Project/project/TestClass.cs

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

namespace project
{
    public class Testovik
    {
        DataBase dataBase = new DataBase();
        public string autoRization(string login, string password)
        {
            if (login == null)
                return "Введите логин";
            if (password == null)
                return "Введите пароль";
            SqlConnection sqlconn = new SqlConnection(@"Data Source=ngknn.ru;Initial Catalog=Project_GorodskoiSpravochnik;Persist Security Info=True;User ID=31П;Password=12357");
            string query = $"Select *from Employee where login = '{login}' and password = '{password}'";
            SqlDataAdapter sda = new SqlDataAdapter(query, sqlconn);
            DataTable dtbl = new DataTable();
            sda.Fill(dtbl);
            if (dtbl.Rows.Count == 1)
            {
                Login.login = login;
                return "Вы успешно вошли";
            }
            else
            {
                return "Неверный логин и/или пароль";
            }
        }
    }
}