DeanOfficeApp/LoginForm.cs

using Npgsql;
using System;
using System.Security.Cryptography;
using System.Text;
using System.Windows.Forms;

namespace DeanOfficeApp
{
    public partial class LoginForm : Form
    {
        private readonly string _connString = "Host=245e1-rw.db.pub.dbaas.postgrespro.ru;Port=5432;Database=dbpractice;Username=mardasov_mv;Password=A4%2x$$2Ynr";

        public string UserRole { get; private set; }
        public string Username { get; private set; }
        public string FullName { get; private set; }

        public LoginForm()
        {
            InitializeComponent();
            txtPassword.PasswordChar = '*';
        }

        private async void btnLogin_Click(object sender, EventArgs e)
        {
            string username = txtUsername.Text.Trim();
            string password = txtPassword.Text;
            if (string.IsNullOrEmpty(username) || string.IsNullOrEmpty(password))
            {
                MessageBox.Show("Введите логин и пароль!", "Ошибка", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                return;
            }

            string hash = ComputeMd5(password);

            try
            {
                using (var conn = new NpgsqlConnection(_connString))
                {
                    await conn.OpenAsync();
                    string sql = "SELECT role, full_name FROM appuser WHERE username = @user AND password_hash = @hash";
                    using (var cmd = new NpgsqlCommand(sql, conn))
                    {
                        cmd.Parameters.AddWithValue("@user", username);
                        cmd.Parameters.AddWithValue("@hash", hash);
                        using (var reader = await cmd.ExecuteReaderAsync())
                        {
                            if (await reader.ReadAsync())
                            {
                                UserRole = reader.GetString(0);
                                FullName = reader.GetString(1);
                                Username = username;
                                DialogResult = DialogResult.OK;
                                Close();
                            }
                            else
                            {
                                MessageBox.Show("Неверный логин или пароль!", "Ошибка", MessageBoxButtons.OK, MessageBoxIcon.Error);
                            }
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show($"Ошибка подключения: {ex.Message}", "Ошибка", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        private string ComputeMd5(string input)
        {
            using (MD5 md5 = MD5.Create())
            {
                byte[] hash = md5.ComputeHash(Encoding.UTF8.GetBytes(input));
                StringBuilder sb = new StringBuilder();
                foreach (byte b in hash) sb.Append(b.ToString("x2"));
                return sb.ToString();
            }
        }

        private void btnExit_Click(object sender, EventArgs e) => Application.Exit();
        private void chkShowPassword_CheckedChanged(object sender, EventArgs e) => txtPassword.PasswordChar = chkShowPassword.Checked ? '\0' : '*';

        private void panelLogin_Paint(object sender, PaintEventArgs e)
        {

        }
    }
}