CollectionChanged event is not fired

Do you know why Collection Changed is not fired when I add a new item in the collection?
RibbonTab newTab= new RibbonTab
{
    Title = "newTab",
    Id = "newTab",
    Name = "newTab",
    IsVisible = true
};
ComponentManager.Ribbon.Tabs.CollectionChanged += (sender, e) => { 
   //Handle the event...
};
ComponentManager.Ribbon.Tabs.Add(newTab)

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