please help with FileStream reading.

private void Login_btn_Click(object sender, EventArgs e)
        {
            User u1 = new User();
            u1.Username = Username_login_Textbox.Text;
            u1.Password = Password_login_Textbox.Text;

            FileStream fs1 = new FileStream(@"C:\Users\QEBO\Desktop\Exam_System_Users\test.txt", FileMode.OpenOrCreate, FileAccess.Write);
            BinaryReader br = new BinaryReader(fs1); // here appears THE STREAM WAS NOT READABLE :(

            if (u1.Username == fs1.Length.ToString())
            {
                if (u1.Password == fs1.Length.ToString())
                {
                    ExamSystem es = new ExamSystem();
                    es.ShowDialog();
                }
            }
            else
            {
                MessageBox.Show("Invalid username or password");
            }
            fs1.Flush();
            fs1.Close();
            br.Close();
        }