WebClient Class

MSDN
英：http://msdn.microsoft.com/en-us/library/system.net.webclient.aspx
中：http://msdn.microsoft.com/zh-tw/library/system.net.webclient.aspx

先做個簡單的程式來學學網路應用程式，System.Net內的類別當中，我先用最高層次的類別來玩玩看，這個類別是WebClient，只能讓你當作client來使用。

控制項

• requestTextBox=輸入的網址
• messageTextBox=訊息輸出

程式

    try
    {
        //取得URI識別
        Uri thisUri = new Uri(requestTextBox.Text);

        WebClient thisWebClient = new WebClient();
        //設定快取原則
        thisWebClient.CachePolicy = new RequestCachePolicy(RequestCacheLevel.NoCacheNoStore);
        //設定用於上載和下載字串的Encoding
        thisWebClient.Encoding = Encoding.UTF8;

        //下載回應的body資料為string，呼叫DownloadString這會開始執行下載動作
        String thisDownloadString = thisWebClient.DownloadString(thisUri);

        //取得回應的header資料
        messageTextBox.Text += thisWebClient.ResponseHeaders.ToString() + Environment.NewLine;
        messageTextBox.Text += thisDownloadString + Environment.NewLine;

        //將多行的TextBox移到最後一列
        messageTextBox.Select(messageTextBox.Text.Length, 0);
        messageTextBox.ScrollToCaret();//將控制項的內容捲動到目前插入號的位置

    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
    }
    finally
    {
        //clean up
    }

上述是同步(Synchronous)的方式，程式需等待下載命令執行完之後，才能繼續run，你的成是有可能會當掉鎖住，解決方式就是使用非同步(Asynchronous )方式，非同步方法不會封鎖呼叫執行緒。

    try
    {
        //取得URI識別
        Uri thisUri = new Uri(requestTextBox.Text);

        WebClient thisWebClient = new WebClient();
        //設定快取原則
        thisWebClient.CachePolicy = new RequestCachePolicy(RequestCacheLevel.NoCacheNoStore);
        //設定用於上載和下載字串的Encoding
        thisWebClient.Encoding = Encoding.UTF8;

        //委派處理WebClient的DownloadStringCompleted 事件的方法
        thisWebClient.DownloadStringCompleted += new DownloadStringCompletedEventHandler(thisWebClient_DownloadStringCompleted);

        //呼叫DownloadString這會開始執行下載動作
        thisWebClient.DownloadStringAsync(thisUri);

    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.ToString());
    }
    finally
    {
        //clean up
    }

    void thisWebClient_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        //throw new NotImplementedException();
        try
        {
            //取得回應的body資料
            String thisDownloadString = e.Result.ToString() + Environment.NewLine;

            //取得回應的header資料
            messageTextBox.Text += ((WebClient)sender).ResponseHeaders.ToString() + Environment.NewLine;
            messageTextBox.Text += thisDownloadString + Environment.NewLine;

            //將多行的TextBox移到最後一列
            messageTextBox.Select(messageTextBox.Text.Length, 0);
            messageTextBox.ScrollToCaret();//將控制項的內容捲動到目前插入號的位置

        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }
        finally
        {
            //clean up
        }
    }