Replace a string using two forms

Here is a quick basic example

 public partial class Form1 : Form
    {

        public Form1()
        {
            InitializeComponent();
        }

        internal string StrVariable = string.Empty;
        private void button1_Click(object sender, EventArgs e)
        {
            using (Form2 FrmObj = new Form2(this))
            {
                FrmObj.ShowDialog();
            }
        }
    }

-------------------------------------------------------------
public partial class Form2 : Form
    {
        Form1 FrmRef;
        public Form2()
        {
            InitializeComponent();
        }
        public Form2(Form1 frmref)
            : this()
        {
            this.FrmRef = frmref;
        }

        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            this.FrmRef.StrVariable = this.textBox1.Text;
        }
    }