異步編程，採用WorkgroupWorker，async和await關鍵字

金科玉律：不要在UI線程上執行耗時的操做；不要在除了UI線程以外的其餘線程上訪問UI控件！

NET1.1的BeginInvoke異步調用，須要準備3個方法：功能方法GetWebsiteLength，結果方法DownloadComplete，呼叫方法BeginInvoke！

但很不幸，在UI線程以外訪問UI線程控件！調用失敗。線程同步必須在線程所屬進程的公共區域保留同步區，以此實現線程間的通信。

2、C#2.0引入了BackgroundWorker，從而極大的簡化了線程間通信。

3、C#4.0引入的async和await關鍵字，使得一切變得如此簡單！

async和await關鍵字

using System;
using System.Windows.Forms;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AsyncDemo
{
    class MyForm:Form
    {
        Label lblInfo;
        Button btnCaculate;
        public MyForm()
        {
            lblInfo = new Label { Location = new System.Drawing.Point(10, 20), Text = "Length" };
            btnCaculate = new Button { Location = new System.Drawing.Point(10, 50), Text = "GetLength" };
            btnCaculate.Click += DisplayWebsiteLength;
            AutoSize = true;
            this.Controls.Add(lblInfo);
            this.Controls.Add(btnCaculate);
        }
    
        async void DisplayWebsiteLength(object sender,EventArgs e)
        {
            lblInfo.Text = "Fetching...";
            using (System.Net.Http.HttpClient client = new System.Net.Http.HttpClient())
            {
 //string text = await client.GetStringAsync("http://flaaash.cnblogs.com"); //lblInfo.Text = text.Length.ToString();

 Task<string> task = client.GetStringAsync("http://www.sina.com"); string contents = await task;
                lblInfo.Text = contents.Length.ToString();
            }
        }
        static void Main(string[] args)
        {
            Application.Run(new MyForm());

        }
    }
}

BackgroundWorker

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace AsyncDemo
{
    class BackWorker : Form
    {
        Label lblInfo;
        Button btnCaculate;
        System.ComponentModel.BackgroundWorker worker;

        BackWorker()
        {
            lblInfo = new Label { Location = new System.Drawing.Point(10, 20), Text = "Length" };
            btnCaculate = new Button { Location = new System.Drawing.Point(10, 50), Text = "GetLength" };
            btnCaculate.Click += (o, e) => { worker.RunWorkerAsync(); };
            this.Controls.Add(lblInfo);
            this.Controls.Add(btnCaculate);

 worker = new System.ComponentModel.BackgroundWorker(); worker.DoWork += (o, e) =>
 { System.Net.WebClient client = new System.Net.WebClient(); string contents = client.DownloadString("http://www.sina.com"); e.Result = contents.Length; }; worker.RunWorkerCompleted += (o, e) => lblInfo.Text = e.Result.ToString();
        }

        public static void MainTest(string[] args)
        {
            Application.Run(new BackWorker());
        }
    }
}