Gostinitsa/Registration.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 Khozeev8
{
    public partial class Registration : Form
    {
        Database database = new Database();
        public Registration()
        {
            InitializeComponent();
            StartPosition = FormStartPosition.CenterScreen;
        }

        private void Registration_Load(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            var login = textBox1.Text;
            var password = textBox2.Text;

            string querystring = $"insert into Пользователи(Логин, Пароль) values ('{login}' , '{password}')";

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

            database.openConnection();

            if (command.ExecuteNonQuery() == 1)
            {
                MessageBox.Show("Аккаунт успешно создан!", "Успех!");
                Aithorization frm = new Aithorization();
                this.Hide();
                frm.Show();
            }
            else
            {
                MessageBox.Show("Аккаунт не создан!");
            }
            database.closeConnection();
        }

        private Boolean checkuser()

        {
            var loginuser = textBox1.Text;
            var passuser = textBox2.Text;

            SqlDataAdapter adapter = new SqlDataAdapter();
            DataTable table = new DataTable();
            string querystring = $"select Логин, Пароль from Пользователи where Логин = '{loginuser}' and Пароль = '{passuser}'";

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

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

            if (table.Rows.Count > 0)
            {
                MessageBox.Show("Пользователь уже существует!");
                return true;
            }
            else
            {
                return false;
            }
        }
    }
}