Copy大文件时，显示进度条

int totalSize;  //Total Size
        int position;   //每次向新文件写入数据后的位置
        const int BUFFER_SIZE = 4096; 
        byte[] buffer;
        Stream stream;

        private void btnCopy_Click(object sender, System.EventArgs e)
        ...{
            string strFile = "";

            OpenFileDialog dlg = new OpenFileDialog();
            if ( dlg.ShowDialog() ==  DialogResult.OK )
            ...{
                strFile = dlg.FileName ;
            }
            else
            ...{
                return ;
            }

            FileStream fs = new FileStream( strFile , FileMode.Open , FileAccess.Read ) ;
            totalSize = (int)fs.Length ;
            stream = fs;

            //Delete file which aready exists.
            if ( File.Exists( "D:/123.txt" ) )
                File.Delete( "D:/123.txt" );
             //Copy file while larger than 4KB.



            if ( totalSize > BUFFER_SIZE )
            ...{
                buffer = new byte[ BUFFER_SIZE ]; 

                // 将数据异步读取到buffer中
                stream.BeginRead( buffer , 0 , BUFFER_SIZE , new AsyncCallback( AsyncCopyFile ) , null );   
            }
            else
            ...{
                fs.Close();
            }
        }
        
        /**//// <summary>
        /// Asynchronously copy file
        /// </summary>
        /// <param name="ar"></param>
        private void AsyncCopyFile( IAsyncResult ar )
        ...{
            int readedLength ;
   
            // Lock FileStream
            lock( stream )
            ...{
                readedLength = stream.EndRead( ar );   // When stream endread, get readed length
            }

            // Write to disk
            FileStream fsWriter = new FileStream( "D:/123.txt" , FileMode.Append , FileAccess.Write );
            fsWriter.Write( buffer , 0 , buffer.Length );
            fsWriter.Close();
 
            // Current stream position 
            position += readedLength;
     

            // 通过系统委托更新前台进度条
            MethodInvoker m = new MethodInvoker( SynchProgressBar );
            m.BeginInvoke( null , null );
 
            if ( position >= totalSize ) // Read over.
            ...{
                stream.Close();        //Close FileStream  
                return ;
            }

            // Continue to read and write
            lock ( stream )
            ...{
                int leftSize = totalSize - position;

                if ( leftSize < BUFFER_SIZE ) 
                    buffer = new byte[ leftSize ];
                                                                                     //以递归的方式继续异步读取后执行AsyncCopyFile操作
                stream.BeginRead( buffer , 0 , buffer.Length , new AsyncCallback( AsyncCopyFile ) , null );     
      
            }
        }

        private void SynchProgressBar()
        ...{
            this.progressBar1.Maximum = totalSize;
            this.progressBar1.Value   = position ; 
        }