ContentControl.ContentTemplateSelector dynamically select template

I set a ContentControl in the right of Window, and set Content binding Items (it's type is ObservableCollection). Now I want to achieve it: if there are no item, ContentControl select first DataTemplate, and add a item into items, ContentControl will select second DataTemplate to display some info.

Like this:

The problem is when I add one item into items, ContentControl didnot update and change DataTemplate, I have a try to set mode, UpdateSourceTrigger, etc., but failed. In ViewModel, after delete a item, I use this statements, it will work well <1>:

private void ExecuteDeleteClientCommand()
{
    ...
    if (DeleteClient(item))
    {
        ObservableCollection<MyViewModel> tmp = TabItems;
        TabItems = null;
        TabItems = tmp;
    }
}

.

<ContentControl 
    ContentTemplateSelector="{StaticResource MyDataTemplateSelector}" 
    Content="{Binding Items}"/>

.

public class SingleClientDataTemplateSelector : DataTemplateSelector
{
    public override DataTemplate SelectTemplate(object item, 
        DependencyObject container)
    {
        ObservableCollection<MyViewModel> obj = 
            item as ObservableCollection<MyViewModel>;
        if (null == obj || 0 == obj.Count)
        {
            return App.Current.FindResource("NullItemDataTemplate") as DataTemplate;
        }
        return App.Current.FindResource("DefaultDataTemplate") as DataTemplate;
    }
}

Edited: use this way is also failed after delete one item:

RaisePropertyChanging(ItemsPropertyName);
RaisePropertyChanged(ItemsPropertyName);

but I wonder why it work well with <1>.

Edited2 It's the delcaration:

public const string ItemsPropertyName = "Items";
private ObservableCollection<MyViewModel> items = new ObservableCollection<MyViewModel>();
public ObservableCollection<SingleClientDetailViewModel> TabItems
{
    get { return items; }
    set 
    { 
        if (items == value) { return;}
        RaisePropertyChanging(ItemsPropertyName);
    items = value;
    RaisePropertyChanged(ItemsPropertyName);
    }
}