ASP.NET Core Web API - Put action with DTO didn't update the existing object

Question

I've been trying to write Put (update) action without the id property, by using DTO. But every time I'm trying I'm getting the existing object and not the updated one, and I can't figure out why and how to change it so it will work.

My repository:

public User Update(Guid id, User user)
{
   var userToUpdate=_context.Users.FirstOrDefault(x => x.Id == id);
   _context.Entry(userToUpdate).State = EntityState.Modified;
   _context.SaveChanges();
   return userToUpdate;
}

My DTO:

public class UserPostDTO
{
    public UserPostDTO()
    {

    }
    public UserPostDTO(User user)
    {
        UserName= user.UserName;
        Password= user.Password;
        LastLogin= user.LastLogin;
    }
    [StringLength(255)]
    public string UserName { get; set; } = null!;
    [StringLength(255)]
    public string Password { get; set; } = null!;
    [Column(TypeName = "datetime")]
    public DateTime? LastLogin { get; set; }
    public User ToPostUser()
    {
        var user = new User();
        user.UserName = UserName;
        user.Password = Password;
        user.LastLogin = LastLogin;
        return user;
    }
}

My Controller:

public class UserController : ControllerBase
{
    private readonly IUserRepository _userRepository;
    public UserController(IUserRepository userRepository)
    {
        _userRepository = userRepository;
    }

    [HttpPut("{id}")]
    public IActionResult Put(Guid id, [FromBody] UserPostDTO user)
    {
        _userRepository.Update(id, user.ToPostUser());
        return Ok();
    }

Answer 1

Didn't see you updating the User object with the new value.

Probably this is what you need:

public User Update(Guid id, User user)
{
    var userToUpdate = _context.Users.FirstOrDefault(x => x.Id == id)
        .AsNoTracking();
    if (userToUpdate == null)
    {
        // Handle ID is not existed
        throw new ArgumentNullException("ID is not existed");
    }

    user.Id = userToUpdate.Id;
    _context.Entry(user).State = EntityState.Modified;
    _context.SaveChanges();
    return user;
}