以前用Response.WriteFile(filename)，但当遇到大文件时无法完整下载。
该方法最大的问题，它不是直接将数据抛到客户端，而是在服务器端（IIS）上缓存。当下载文件比较大时，服务器压力会很大，iis虽然支持2G大小的文件下载，但当文件上了很多M时，由于服务器以及网络等因素的影响，异常概率相当大。所以当需要下载大文件时就不能使用上面的方法了。
微软推荐以下方法代替之：
■将数据分成较小的部分，然后将其移动到输出流以供下载，从而获取这些数据。
■为用户提供用于下载文件的链接。 ■使用 Microsoft ASP 3.0 进行下载或者与 ASP 一起使用 Software Artisans FileUp。 ■创建 ISAPI 扩展以下载文件。 ■使用 FTP 下载文件。参考文档：C#相关代码如下：
public class FileDown { public FileDown() { // //TODO: 在此处添加构造函数逻辑 // } /// /// 参数为虚拟路径 /// /// /// /// 获取物理地址 /// /// /// ///使用WriteFile下载文件，参数为文件虚拟路径 /// /// public static void DownLoadold(string FileName) { string destFileName = MapPathFile(FileName); // Labelmsg.Text = destFileName; if (File.Exists(destFileName)) { FileInfo fi = new FileInfo(destFileName); HttpContext.Current.Response.Clear(); HttpContext.Current.Response.ClearHeaders(); HttpContext.Current.Response.Buffer = false; //HttpContext.Current.Response.AppendHeader("Content-Disposition","attachment;filename=" +HttpUtility.UrlEncode(Path.GetFileName(destFileName),System.Text.Encoding.Default)); HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(Path.GetFileName(destFileName), System.Text.Encoding.UTF8)); HttpContext.Current.Response.AppendHeader("Content-Length", fi.Length.ToString()); HttpContext.Current.Response.ContentType = "application/octet-stream"; HttpContext.Current.Response.WriteFile(destFileName); HttpContext.Current.Response.Flush(); HttpContext.Current.Response.End(); } } /// /// 使用OutputStream.Write分块下载文件，参数为文件虚拟路径 /// /// public static void DownLoad(string FileName) { string filePath = MapPathFile(FileName); //指定块大小 long chunkSize = 204800; //建立一个200K的缓冲区 byte[] buffer = new byte[chunkSize]; //已读的字节数 long dataToRead = 0; FileStream stream = null; try { //打开文件 stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read); dataToRead = stream.Length; //添加Http头 HttpContext.Current.Response.ContentType = "application/octet-stream"; HttpContext.Current.Response.AddHeader("Content-Disposition", "attachement;filename=" + HttpUtility.UrlEncode(Path.GetFileName(filePath))); HttpContext.Current.Response.AddHeader("Content-Length", dataToRead.ToString()); while (dataToRead > 0) { if (HttpContext.Current.Response.IsClientConnected) { int length = stream.Read(buffer, 0, Convert.ToInt32(chunkSize)); HttpContext.Current.Response.OutputStream.Write(buffer, 0, length); HttpContext.Current.Response.Flush(); HttpContext.Current.Response.Clear(); dataToRead -= length; } else { //防止client失去连接 dataToRead = -1; } } } catch (Exception ex) { HttpContext.Current.Response.Write("Error:" + ex.Message); } finally { if (stream != null) { stream.Close(); } HttpContext.Current.Response.Close(); } } /// /// 使用OutputStream.Write分块下载文件，参数为文件绝对路径 /// /// public static void DownLoadFile(string filePath) { //string filePath = MapPathFile(FileName); //指定块大小 long chunkSize = 204800; //建立一个200K的缓冲区 byte[] buffer = new byte[chunkSize]; //已读的字节数 long dataToRead = 0; FileStream stream = null; try { //打开文件 stream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.Read); dataToRead = stream.Length; //添加Http头 HttpContext.Current.Response.ContentType = "application/octet-stream"; HttpContext.Current.Response.AddHeader("Content-Disposition", "attachement;filename=" + HttpUtility.UrlEncode(Path.GetFileName(filePath))); HttpContext.Current.Response.AddHeader("Content-Length", dataToRead.ToString()); while (dataToRead > 0) { if (HttpContext.Current.Response.IsClientConnected) { int length = stream.Read(buffer, 0, Convert.ToInt32(chunkSize)); HttpContext.Current.Response.OutputStream.Write(buffer, 0, length); HttpContext.Current.Response.Flush(); //HttpContext.Current.Response.Clear(); buffer = new Byte[chunkSize]; dataToRead = dataToRead - length; } else { //防止client失去连接 dataToRead = -1; } } } catch (Exception ex) { throw ex; //HttpContext.Current.Response.Write("Error:" + ex.Message); } finally { if (stream != null) { stream.Close(); } HttpContext.Current.Response.Close(); } } }