How to serialize MapPolyline in WPF Bing map?

Hi everyone, I want to serialize a MapPolyline on WPF Bing map to a file, close the program, then start the program again and load the file in order to restore that MapPolyline on the map. I couldn't figure how to make this work. Could you please help me to solve this? Thanks in advance.

Here is my code:

private void Save(object sender, RoutedEventArgs e)
{
System.Windows.Forms.SaveFileDialog saveFile = new System.Windows.Forms.SaveFileDialog();
            saveFile.Filter = "XML Files (*.xml)|*.xml|All Files (*.*)|*.*";
            saveFile.FileName = string.Format("{0:MM-dd-yyyy HH.mm.ss}", DateTime.Now);
            saveFile.DefaultExt = ".xml";
            saveFile.InitialDirectory = @"c:\BingMapApp\BingMapState";            

            if (saveFile.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                string filename = saveFile.FileName;
                FileStream Fs = new FileStream(filename, FileMode.Create);               

                foreach (UIElement element in MainMap.Children)
                {
                    if (element is MapPolyline)
                    {
                        MapPolyline polyline = element as MapPolyline;
                        XamlWriter.Save(polyline, Fs);
                    }
                }
                Fs.Close();
}
}

private void Load(object sender, RoutedEventArgs e)
{

System.Windows.Forms.OpenFileDialog openFile = new System.Windows.Forms.OpenFileDialog();
            openFile.Filter = "XML Files (*.xml)|*.xml|All Files (*.*)|*.*";

            if (openFile.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {                
                FileStream xamlFile = openFile.OpenFile() as FileStream;

                if (xamlFile == null) 
                    return;
                else
                {                    
                        MapPolyline element = XamlReader.Load(xamlFile) as MapPolyline;
                                               
                        xamlFile.Close();                     

                    }

}

There's an error with this line: MapPolyline element = XamlReader.Load(xamlFile) as MapPolyline;

It said: "'Failed to create a 'Locations' from the text 'Microsoft.Maps.MapControl.WPF.LocationCollection'."