✅ Just curious about something

using Diary.Models;
using Diary.Services;
using ReactiveUI;
using System;
using System.Reactive;
using System.Reactive.Linq;

namespace Diary.ViewModels;

public class LoginViewModel : ViewModelBase
{
    private Database _db;
    private Helpers _hp;
    private string _username;
    private string _password;
    private User _user;

    public LoginViewModel(Database db, Helpers hp)
    {
        _db = db;
        _hp = hp;

        IObservable<bool> usernameOk = this.WhenAnyValue(
            x => x.Username,
            x => !string.IsNullOrEmpty(x) && CheckUsername());

        IObservable<bool> passwordOk = this.WhenAnyValue(
            x => x.Password,
            x => !string.IsNullOrEmpty(x) && CheckPassword());
        IObservable<bool> okEnabled = passwordOk
            .Concat(usernameOk);

        Login = ReactiveCommand.Create(CreateUser, okEnabled);
        Exit = ReactiveCommand.Create(() => { });
    }

    public User CreateUser()
    {
        User currentUser = _db.GetUser(Username);

        return new User
        {
            Id = currentUser.Id,
            FirstName = currentUser.FirstName,
            LastName = currentUser.LastName,
            Email = currentUser.Email,
            Username = currentUser.Username,
            Password = currentUser.Password
        };
    }