Response.AddHeader "content-disposition","attachment; filename=fname.ext"

public static void ToDownload(string serverfilpath,string filename)
{
FileStream fileStream = new FileStream(serverfilpath, FileMode.Open);
long fileSize = fileStream.Length;
HttpContext.Current.Response.ContentType = "application/octet-stream";
HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment; filename=\"" + UTF_FileName(filename) + ".doc\";");
////attachment --- 作为附件下载
////inline --- 在线打开
HttpContext.Current.Response.AddHeader("Content-Length", fileSize.ToString());
byte[] fileBuffer = new byte[fileSize];
fileStream.Read(fileBuffer, 0, (int)fileSize);
HttpContext.Current.Response.BinaryWrite(fileBuffer);
fileStream.Close();
HttpContext.Current.Response.End();
}
public static void ToOpen(string serverfilpath, string filename)
{
FileStream fileStream = new FileStream(serverfilpath, FileMode.Open);
long fileSize = fileStream.Length;
HttpContext.Current.Response.ContentType = "application/octet-stream";
HttpContext.Current.Response.AddHeader("Content-Disposition", "inline; filename=\"" + UTF_FileName(filename) + ".doc\";");

    HttpContext.Current.Response.AddHeader("Content-Length", fileSize.ToString());
byte[] fileBuffer = new byte[fileSize];
fileStream.Read(fileBuffer, 0, (int)fileSize);
HttpContext.Current.Response.BinaryWrite(fileBuffer);
fileStream.Close();
HttpContext.Current.Response.End();
}
private static string UTF_FileName(string filename)
{
return HttpUtility.UrlEncode(filename, System.Text.Encoding.UTF8);
}

简单的对上述代码做一下解析，ToDownload方法为将一个服务器上的文件（serverfilpath为服务器上的物理地址），以某文件名(filename)在浏览器上弹出“文件下载”对话框，而ToOpen是将服务器上的某文件以某文件名在浏览器中显示/打开的。注意其中我使用了UTF_FileName方法，该方法很简单，主要为了解决包含非英文/数字名称的问题，比如说文件名为“衣明志.doc”，使用该方法客户端就不会出现乱码了。
//*---------------------------end----------------------------------

园子里liping13599168的下载函数也不错，顺便也贴一下：
出处：http://www.cnblogs.com/liping13599168/archive/2008/07/31/672025.html