初次接触C#异步编程

C#异步编程有四种方式,可参考下面的文章
MSDN-异步编程概述 [C#]
并且在MSDN帮助文档中也有相关介绍
MSDN:ms-help://MS.VSCC.2003/MS.MSDNQTR.2003FEB.2052/cpgenref/html/cpconasynchronousexecution.htm
另外这样也可以实现相同的效果(我还没有使用过)：
//System.Threading.ThreadPool.QueueUserWorkItem(new System.Threading.WaitCallback(DoDownloadWork), null);

下面是一段实际程序的片段(很多属性字段被我省去了)：

namespace Hooyee.Ftp {
    public class FileDownloader {
        public FileDownloader() {
        }

        IFtpDownloader Members#region IFtpDownloader Members

        private delegate void AsyncDownloadDelegate(FileStream fileStream);

        public void Start() {
            IResult<FileStream> resultStream;
            resultStream = (new LocalFileAccess()).AppendFile(_saveAs); //_saveAs是类的一个字段，定义都被省去

            AsyncDownloadDelegate asyncDownLoadDlg = new AsyncDownloadDelegate(DoDownloadWork);
            IAsyncResult ar = asyncDownLoadDlg.BeginInvoke(resultStream.Object, new AsyncCallback(DownloadFinished), asyncDownLoadDlg);
            
            //System.Threading.ThreadPool.QueueUserWorkItem(new System.Threading.WaitCallback(DoDownloadWork), null);
        }

        
        private void DoDownloadWork(FileStream fileStream) {
            FtpWebResponse response = null;
            Stream stream = null;
            try {
                response = (FtpWebResponse)_request.GetResponse();
                stream = response.GetResponseStream();
                int reads = 0;
                Byte[] buffer = new Byte[BUFFER_SIZE];
                BinaryWriter writer = new BinaryWriter(fileStream);
                while ((reads = stream.Read(buffer, 0, BUFFER_SIZE)) > 0) {
                    writer.Write(buffer, 0, reads);
                }
            } catch (IOException ex) {
                //FtpLog log = new FtpLog();
                //log.Write
            } catch (Exception ex) {
            } finally {
                if (stream != null) stream.Close();
                if (fileStream != null) fileStream.Close();
                if (response != null) response.Close();
            }
        }

        private void DownloadFinished(IAsyncResult ar) {
            AsyncDownloadDelegate dlgt = (AsyncDownloadDelegate)ar.AsyncState;
            dlgt.EndInvoke(ar);
            try {
                //下载完成后修改文件名
                LocalFileAccess.Rename(_saveAs);
            } catch {
            }
            if (OnFinished != null) {
                FtpNoticeEventArgs e = new FtpNoticeEventArgs(_remoteFile, 0, FtpDownloadActions.Finish);
                OnFinished(this, e);
            }
        }
        #endregion
    }
}