12/WindowsFormsAppGrishina/Form1.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsAppGrishina
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 Form2 = new Form2();
            Form2.Show();

        }

        private void button2_Click(object sender, EventArgs e)
        {
            if (string.IsNullOrEmpty(TextBoxLogin.Text) || string.IsNullOrEmpty(PasswordBox.Text))
            {
                MessageBox.Show("Введите логин и пароль");
                return;
            }

            using (var db = new UserEntities())
            {
                var user = db.User
                    .AsNoTracking()
                    .FirstOrDefault(u => u.Login == TextBoxLogin.Text && u.Pass == PasswordBox.Text);
                if (user == null)
                {
                    MessageBox.Show("Пользователь с такими данными не найден");
                    return;
                }
                MessageBox.Show("Пользователь найден");


            }
        }
    }
}