c#中子线程控制进度条的一个简单例子

这个问题来自社区提问，代码保留一份用来以后回答

using  System;
using  System.ComponentModel;
using  System.Windows.Forms;
namespace  WindowsApplication4
... {
     /**/ ///   <summary>
     ///  gui 类
     ///   </summary>
     public   partial   class  Form1 : Form
     ... {
         public  Form1()
         ... {
            InitializeComponent();
        }
         private   void  button1_Click( object  sender, EventArgs e)
         ... {
             // 用子线程工作
             new  System.Threading.Thread( new  System.Threading.ThreadStart(StartDownload)).Start();
        }
         // 开始下载
         public   void  StartDownload()
         ... {
            Downloader downloader  =   new  Downloader();
            downloader.onDownLoadProgress  +=   new  Downloader.dDownloadProgress(downloader_onDownLoadProgress);
            downloader.Start();
        }
         // 同步更新ui
         void  downloader_onDownLoadProgress( long  total,  long  current)
         ... {
             if  ( this .InvokeRequired)
             ... {
                 this .Invoke( new  Downloader.dDownloadProgress(downloader_onDownLoadProgress),  new   object []  ... { total, current } );
            }
             else
             ... {
                 this .progressBar1.Maximum  =  ( int )total;
                 this .progressBar1.Value  =  ( int )current;
            }
        }
    }

     /**/ ///   <summary>
     ///  下载类
     ///   </summary>
     public   class  Downloader
     ... {
         // 委托
         public   delegate   void  dDownloadProgress( long  total, long  current);
         // 事件
         public   event  dDownloadProgress onDownLoadProgress;
         // 开始模拟工作
         public   void  Start()
         ... {
             for  ( int  i  =   0 ; i  <   100 ; i ++ )
             ... {
                 if  (onDownLoadProgress  !=   null )
                    onDownLoadProgress( 100 , i);
                System.Threading.Thread.Sleep( 100 );
            }
        }
    }
}