Code

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="50" />
            <RowDefinition />
        </Grid.RowDefinitions>
        <diag:TabbedDiagramView Name="diag" Grid.Row="1" />
        <Button Name="Load"  Height="50" Click="AddCustomShape" HorizontalAlignment="Left">
            Add Custom Shape
        </Button>
        <Button Name="Save"  Height="50" Click="ReloadDocument" HorizontalAlignment="Center">
            Reload the document
        </Button>
    </Grid>
 

Code

public MainWindow()
        {
            InitializeComponent();
            //Diagram.RegisterItemClass(typeof(DiagramCustomNode), "my:DiagramCustomNode", 1);
            diag.Document = new DiagramDocument();
        }

        private void AddCustomShape(object sender, System.Windows.RoutedEventArgs e)
        {
            string xml = "<Shape id=\"Monitor\" fill-mode=\"Winding\"> <outline> <round-rectangle dash-style=\"Solid\" width=\"-1\" x=\"14\" y=\"8.25\" w=\"76.5\" h=\"63\" r=\"5\" /> </outline> <decorations> <round-rectangle dash-style=\"Solid\" width=\"-1\" x=\"20\" y=\"14.75\" w=\"63.5\" h=\"48\" r=\"5\" /> <round-rectangle dash-style=\"Solid\" width=\"-1\" x=\"39\" y=\"74.75\" w=\"29.5\" h=\"13\" r=\"5\" /> </decorations> <shape-decorations /></Shape>";
            XmlDocument doc = new XmlDocument();
            doc.LoadXml(xml);
            Shape s = new Shape(new byte[] { 0, 0, 1, 1, 2, 2 });
            if (diag.Document == null)
                diag.Document = new DiagramDocument();
            if (diag.Document.Pages.Count == 0)
                diag.Document.Pages.Add(new DiagramPage());
            s.LoadFromXml(doc.ChildNodes[0] as XmlElement, new XmlPersistContext(diag.Document.Pages[0], doc, 1));
            diag.Document.Pages[0].Items.Add(new ShapeNode() { Shape = s });
        }

        private void ReloadDocument(object sender, RoutedEventArgs e)
        {
            string s = diag.Document.SaveToString();
            diag.Document.LoadFromString(s);
        }