using System;
using System.Windows.Forms;
using System.Threading;

class MainForm : Form {
  TextBox textBox;
  Thread thread;
  volatile bool abortRequest = false;
  volatile bool activeBackground = false;
  void DoBackgroundWork() {
    activeBackground = true;
    while (!abortRequest) {
       Thread.Sleep(1000);
       this.BeginInvoke(new EventHandler((sender, e) => { textBox.AppendText("."); }),
         new object [] { this, EventArgs.Empty });
    }
    this.Invoke(new EventHandler((sender, e) => {
        textBox.AppendText(Environment.NewLine + "UI同期が必要な終了処理");
        activeBackground = false;
        this.Close();
       }), new object [] { this, EventArgs.Empty });
  }
  public MainForm() {
    thread = new Thread(DoBackgroundWork);
    textBox = new TextBox() { Multiline= true, Dock = DockStyle.Fill };
    this.Controls.Add(textBox);
    this.Load += (sender, e) => { thread.Start(); };
    this.Closing += (sender, e) => {
      if (activeBackground) { 
        abortRequest = true;
        e.Cancel = true;
      }
    };
  }
}
class Program {
 static void Main() {
   Application.Run(new MainForm());
 }
}