private static NList<NShape> GetGluedShapes(NPort port)
{
    NList<NShape> gluedShapes = new NList<NShape>();
    if (port.OwnerShape == null)
        return gluedShapes;

    int portIndex = port.GetAggregationInfo().Index;

    // Check incoming shapes
    NList<NShape> shapes = port.OwnerShape.GetIncomingShapes();
    for (int i = 0; i < shapes.Count; i++)
    {
        NShape shape = shapes[i];
        if (shape.EndPointGlue is NEndPointGlueToPort portGlue && portGlue.PortIndex == portIndex)
        {
            gluedShapes.Add(shape);
        }
    }

    // Check outgoing shapes
    shapes = port.OwnerShape.GetOutgoingShapes();
    for (int i = 0; i < shapes.Count; i++)
    {
        NShape shape = shapes[i];
        if (shape.BeginPointGlue is NEndPointGlueToPort portGlue && portGlue.PortIndex == portIndex)
        { 
            gluedShapes.Add(shape);
        }
    }

    return gluedShapes;
}

private static void CreateSampleDiagram(NDrawingView drawingView)
{
    NPage page = drawingView.ActivePage;
    NBasicShapeFactory factory = new NBasicShapeFactory();

    // Create 2 shapes
    NShape shape1 = factory.CreateShape(ENBasicShape.Rectangle);
    shape1.Text = "Rectangle";
    shape1.SetBounds(100, 100, 150, 100);
    shape1.Geometry.Fill = new NColorFill(NColor.Red);
    page.Items.Add(shape1);

    NShape shape2 = factory.CreateShape(ENBasicShape.Circle);
    shape2.Text = "Circle";
    shape2.SetBounds(350, 300, 150, 100);
    shape2.Geometry.Fill = new NColorFill(NColor.Green);
    page.Items.Add(shape2);

    // Create a connector and connect the shapes
    NRoutableConnector connector = new NRoutableConnector();
    connector.UserClass = NDR.StyleSheetNameConnectors;
    page.Items.Add(connector);
    connector.GlueBeginToShape(shape1);
    connector.GlueEndToShape(shape2);

    // Get the right port of the rectangle shape
    NPort port = shape1.Ports.GetPortByName("Right");
    NShape portConnector;
    GetGluedShapes(port).TryGetFirst(null, out portConnector);

    // Subscribe to the deactivated event of the drag handle tool
    NDragHandleTool dragHandleTool = drawingView.Interactor.GetTool<NPointerTool>().GetTool<NDragHandleTool>();
    dragHandleTool.Deactivated += (NEventArgs arg) =>
    {
        NShape curPortConnector;
        GetGluedShapes(port).TryGetFirst(null, out curPortConnector);
        if (curPortConnector != null && curPortConnector != portConnector)
        {
            NMessageBox.Show("New connector connected to port.", "New Connection");
        }
    };
}