//ID自增但不是主键的情况
 public int Update_join<TEntity>(TEntity entity) where TEntity : class
        {
            dbcontext.Set<TEntity>().Attach(entity);
            PropertyInfo[] props = entity.GetType().GetProperties();
            foreach (PropertyInfo prop in props)
            {
                if(prop.Name=="ID")continue;
                if (prop.GetValue(entity, null) != null)
                {
                    if (prop.GetValue(entity, null).ToString() == "&nbsp;")//一些属性由于前台的原因  传过来的值会是&nbsp;  这里做一下处理
                        dbcontext.Entry(entity).Property(prop.Name).CurrentValue = http://www.mamicode.com/null;
                    dbcontext.Entry(entity).Property(prop.Name).IsModified = true;
                }
            }
            return dbcontext.SaveChanges();
        }

 //根据ViewModel更新实体的部分属性
public int UpdateEntityFields<TEntity>(TEntity entity, List<string> fileds) where TEntity : class
        {
            if (entity != null && fileds != null)
            {
                dbcontext.Set<TEntity>().Attach(entity);
                var SetEntry = ((IObjectContextAdapter)dbcontext).ObjectContext.
                    ObjectStateManager.GetObjectStateEntry(entity);
                foreach (var t in fileds)
                {
                    SetEntry.SetModifiedProperty(t);
                }
            }
            return dbcontext.SaveChanges();

}
//上面的参数list是将ViewModel的属性整理成的集合，方法如下

 public static List<string> GetDeleteFiles()
        {
            List<string> list = new List<string>();
            PropertyInfo[] props = new DeleteEntity().GetType().GetProperties();//得到ViewModel属性集合
            foreach (PropertyInfo propertyInfo in props)
            {
                list.Add(propertyInfo.Name);
            }
            return list;
        }

 public class DeleteEntity : INDeleteAudited
    {
        public bool F_DeleteMark { get; set; }

        /// <summary>
        /// 删除实体的用户
        /// </summary>
        public string F_DeleteUserId { get; set; }

        /// <summary>
        /// 删除实体时间
        /// </summary>
        public DateTime? F_DeleteTime { get; set; }

    }