通过url下载文件

  public ActionResult GetFileFromNetUrldown(string idcardup)
        {
            try
            {
                System.Net.WebRequest req = System.Net.WebRequest.Create(idcardup);
                req.Method = "GET";

                byte[] fileBytes;
                using (WebResponse webRes = req.GetResponse())
                {
                    int length = (int)webRes.ContentLength;
                    HttpWebResponse response = webRes as HttpWebResponse;
                    Stream stream = response.GetResponseStream();

                    //读取到内存
                    MemoryStream stmMemory = new MemoryStream();
                    byte[] buffer = new byte[length];
                    int i;
                    //将字节逐个放入到Byte中
                    while ((i = stream.Read(buffer, 0, buffer.Length)) > 0)
                    {
                        stmMemory.Write(buffer, 0, i);
                    }
                    fileBytes = stmMemory.ToArray();//文件流Byte
                    stmMemory.Close();
                }
                return File(fileBytes, "image/jpeg", filename);

            }
            catch (System.Exception ex)
            {
                throw ex;
            }
        }