I`m creating apllication on WPF with using MVVM and I need to get an object (node, edge) after mouse click on a graph, to process them with different ways.

But when I click on a graph, I get an object sender equals null after casting it to GraphViewer type. Object sender is retrieving by type Microsoft.Msagl.WpfGraphControl.AutomaticGraphLayoutControl and I do not know how to get this object, because this type do not have this capability. Yes this type (Microsoft.Msagl.WpfGraphControl.AutomaticGraphLayoutControl) have property _graphViewer with type GraphViewer but it is private.
So here is my code.

View xaml:

<Window x:Class="myApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:myApp"
        xmlns:wpfgraphcontrol="http://mlsagl"
        mc:Ignorable="d"
        Title="Main Window" 
        SizeToContent="WidthAndHeight"
        MinHeight="800"
        MinWidth="800" 
        x:Name="mainWindow">
   <Grid>
      <wpfgraphcontrol:AutomaticGraphLayoutControl
                             Graph="{Binding CurrentGraph.GraphObject,  UpdateSourceTrigger=PropertyChanged}" 
                             MouseDown="graphDisplay_MouseDown">
      </wpfgraphcontrol:AutomaticGraphLayoutControl>
    </Grid>
</Window>

View codebehind:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new ApplicationViewModel(new DefaultDialogService(), new MSAGLFileService());
    }
    private void graphDisplay_MouseDown(object sender, MouseButtonEventArgs e)
    {
        var graphViewer = sender as GraphViewer;
        if (graphViewer != null)
        { 
           if(graphViewer.ObjectUnderMouseCursor as IViewerNode != null)
               MessageBox.Show("You click on node");
           else if(graphViewer.ObjectUnderMouseCursor as IViewerEdge != null)
               MessageBox.Show("You click on edge");
           else
               MessageBox.Show("You click not on edge or not on node");
        }
        else
           MessageBox.Show("sender is null");
    }
}

ApplicationViewModel:

public class ApplicationViewModel 
{
    private GraphModel currentGraph;
    public GraphModel CurrentGraph
    {
        get { return currentGraph; }
        set { currentGraph = value; OnPropertyChanged(nameof(CurrentGraph)); }
    }
    public ApplicationViewModel(IDialogService dialogService, IFileService fileService)
    {
        currentGraph = new GraphModel();
    }
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = "")
    {
        if (PropertyChanged != null)
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

GraphModel:

    private string name { get; set; }
    private Graph graphObject { get; set; }
    private bool isInitialized { get; set; }
    public string Name
    {
        get{ return name; }
        set { name = value; OnPropertyChanged(nameof(Name)); }
    }        
    public Graph GraphObject
    {
        get{ return graphObject; }
        set{ graphObject = value; OnPropertyChanged(nameof(GraphObject));}   
    }    
    public bool IsInitialized
    {
        get{ return isInitialized; }
        
        set{i sInitialized = value; OnPropertyChanged(nameof(IsInitialized));}
    }
    public GraphModel()
    {
        IsInitialized = false;
        Name = " - ";
        GraphObject = new Graph(Name);
    }
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = "")
    {
        if (PropertyChanged != null)
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }