用Visual C#实现文件下载

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
using System.Net;
using System.IO;
using System.Threading;
  
namespace MyGetCar
{
 ///
 /// Form1 的摘要说明。
 ///
 public class Form1 : System.Windows.Forms.Form
 {
  private System.Windows.Forms.Label label1;
  private System.Windows.Forms.Label label2;
  private System.Windows.Forms.TextBox srcAddress;
  private System.Windows.Forms.TextBox tarAddress;
  private System.Windows.Forms.StatusBar statusBar;
  private System.Windows.Forms.Button Start;
  
  private WebClient client = new WebClient();
  
   
  ///
  /// 必需的设计器变量。
  ///
  private System.ComponentModel.Container components = null;
  
  public Form1()
  {
   //
   // Windows 窗体设计器支持所必需的
   //
   InitializeComponent();
  
   //
   // TODO: 在 InitializeComponent 调用后添加任何构造函数代码
   //
  }
  
  ///
  /// 清理所有正在使用的资源。
  ///
  protected override void Dispose( bool disposing )
  {
   if( disposing )
   {
    if (components != null)
    {
     components.Dispose();
    }
   }
   base.Dispose( disposing );
  }
  
  #region Windows Form Designer generated code
  ///
  /// 设计器支持所需的方法 - 不要使用代码编辑器修改
  /// 此方法的内容。
  ///
  private void InitializeComponent()
  {
   this.label1 = new System.Windows.Forms.Label();
   this.label2 = new System.Windows.Forms.Label();
   this.srcAddress = new System.Windows.Forms.TextBox();
   this.tarAddress = new System.Windows.Forms.TextBox();
   this.statusBar = new System.Windows.Forms.StatusBar();
   this.Start = new System.Windows.Forms.Button();
   this.button1 = new System.Windows.Forms.Button();
   this.SuspendLayout();
   //
   // label1
   //
   this.label1.Location = new System.Drawing.Point(8, 32);
   this.label1.Name = "label1";
   this.label1.Size = new System.Drawing.Size(72, 23);
   this.label1.TabIndex = 0;
   this.label1.Text = "文件地址：";
   this.label1.TextAlign = System.Drawing.ContentAlignment.MiddleRight;
   //
   // label2
   //
   this.label2.Location = new System.Drawing.Point(8, 72);
   this.label2.Name = "label2";
   this.label2.Size = new System.Drawing.Size(72, 23);
   this.label2.TabIndex = 1;
   this.label2.Text = "另存到：";
   this.label2.TextAlign = System.Drawing.ContentAlignment.MiddleRight;
   //
   // srcAddress
   //
   this.srcAd