C# ListCollectionView.AddNewItem 没有成功添加对象

我在执行ListCollectionView.AddNewItem添加对象后，IndexOf返回的索引为-1，不知道是什么原因？

代码如下：

List PortNoindexData = new List()

{

    new ComboBoxItem() { Content = 1 , VerticalAlignment = VerticalAlignment. Center,FontSize = 14 },
    new ComboBoxItem() { Content = 2 , VerticalAlignment = VerticalAlignment. Center,FontSize = 14 },
    new ComboBoxItem() { Content = 3 , VerticalAlignment = VerticalAlignment. Center,FontSize = 14 },
    new ComboBoxItem() { Content = 4 , VerticalAlignment = VerticalAlignment. Center,FontSize = 14 },
    new ComboBoxItem() { Content = 5 , VerticalAlignment = VerticalAlignment. Center,FontSize = 14 },
    new ComboBoxItem() { Content = 6 , VerticalAlignment = VerticalAlignment. Center,FontSize = 14 },

};

    ComboBox_PortNo.ItemsSource = PortNoindexData;

    ListCollectionView viewPort = (ListCollectionView)CollectionViewSource.GetDefaultView(ComboBox_PortNo.ItemsSource);

    //viewPort已经删除了PortNoindexData[2]
    viewPort.AddNewItem(PortNoindexData[2]); 
    viewPort.CommitNew();

    viewPort.IndexOf(PortNoindexData[2]);

IndexOf是引用匹配，也就是只有指向同一个对象的引用才能匹配，不是一个对象，就算所有字段都相等，也不会匹配。