public partial class Form2 : Form
    {
        private Form1 _f1;
        public Form2(Form1 f1)
        {           
            _f1 = f1;
        }

        private void Form2_Load(object sender, EventArgs e)
        {
            richTextBox1.Text = _f1.getString();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            _f1.Show();
            this.Close();
        }
    }