C#

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
       
        public string WriteMainMessage
        {
            get { return textBox1.Text; }
            set
            {
                textBox1.Text = value;
                MessageBox.Show("from set " + textBox1.Text);
            }
        }
    }

public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
            form1 = new Form1();
            form1.Show();
        }
        Form1 form1;
        private void button1_Click(object sender, EventArgs e)
        {
            form1.WriteMainMessage = "This is accessor";
            MessageBox.Show("accessor says " + form1.WriteMainMessage);
        }
    }

// Form1.cs
using System;
using System.ComponentModel;
using System.Drawing;
using System.Windows.Forms;

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private Button button1;

    private IContainer components = null;

    protected override void Dispose(bool disposing)
    {
        if (disposing && (components != null))
        {
            components.Dispose();
        }
        base.Dispose(disposing);
    }

    private void InitializeComponent()
    {
        button1 = new Button();
        SuspendLayout();

        button1.Location = new Point(13, 13);
        button1.Text = "Show Form2";
        button1.Click += new EventHandler(this.button1_Click);

        Controls.Add(this.button1);
        ResumeLayout(false);

    }

    private void button1_Click(object sender, EventArgs e)
    {
        Form2 form2 = new Form2();
        form2.UpdateText += new Form2.UpdateTextEventHandler(form2_UpdateText);
        form2.ShowDialog();
        form2.UpdateText -= form2_UpdateText;
        form2.Dispose();
    }

    void form2_UpdateText(string text)
    {
        Text = text;
    }
}

// Form2.cs
using System;
using System.ComponentModel;
using System.Windows.Forms;

public class Form2 : Form
{
    public delegate void UpdateTextEventHandler(string text);
    public event UpdateTextEventHandler UpdateText;

    public Form2()
    {
        InitializeComponent();
    }

    private TextBox textBox1;
    private Button button1;

    private IContainer components = null;

    protected override void Dispose(bool disposing)
    {
        if (disposing && (components != null))
        {
            components.Dispose();
        }
        base.Dispose(disposing);
    }

    private void InitializeComponent()
    {
        textBox1 = new System.Windows.Forms.TextBox();
        button1 = new System.Windows.Forms.Button();

        SuspendLayout();

        textBox1.Location = new System.Drawing.Point(12, 12);

        button1.Location = new System.Drawing.Point(12, 38);
        button1.Text = "Update";
        button1.Click += new System.EventHandler(this.button1_Click);

        Controls.Add(this.button1);
        Controls.Add(this.textBox1);

        ResumeLayout(false);
        PerformLayout();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        OnUpdateText(textBox1.Text);
    }

    protected virtual void OnUpdateText(string text)
    {
        UpdateTextEventHandler eh = UpdateText;
        if (eh != null)
            eh(text);
    }
}