LoginCotroller.cs

using System.Collections.Generic;
using System.Linq;
using Microsoft.AspNetCore.Mvc;

namespace Login.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    public class LoginController : ControllerBase
    {
        private readonly List<Member> _members = new List<Member>
        {
            new Member { Username = "Sam", Password = "1234"}
        };
        
        // POST api/login
        [HttpPost]
        public ActionResult<Response> Post([FromBody] Member member)
        {
            return _members
                .Where(IsMember)
                .Select(Result)
                .FirstOrDefault();
            
            bool IsMember(Member x) => x.Username == member.Username && x.Password == member.Password;
            
            Response Result(Member x) => new Response
            {
                Success = x.Username != string.Empty,
                Username = x.Username ?? ""
            };
        }
    }
}