1314/prakt1314/ViewModels/AuthPageViewModel.cs

using System;
using System.Collections.Generic;
using MsBox.Avalonia.Enums;
using MsBox.Avalonia;
using System.Reactive;
using ReactiveUI;
using Microsoft.EntityFrameworkCore;
using System.Threading.Tasks;
using System.Linq;

namespace prakt1314.ViewModels
{
	public class AuthPageViewModel : ReactiveObject
	{

        public AuthPageViewModel()
        {
            AuthorizeCommand = ReactiveCommand.Create<string>(AuthorizeInSystemCommandMethod);
        }


        public event Action<int> NotifyUserWasSuccessfulAuthorize;


        public string Login
        {
            get => _login;
            set => this.RaiseAndSetIfChanged(ref _login, value);
        }
        public string Password
        {
            get => _password;
            set => this.RaiseAndSetIfChanged(ref _password, value);
        }

        public ReactiveCommand<string, Unit> AuthorizeCommand { get; }


        private string _login;
        private string _password;


        private async void AuthorizeInSystemCommandMethod(string param)
        {
            UserLogin? user;
            using (AvaloniaPrakt13DbContext db = new AvaloniaPrakt13DbContext())
            {
                user = db.UserLogins.FirstOrDefault(a => a.Login == Login && a.Password == Password);
            }
            if (user is null)
            {
                MessageBoxManager.GetMessageBoxStandard("Îøèáêà àâòîðèçàöèè!"
                                                     , "Ïîëüçîâàòåëü íå ñóùåñòâóåò"
                                                     , ButtonEnum.Ok).ShowAsync();
                return;
            }
            NotifyUserWasSuccessfulAuthorize(user.IdUser);
        }
    }

}