How to create parent and child relationship by drag and drop nodes in WPF Diagram (SfDiagram)?

public class DiagramVM : DiagramViewModel
{
  public DiagramVM()
  {
    Nodes = new ObservableCollection<CustomNode>();
 
    // Initialize Commands
    ItemAddedCommand = new Command(OnItemAdded);
    DropCommand = new Command(OnItemDrop);
  }
 
  // Method used to add the Allowdrop constraints to the dropped node
  // Allowdrop constraints is used to allow the itemdropped event to get the element as target element.
  private void OnItemAdded(object obj)
  {
    ItemAddedEventArgs args = obj as ItemAddedEventArgs;
    if (args.Item is CustomNode)
    {
      (args.Item as CustomNode).Constraints = (args.Item as CustomNode).Constraints.Add(NodeConstraints.AllowDrop);
    }
  }
 
  // Mehtod to create relation between drag and dropped nodes
  private void OnItemDrop(object obj)
  {
    ItemDropEventArgs args = obj as ItemDropEventArgs;
    if (!(args.Target is SfDiagram))
    {
      foreach (object targetElement in args.Target as IEnumerable<object>)
      {
         if (targetElement is CustomNode)
         {
           if ((args.Source as CustomNode).ParentId == null)
           {
             (args.Source as CustomNode).Id = "Node" + (Nodes as ObservableCollection<CustomNode>).Count.ToString();
             (args.Source as CustomNode).ID = (args.Source as CustomNode).Id;
             (args.Source as CustomNode).ParentId = (targetElement as CustomNode).Id;
 
             CreateConnector((args.Source as CustomNode).ParentId, (args.Source as CustomNode).Id);
             LayoutManager.Layout.UpdateLayout();
           }
         }
      }
    }
 
    else if (args.Target is SfDiagram)
    {
      if ((args.Source as CustomNode).ParentId == null)
      {
        (args.Source as CustomNode).Id = "Node" + (Nodes as ObservableCollection<CustomNode>).Count.ToString();
        (args.Source as CustomNode).ID = (args.Source as CustomNode).Id;
        (args.Source as CustomNode).ParentId = "";
        LayoutManager.Layout.UpdateLayout();
      }
    }
  }
}