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

[llion.j]

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]);