@model DataCollector.Models.User
@{
    ViewData["Title"] = "Home Page";
}

<div class="container">
    <div class="row">
        <div class="col-sm-4"></div>
        <div class="col-sm-4">
            <form class="form-horizontal" asp-action="Index">
                <div class="form-group">
                    <label class="control-label">Имя пользователя</label>
                    <input asp-for="Login" class="form-control" type="text" />
                </div>
                <div class="form-group">
                    <label class="control-label">Пароль</label>
                    <input asp-for="Password" class="form-control" type="password" />
                </div>
                <div class="form-group">
                    <input type="submit" value="Войти" />
                </div>
            </form>
        </div>
        <div class="col-sm-4"></div>
    </div>
</div>

@{
    if (TempData["msg"] != null)
    {
        <script>
            alert('@TempData["msg"]')
        </script>
    }
}

public class HomeController : Controller
    {
        private readonly Db_usersContext _context;

        public HomeController(Db_usersContext context, ILogger<HomeController> logger)
        {
            _context = context;
        }

        public IActionResult Index()
        {
            return View();
        }

        [HttpPost]
        [ValidateAntiForgeryToken]
        public IActionResult Index([Bind("Login, Password")] User user)
        {
            var db_usersContext = _context.User.Where(u => u.Login.Equals(user.Login) & u.Password.Equals(user.Password));

            User userFind = null;
            if (db_usersContext.Count() > 0)
            {
                userFind = db_usersContext.First();
                return RedirectToAction("Index", "FriendsController", new { login = userFind.Login });
            }
            else
            {
                TempData["msg"] = "Данный пользователь был добавлен ранее";
                return NotFound();
            }               
        }

    }

public class FriendsController : Controller
    {
        private readonly Db_usersContext _context;

        public FriendsController(Db_usersContext context)
        {
            _context = context;
        }

        // GET: Friends
        public async Task<IActionResult> Index(string login)
        {
            var db_usersContext = _context.Friend.Include(f => f.User).Where(f => f.User.Login.Equals(login));
            return View(await db_usersContext.ToListAsync());
        }
}

 return RedirectToAction("Index", "Friends", new { login = userFind.Login });

TempData["UserName"] = loginViewModel.UserName;
return Redirect(loginViewModel.ReturnUrl);