错误是因为列表未定义为 EF 实体.

The error was because the list isn't defined as an EF Entity.

最后，您不需要自己修改状态.

In the end, you don't need to modify the state youself.

List<User> usersList = _context.Users.Where(u => u.age >18).ToList(); 
usersList.ForEach(a => { a.isAdult = 1; });
 _context.SaveChanges();