Asp.net 断点续传 下载大文件

        /// 支持断点续传的下载大文件的函数
        /// </summary>
        /// <param name="downloadFilePath">被下载文件的路径，相对和绝对都可以</param>
        /// <param name="showFileName">显示给用户的文件名，可以为空</param>
        /// <param name="fileType">要下载的文件类型，如果不确定，请留空</param>
        /// <remarks>用于大文件的下载</remarks>
        public static void BigFileDownload(string downloadFilePath, string showFileName, string fileType)
{
            downloadFilePath = downloadFilePath.Trim();
            if(string.Empty == downloadFilePath)
                ShowErrorMessage("没有指定要下载的文件路径！");
            
            if(downloadFilePath.IndexOf(":") < 0)
                downloadFilePath = System.Web.HttpContext.Current.Server.MapPath(downloadFilePath);

            showFileName = showFileName.Trim();
            if(string.Empty == showFileName)
                showFileName = Path.GetFileName(downloadFilePath);

            showFileName = System.Web.HttpUtility.UrlEncode(System.Web.HttpContext.Current.Request.ContentEncoding.GetBytes(showFileName));
            
            fileType = fileType.Trim();
            if(string.Empty == fileType)
                fileType = "application/octet-stream";

            System.Web.HttpContext.Current.Response.Clear();
            Stream objStream = null;
            byte[] btBuffer = new byte[8192];  // 1024*8 = 8Kb
            int intLength = 0;
            long lngData2Read = 0;
            try
{
                using(objStream = new FileStream(downloadFilePath, FileMode.Open, FileAccess.Read, FileShare.Read))
{
                    lngData2Read = objStream.Length;
                    

                    
                    long lngP = 0;
                    if(null != System.Web.HttpContext.Current.Request.Headers["Range"])
{
                        System.Web.HttpContext.Current.Response.StatusCode = 206;
                        lngP = long.Parse(System.Web.HttpContext.Current.Request.Headers["Range"].Replace("bytes=","").Replace("-",""));
                    }
                    if(0 != lngP)
{
                        System.Web.HttpContext.Current.Response.AddHeader("Content-Range","bytes " + lngP.ToString() + "-" + ((long)(lngData2Read - 1)).ToString() + "/" + lngData2Read.ToString());                    
                    }
                    System.Web.HttpContext.Current.Response.AddHeader("Content-Length",((long)(lngData2Read - lngP)).ToString());

                    System.Web.HttpContext.Current.Response.ContentType = fileType;
                    System.Web.HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment;filename=" + showFileName);
                    
                    #endregion
                    
                    while(lngData2Read > 0)
{
                        if(HttpContext.Current.Response.IsClientConnected)
{
                            intLength = objStream.Read(btBuffer, 0, 8192);
                            HttpContext.Current.Response.OutputStream.Write(btBuffer, 0, intLength);
                            HttpContext.Current.Response.Flush();
                            lngData2Read -= intLength;
                            btBuffer = new byte[8192];
                        }
                        else
{
                            lngData2Read = -1;
                        }
                    }
                }
            }
            catch(Exception ex)
{
                ShowErrorMessage(ex.Message);
            }
            // 结束文件下载，此句不可缺少，否则会把本页的HTML代码一起下载文件里
            HttpContext.Current.Response.End();
        }


        /// 输出错误信息给用户，采用客户端弹出框的方式
        /// </summary>
        /// <param name="errorMessage">错误信息的内容</param>
        internal static void ShowErrorMessage(string errorMessage)
{
            errorMessage = errorMessage.Replace("'", "\"");
            System.Web.HttpContext.Current.Response.Write("<script language='javascript'>");
            System.Web.HttpContext.Current.Response.Write("alert('");
            System.Web.HttpContext.Current.Response.Write(errorMessage);
            System.Web.HttpContext.Current.Response.Write("');</script>");
            System.Web.HttpContext.Current.Response.End();
        }