Taller de Programacion (Esqueleto de clases)

El "esqueleto" y las clases de mi proyecto son:

ALTA DE AUTO
public partial class altaAuto : Form
    {
        public altaAuto()
        {
            InitializeComponent();
        }
        int a;
        private void pictureBox1_Click(object sender, EventArgs e)
        {
            try
            {
                OpenFileDialog openFileDialog1 = new OpenFileDialog();
                openFileDialog1.InitialDirectory = "C:\\";
                openFileDialog1.Filter = "Jpeg|*.jpg";
                openFileDialog1.FilterIndex = 1;
                openFileDialog1.RestoreDirectory = true;
                if (openFileDialog1.ShowDialog() == DialogResult.OK)
                {
                    string fullPathname = openFileDialog1.FileName;
                    pictureBox1.Image = System.Drawing.Image.FromFile(openFileDialog1.FileName);
                }
            }
            catch
            {
                MessageBox.Show("Error al elegir la imagen", "ERROR");
            }
        }


BUSQUEDA DE AUTO

 public partial class buscaAuto : Form
    {
        public buscaAuto()
        {
           InitializeComponent();           
        }
        Vender mos2;
        private string cadena;
        public string datoAEnviar
        {
            get { return cadena; }
            set { cadena = value; }
        }

REGISTRO
public partial class Registrar : Form
    {
        public Registrar()
        {
            InitializeComponent();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            MessageBox.Show("Has cancelado la accion de crear una cuenta nueva", "CANCELAR");
            Visible = false;
            Form mos = new Principal ();
            mos.Show();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            saveFileDialog1.FileName = "C:\\agenciaAutos\\Autos\\Contraseñas\\" + id.Text;
            using (StreamWriter sw = new StreamWriter(saveFileDialog1.FileName + ".txt"))
            {
                sw.Write(contraseña.Text);
            }
   }