WPF How to set a maximum amount of results from CollectionViewSource

Question

I am trying to display a set of data using a DataTemplate in an ItemsControl, with the ItemsPanel set as a uniform grid of size 3 x 3. The ItemsSource of the ItemsControl is set to bind to a CollectionViewSource which filters the source collection based on a search term. This all works fine.

The list I am binding to is of an arbitrary size, but I only want 9 results to be displayed, but I can't for the life of me work out how to either:
a) limit the CollectionViewSource to output the first 9 items
b) limit the UniformPanel to only 3 x 3 and never to create new rows
c) limit the ItemsControl to only allow 9 data templates to be created at once.

I'm really scratching my head because I'm sure this is a common databinding scenario but I can't find anything on the web about it.

Conlan

Based on the answer of cwap, I have scrapped the CollectionViewSource entirely and just created a master list and a filtered list in the view model, the latter being updated when the search term changes. Using a converter is a good idea and probably more "pure" MVVM than including the maximum number of outputs in the VM (why I marked it as answer), but for now I'm happy with doing it in the VM. Cheers for the replies all. – Conlan

Answer 1

Two solutions I can think of:

If you are using View Model, put the logic in there to create a property that you can bind to that only ever has 9 elements. Make sure it's unit tested to ensure more than 9 elements never sneak into the collection.

Alternatively you could use a converter when binding the items source to convert the full list to a reduced list of the first 9.

Farny

It is worth pointing out that the converter approach prevents change events propagating. If you add or remove from your bound data source it will not be reflected. – Frederik

Answer 2

This is just one of those things you can't do in XAML (yet).

You could create a "CollectionViewSourceView" (:P), which will be updated whenever CollectionViewSource is updated, to only output the first 9 items. Still, this is going into the codebehind (or better, the viewmodel).

Cavanagh

Answer 3

I was about to say the same as James Hay (use a Converter), but I would also add that you can databind a ConverterParameter in the XAML to dynamically specify how many items are returned.

Kimon