C#

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using System;
using System.Drawing;
using System.Windows.Forms;
 
class Parent : Form
{
  private TextBox txt;
  private Button btn;
 
  static void Main()
  {
    Application.Run(new Parent());
  }
 
  public Parent()
  {
    this.Text = "Example: Parent Form";
 
    this.txt = new TextBox();
    this.txt.Location = new Point(8, 8);
 
    this.btn = new Button();
    this.btn.Location = new Point(8, this.txt.Bottom + 8);
    this.btn.Text = "Show Child";
    this.btn.Click += new EventHandler(this.btn_Click);
 
    this.Controls.Add(this.txt);
    this.Controls.Add(this.btn);
  }
 
  private void btn_Click(object sender, EventArgs e)
  {
    using (Child form = new Child()) // Make sure 'form' gets disposed
      form.ShowDialog(this);
  }
 
  public string ImportantText
  {
    get { return this.txt.Text; } // Do not expose 'txt' directly.
  }
}
 
class Child : Form
{
  private Label lbl;
 
  public Child()
  {
    this.Text = "Example: Child Form";
 
    this.lbl = new Label();
    this.lbl.Location = new Point(8, 8);
 
    this.Controls.Add(this.lbl);
  }
 
  // Owner is not available during construction.
  protected override void OnLoad(EventArgs e)
  {
    this.lbl.Text = ((Parent)this.Owner).ImportantText;
  }
}