yrok3/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;
using System.Data.SqlClient;

namespace yrok2
{
    public partial class Form1 : Form
    {
        Class1 database = new Class1();
        public Form1()
        {
            InitializeComponent();

          
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            string usern = textBox1.Text;
            string pass = textBox2.Text;

            SqlDataAdapter adapter = new SqlDataAdapter();
            DataTable table = new DataTable();

            string querystring = $"SELECT Login, Password FROM LoginPassword WHERE Login = '{usern}' and Password = '{pass}'";

            SqlCommand command = new SqlCommand(querystring, database.GetConnection());

            adapter.SelectCommand = command;
            adapter.Fill(table);

            if (table.Rows.Count == 1)
            {
                MessageBox.Show("Успешно!");
                Form2 frm = new Form2();
                this.Hide();
                frm.Show();
            }
            else
                MessageBox.Show("Неверные данные");


        }
    }
}