dispatch.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication11
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            //WORKS
            var window1 = new Window1();
            window1.Show();

            //DOESNT WORK
            var bw = new BackgroundWorker();
            bw.DoWork += bw_DoWork;
            bw.RunWorkerAsync();

            //WORKS AGAIN!
            var bw2 = new BackgroundWorker();
            bw2.DoWork += bw_DoWork2;
            bw2.RunWorkerAsync();
        }

        void bw_DoWork(object sender, DoWorkEventArgs e)
        {
            var window1 = new Window1();
            window1.Show();
        }

        void bw_DoWork2(object sender, DoWorkEventArgs e)
        {
            Application.Current.Dispatcher.Invoke((Action) delegate
            {
                var window1 = new Window1();
                window1.Show();
            });
        }
    }
}