这样写角色权限的编辑，判断以前有的，现在没有了，删掉它。新加的，add.

public bool AssignMenuForRole(List<RoleMenu> newList)
        {
            if (newList != null)
            {
                int roleId = (int)newList[0].RoleId;
                using (ePRGEntities dbContext = new ePRGEntities())
                {
                    var oldEntitys_query = from item in dbContext.RoleMenu where item.RoleId == roleId select item;
                    var oldEntitys = oldEntitys_query.ToList();
                    //delete no need olds (in old but cancel checked now)
                    foreach (var oldEntity in oldEntitys)
                    {
                        if (newList.Where(o => o.MenuId == oldEntity.MenuId).Count() == 0)
                        {
                            dbContext.RoleMenu.Remove(oldEntity);
                            dbContext.SaveChanges();
                        }
                    }
                    //add new adds (don't exists yet)
                    foreach (var newEntity in newList)
                    {
                        if (oldEntitys.Where(o => o.MenuId == newEntity.MenuId).Count() == 0)
                        {
                            dbContext.RoleMenu.Add(newEntity);
                            dbContext.SaveChanges();
                        }
                    }
                    return true;
                }
            }

            return false;

        }