requestState);
            }
            catch (Exception e)
            {
                Console.WriteLine("Error Message is:{0}",e.Message);
            }
        }

        // The following method is called when each asynchronous operation completes. 
        private static void ResponseCallback(IAsyncResult callbackresult)
        {
            // Get RequestState object
            RequestState myRequestState = (RequestState)callbackresult.AsyncState;

            HttpWebRequest myHttpRequest = myRequestState.request;

            // End an Asynchronous request to the Internet resource
            myRequestState.response = (HttpWebResponse)myHttpRequest.EndGetResponse(callbackresult);
            
            // Get Response Stream from Server
            Stream responseStream = myRequestState.response.GetResponseStream();
            myRequestState.streamResponse = responseStream;

            IAsyncResult asynchronousRead = responseStream.BeginRead(myRequestState.BufferRead, 0, myRequestState.BufferRead.Length, ReadCallBack, myRequestState);         
        }

        // Write bytes to FileStream
        private static void ReadCallBack(IAsyncResult asyncResult)
        {
            try
            {
                // Get RequestState object
                RequestState myRequestState = (RequestState)asyncResult.AsyncState;

                // Get Response Stream from Server
                Stream responserStream = myRequestState.streamResponse;

                // 
                int readSize = responserStream.EndRead(asyncResult);
                if (readSize > 0)
                {
                    myRequestState.filestream.Write(myRequestState.BufferRead, 0, readSize);
                    responserStream.BeginRead(myRequestState.BufferRead, 0, myRequestState.BufferRead.Length, ReadCallBack, myRequestState);
                }
                else
                {
                    Console.WriteLine("\nThe Length of the File is: {0}", myRequestState.filestream.Length);
                    Console.WriteLine("DownLoad Completely, Download path is: {0}", myRequestState.savepath);
                    myRequestState.response.Close();
                    myRequestState.filestream.Close();
                }        
            }
            catch (Exception e)
            {
                Console.WriteLine("Error Message is:{0}", e.Message);
            }
        }
        #endregion

 #region Download File Synchrously
        private static void DownLoadFileSync(string url)
        {
            // Create an instance of the RequestState 
            RequestState requestState=new RequestState();
            try
            {
                // Initialize an HttpWebRequest object
                HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(url);

                // assign HttpWebRequest instance to its request field.
                requestState.request = myHttpWebRequest;
                requestState.response = (HttpWebResponse)myHttpWebRequest.GetResponse();
                requestState.streamResponse = requestState.response.GetResponseStream();
                int readSize = requestState.streamResponse.Read(requestState.BufferRead, 0, requestState.BufferRead.Length);
                while (readSize > 0)
                {
                    requestState.filestream.Write(requestState.BufferRead, 0, readSize);
                    readSize = requestState.streamResponse.Read(requestState.BufferRead, 0, requestState.BufferRead.Length);
                }

                Console.WriteLine("\nThe Length of the File is: {0}", requestState.filestream.Length);
                Console.WriteLine("DownLoad Completely, Download path is: {0}", requestState.savepath);
            }
            catch (Exception e)
            {
                Console.WriteLine("Error Message is:{0}", e.Message);
            }
            finally
            {
                requestState.response.Close();
                requestState.filestream.Close();
            }
        }
        #endregion 

 public partial class MainForm : Form
    {
        // 定义用来实现异步编程的委托
        private delegate string AsyncMethodCaller(string fileurl);

          // 定义显示状态的委托
        private delegate void ShowStateDelegate(string value);
        private ShowStateDelegate showStateCallback;

        SynchronizationContext sc;
        public MainForm()
        {
            InitializeComponent();
            txbUrl.Text = "http://download.microsoft.com/download/7/0/3/703455ee-a747-4cc8-bd3e-98a615c3aedb/dotNetFx35setup.exe";
            showStateCallback = new ShowStateDelegate(ShowState);
        }

        private void btnDownLoad_Click(object sender, EventArgs e)
        {
            rtbState.Text = "Download............";
            btnDownLoad.Enabled = false;
            if (txbUrl.Text == string.Empty)
            {
                MessageBox.Show("Please input valid download file url");
                return;
            }

            AsyncMethodCaller methodCaller = new AsyncMethodCaller(DownLoadFileSync);
            methodCaller.BeginInvoke(txbUrl.Text.Trim(), GetResult, null);

            // 捕捉调用线程的同步上下文派生对象
            sc = SynchronizationContext.Current;
        }

        // 同步下载文件的方法
        // 该方法会阻塞主线程，使用户无法对界面进行操作
        // 在文件下载完成之前，用户甚至都不能关闭运行的程序。
        private string DownLoadFileSync(string url)
        {
            // Create an instance of the RequestState 
            RequestState requestState = new RequestState();
            try
            {
                // Initialize an HttpWebRequest object
                HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(url);

                // assign HttpWebRequest instance to its request field.
                requestState.request = myHttpWebRequest;
                requestState.response = (HttpWebResponse)myHttpWebRequest.GetResponse();
                requestState.streamResponse = requestState.response.GetResponseStream();
                int readSize = requestState.streamResponse.Read(requestState.BufferRead, 0, requestState.BufferRead.Length);

                while (readSize > 0)
                {
                    requestState.filestream.Write(requestState.BufferRead, 0, readSize);
                    readSize = requestState.streamResponse.Read(requestState.BufferRead, 0, requestState.BufferRead.Length);
                }

                // 执行该方法的线程是线程池线程，该线程不是与创建richTextBox控件的线程不是一个线程
                // 如果不把 CheckForIllegalCrossThreadCalls 设置为false，该程序会出现“不能跨线程访问控件”的异常
                return string.Format("The Length of the File is: {0}", requestState.filestream.Length) + string.Format("\nDownLoad Completely, Download path is: {0}", requestState.savepath);
            }
            catch (Exception e)
            {
                return string.Format("Exception occurs in DownLoadFileSync method, Error Message is:{0}", e.Message);
            }
            finally
            {
                requestState.response.Close();
                requestState.filestream.Close();
            }
        }

        // 异步操作完成时执行的方法
        private void GetResult(IAsyncResult result)
        {
            AsyncMethodCaller caller = (AsyncMethodCaller)((AsyncResult)result).AsyncDelegate;
            // 调用EndInvoke去等待异步调用完成并且获得返回值
            // 如果异步调用尚未完成，则 EndInvoke 会一直阻止调用线程，直到异步调用完成
            string returnstring = caller.EndInvoke(result);

            // 通过获得GUI线程的同步上下文的派生对象，
            // 然后调用Post方法来使更新GUI操作方法由GUI 线程去执行
            sc.Post(ShowState,returnstring);      
        }

        // 显示结果到richTextBox
        // 因为该方法是由GUI线程执行的，所以当然就可以访问窗体控件了
        private void ShowState(object result)
        {
            rtbState.Text = result.ToString();
            btnDownLoad.Enabled = true;
        }
    }