WPF: How to bind to only one item in a collection,

问题:

I have this requirement, that I have a collection of items (ObservableCollection), but I only want to display the first item. The requirement comes from the fact that in most of the case, the collection only contains one item. And due to the space limit, even if there is more than one items in the collection, we'd like to display the number of the items, details of the first one (same presentation as prior situation) and a ... symbol to indicate to the user that there is more items. And when the mouse is over the UI element a popup will eventually display all items.

The first solution I can think of (please suggest others if they are better) is to bind to this collection (but not using an ItemsControl) and define a DataTemplateSelector derived class (which return either the DataTemplate to display the only one item, or the DateTemplate which has the ... and the popup for more details, based on the number of items in the collection) and use it as ContentTemplateSelector.

But now my question: how both of my DataTemplate would look like in XAML, so that they can display only the first item in the collection? Obviously I can't have a ItemsControl.

UPDATE:

Now I have managed to make it work and agree this question can be closed (I can't delete it anymore since there is already some answers).

I actually knew how to bind to one certain item in the collection, but this was not where I am confused. I felt I should use ContentControl as one answer suggests. But I thought since I need to bind to the whole collection (not to single indexed item), and use a DataTemplateSelector to select the proper DataTemplate based on the number of items in the collection. The code would look like this:

<ContentControl Content="{Binding MyCollection}"
          ContentTemplateSelector="{StaticResource MyTemplateSelector}" />

And in MyTemplateSelector I wasn't sure how to use it since there is no reference to my collection because it is defined as resource and it doesn't have the information of MyCollection. However, it turned out to be very simple, the DataTemplate can refer to an indexed item without knowing the name or any other reference. Simply like this:

<DataTemplate>
   <TextBlock Text="{Binding [0].PropertyName}" />
<DataTemplate />

回答1:

To bind to just one item from a collection, you can use the following syntax:

{Binding Items[0]}

Or to bind to a property of a single item from the collection:

{Binding Items[0].Property}

You can find out more about property path syntax from the Binding.Path Property page at MSDN... from the linked page:

• Indexers of a property can be specified within square brackets following the property name where the indexer is applied. For instance, the clause Path=ShoppingCart[0] sets the binding to the index that corresponds to how your property's internal indexing handles the literal string "0". Multiple indexers are also supported.

回答2:

Try this

<ContentControl Content="{Binding YourCollection[0]}">
    <ContentControl.ContentTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Name}"></TextBlock>
        </DataTemplate>
    </ContentControl.ContentTemplate>
</ContentControl>

回答3:

Ok, late to the party but I thought I'd share my 2 cents anyway: I'd better go with a dumber (XAML-)view and a view-model closer to your presentation needs.

Translated: instead of mapping your existing view-model (or raw data) and its collection of items directly to the view, I suggest to map that to an appropriate view-model showing something like a YourItemViewModel FirstItem property and a bool HasMore property. That second view-model would be easily unit-testable to make sure it behaves propertly, and would be easily mapped to a view with less logic, so to avoid possible hard-to-test problems in view.