bool flagStop = false;
void Run ()
    {
        flagStop = false;
        ManualResetEvent waitHandle = new ManualResetEvent(false);
        for ( int i = 1; i <= 100000; i++ )
        {
            Dispatcher.BeginInvoke(new ThreadStart(delegate { lb.Text = i.ToString(); }));
            waitHandle.WaitOne(1);
            if ( flagStop )
            {
                break;
            }
        }
    }

private void buttonStartFor_Click (object sender, RoutedEventArgs e)
{
        Task ts = new Task(Run);
        ts.Start();
}

private void buttonEndFor_Click (object sender, RoutedEventArgs e)
{
        flagStop = true;
}