Alek5

MySqlConnection con = new MySqlConnection(@"host=localhost; port = your.PORT;User id = root; password = 'your.PASS'; SslMode = true; Database ='your.DATABASE'");
public int I;

private void Login_Click(object sender, RoutedEventArgs e)
{
    I = 0;
    con.Open();
    MySqlCommand cmd = con.CreateCommand();
    cmd.CommandType = CommandType.Text;
    cmd.CommandText = "select * from user where username = '"+username.Text+"'and pass= '"+PasswordBox.Password+"'";
    DataTable dt = new DataTable();
    MySqlDataAdapter da = new MySqlDataAdapter(cmd);
    da.Fill(dt);
    I = Convert.ToInt32(dt.Rows.Count.ToString());
    if (I == 0)
    {
        //Выдать ошибку
     }
    else
     {
           this.Hide();
           MainPage mp = new MainPage();
           mp.Show();
      }
           con.Close();
}