How to use a FolderBrowserDialog from a WPF application

public static class MyWpfExtensions
{
    public static System.Windows.Forms.IWin32Window GetIWin32Window(this System.Windows.Media.Visual visual)
    {
        var source = System.Windows.PresentationSource.FromVisual(visual) as System.Windows.Interop.HwndSource;
        System.Windows.Forms.IWin32Window win = new OldWindow(source.Handle);
        return win;
    }

    private class OldWindow : System.Windows.Forms.IWin32Window
    {
        private readonly System.IntPtr _handle;
        public OldWindow(System.IntPtr handle)
        {
            _handle = handle;
        }

        #region IWin32Window Members
        System.IntPtr System.Windows.Forms.IWin32Window.Handle
        {
            get { return _handle; }
        }
        #endregion
    }
}

var dlg = new FolderBrowserDialog();
System.Windows.Forms.DialogResult result = dlg.ShowDialog(this.GetIWin32Window());

public class OldWindow : System.Windows.Forms.IWin32Window
{
    IntPtr _handle;

    public OldWindow(IntPtr handle)
    {
        _handle = handle;
    }

    #region IWin32Window Members

    IntPtr System.Windows.Forms.IWin32Window.Handle
    {
        get { return _handle; }
    }

    #endregion
}

IntPtr mainWindowPtr = new WindowInteropHelper(this).Handle; // 'this' means WPF Window
        folderBrowserDialog.ShowDialog(new OldWindow(mainWindowPtr));

private class OldWindow : System.Windows.Forms.IWin32Window
{
    IntPtr _handle;
    public OldWindow(IntPtr handle)
    {
        _handle = handle;
    }

    #region IWin32Window Members
    IntPtr System.Windows.Forms.IWin32Window.Handle
    {
        get { return _handle; }
    }
    #endregion
}

System.Windows.Forms.FolderBrowserDialog dlg = new FolderBrowserDialog();
    HwndSource source = PresentationSource.FromVisual(this) as HwndSource;
    System.Windows.Forms.IWin32Window win = new OldWindow(source.Handle);
    System.Windows.Forms.DialogResult result = dlg.ShowDialog(win);

HwndSource source = PresentationSource.FromVisual(this) as HwndSource;
IWin32Window win = source as IWin32Window;

//add a reference to System.Windows.Forms.dll

public partial class MainWindow : Window, System.Windows.Forms.IWin32Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void button_Click(object sender, RoutedEventArgs e)
    {
        var fbd = new FolderBrowserDialog();
        fbd.ShowDialog(this);
    }

    IntPtr System.Windows.Forms.IWin32Window.Handle
    {
        get
        {
            return ((HwndSource)PresentationSource.FromVisual(this)).Handle;
        }
    }
}