C#

using System;
using System.Drawing;
using System.Windows.Forms;
 
class ParentForm : Form // Default access for class is internal
{
  static void Main()
  {
    Application.Run(new ParentForm());
  }
 
  Button openChild; // Default access is private for class members
  internal TextBox childText; // Make accessible to child form in this assembly
  internal ParentForm()
  {
    Text = "Example: Parent Form";
 
    openChild = new Button();
    Controls.Add(openChild);
    openChild.Location = new Point(8, 8);
    openChild.Text = "Open Child";
    openChild.Click += new EventHandler(openChild_Click);
 
    childText = new TextBox();
    Controls.Add(childText);
    childText.Location = new Point(8, openChild.Bottom + 8);
    childText.ReadOnly = true;
  }
 
  void openChild_Click(object sender, EventArgs e)
  {
    using (ChildForm form = new ChildForm())
      form.ShowDialog(this);
  }
}
 
class ChildForm : Form
{
  TextBox myText;
  internal ChildForm()
  {
    Text = "Example: Child Form";
 
    myText = new TextBox();
    Controls.Add(myText);
    myText.Location = new Point(8, 8);
    myText.TextChanged += new EventHandler(myText_TextChanged);
  }
 
  void myText_TextChanged(object sender, EventArgs e)
  {
    ParentForm parent = Owner as ParentForm;
    if (parent != null)
      parent.childText.Text = myText.Text;
  }
}