金科玉律：不要在UI线程上执行耗时的操作；不要在除了UI线程之外的其他线程*问UI控件！
NET1.1的BeginInvoke异步调用，需要准备3个方法：功能方法GetWebsiteLength，结果方法DownloadComplete，呼叫方法BeginInvoke！
但很不幸，在UI线程之外访问UI线程控件！调用失败。线程同步必须在线程所属进程的公共区域保留同步区，以此实现线程间的通讯。
二、C#2.0引入了BackgroundWorker，从而极大的简化了线程间通讯。
三、C#4.0引入的async和await关键字，使得一切变得如此简单！
async和await关键字
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());
}
}
}