How to return message user already exist when calling checku

Question

I make function create user on database by web API asp.net core and it working without

any problem .

I notice more users repeated so that i need to prevent user repeating records on

database .

I make function CheckUserName to check user name exist on database on table user login

if exist prevent clients from add or create new user in case of exist

My problem in case of user exist I need to return "user already exist" on return type

of calling function CheckUserName "user already exist"

but How to return user already exist message

 

What I have tried:


[Produces("application/json")]
    [Route("api/UserLogins")]
    public class UserLoginsController : Controller
    {
        private readonly IUserLogins _services;
        public UserLoginsController(IUserLogins Services)
        {
            this._services = Services;
        }
     }
        [HttpPost]
        public UserLogins Post([FromBody]UserLogins userDto)
        {
            if (_services.CheckUserName(userDto.UserName))
            {
return ?????????????
            }
            return  _services.Create(userDto);

        }
public class UserLogins:SharedValues
    {
        public int UserLoginID { get; set; }
        public string UserName { get; set; }
        public string UserMail { get; set; }
        public string UserPass { get; set; }
        public int FkTeamID { get; set; }
        public Boolean IsAdmin { get; set; }


    }

 public interface IUserLogins
    {
        UserLogins Create(UserLogins user);
        bool CheckUserName(string username);
    }