using System.IO;

  public string UpdatePic(HttpPostedFileBase pic)
        {
            string filePath = null;
            string filename = null;
            try
            {
                string path = Server.MapPath("~/Upload/");

                if (!Directory.Exists(path))
                {
                    Directory.CreateDirectory(path);
                }
                filename = pic.FileName;//获得文件上传全目录
                //filename = DateTime.Now.ToString("yyyyMMddHHmmssfff");
                int place = filename.LastIndexOf(".") + 1;//获得文件扩展名的位置
                string extname = filename.Substring(place).ToLower();//得到文件扩展名              
                string allowedType = "jpg|bmp|jpeg|psd|png";//允许上传文件类型
                long allowedSize = 512000 * 1024;//允许上传最大文件大小
                if (allowedType.Contains(extname) && pic.ContentLength < allowedSize)
                {
                    filePath = path + filename;
                    pic.SaveAs(filePath);
                }
                else
                {
                    return null;
                }
            }
            catch (Exception err)
            {
                throw new Exception(err.Message);
            }
            return filename;
        }

    HttpPostedFileBase file = Request.Files["pic"];    


  if (file.ContentLength != 0)
                    {
                        string filename = UpdatePic(file);
                        model.Url = @"..\..\Upload\" + filename;
                    }

 <tr>
                <th>@Html.LabelFor(model=>model.Url)</th>
                <td>
                   <input type="file" name="pic" id="pic" />
                </td>
              </tr>

    @using (Html.BeginForm("AddActivity", "Activity", FormMethod.Post, new { enctype = "multipart/form-data"}))