ApplicationShapeChanged Event

public event ShapeEventHandler ShapeChanged

Public Event ShapeChanged As ShapeEventHandler

private void AppAddIn_Startup(object sender, EventArgs e)
{
    ShapeChanged += AppAddIn_ShapeChanged;
}

private void AppAddIn_ShapeChanged(object sender, ShapeEventArgs e)
{
    ShapeChangedEventArgs ce = (ShapeChangedEventArgs)e;

    Shape newShape = ce.Shape;
    Shape oldShape = ce.OldShape;

    if (newShape != null && oldShape != null)
    {
        if (newShape.LayerID != oldShape.LayerID)
            MessageBox.Show("Layer of shape has been changed!");
    }
}