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C# - Using An ItemsPanel In WPF [Beginner]


One of the most useful things about WPF is how flexible all of the built in controls are. Don't like how the tabs look on a tab control? Restyle them! Need to have images inside your combo box? Sure, no problem! This flexibility extends deep into a number of controls, and today we are going to take a look at controlling layout in an ItemsControl by setting the ItemsPanel.

An ItemsControl? An ItemsPanel? What are those? Well, an ItemsControl "represents a control that can be used to present a collection of items" (according to MSDN). Things like ListBox, ComboBox, and Toolbar all derive from ItemsControl. The ItemsPanel is the the panel used to layout the items in an ItemsControl, and can be anything that derives from the Panel class, even custom panels that you write. The default panel used in an ItemsControl depends on the control - for instance, the ListBox uses a VirtualizingStackPanel.

So the reason that I first looked into ItemsPanel was actually really simple - I wanted to make a ListBox that scrolled horizontally instead of vertically. I initially expected this to just be some sort of property on the ListBox itself, but that is not actually the case. You actually need to change the ItemsPanel of the ListBox to a VirtualizingStackPanel that has the orientation set to Horizontal.

So, while having this code:
<ListBox>
  <Image Source="Images\Aquarium.jpg" Width="100"/>
  <Image Source="Images\Ascent.jpg" Width="50"/>
  <Image Source="Images\Autumn.jpg" Width="200"/>
  <Image Source="Images\Crystal.jpg" Width="75"/>
  <Image Source="Images\DaVinci.jpg" Width="125"/>
  <Image Source="Images\Follow.jpg" Width="100"/>
  <Image Source="Images\Friend.jpg" Width="50"/>
  <Image Source="Images\Home.jpg" Width="150"/>
  <Image Source="Images\Moon flower.jpg" Width="100"/>
</ListBox> 
 
Gives you the standard vertical look:

Standard List Box with images 

To get it horizontal, like this:

Horizontal ListBox

You need to add an ItemsPanel tag:
<ListBox>
  <ListBox.ItemsPanel>
    <ItemsPanelTemplate>
      <VirtualizingStackPanel Orientation="Horizontal" />
    </ItemsPanelTemplate>
  </ListBox.ItemsPanel>
  <Image Source="Images\Aquarium.jpg" Width="100"/>
  <Image Source="Images\Ascent.jpg" Width="50"/>
  <Image Source="Images\Autumn.jpg" Width="200"/>
  <Image Source="Images\Crystal.jpg" Width="75"/>
  <Image Source="Images\DaVinci.jpg" Width="125"/>
  <Image Source="Images\Follow.jpg" Width="100"/>
  <Image Source="Images\Friend.jpg" Width="50"/>
  <Image Source="Images\Home.jpg" Width="150"/>
  <Image Source="Images\Moon flower.jpg" Width="100"/>
</ListBox> 
 
Not too terrible, but it is a little wordy. At first I was disappointed at having to write that much just to change a simple orientation property, but then I realized what kind of power this gives you over ItemsControls. Say, for instance, I wanted to use that Animated Wrap Panel as the ItemsPanel inside of a ListBox. Well, it really couldn't be any easier:
<ListBox ScrollViewer.HorizontalScrollBarVisibility="Disabled">
  <ListBox.ItemsPanel>
    <ItemsPanelTemplate>
      <ARP:AnimatedWrapPanel />
    </ItemsPanelTemplate>
  </ListBox.ItemsPanel>
  <Image Source="Images\Aquarium.jpg" Width="100"/>
  <Image Source="Images\Ascent.jpg" Width="50"/>
  <Image Source="Images\Autumn.jpg" Width="200"/>
  <Image Source="Images\Crystal.jpg" Width="75"/>
  <Image Source="Images\DaVinci.jpg" Width="125"/>
  <Image Source="Images\Follow.jpg" Width="100"/>
  <Image Source="Images\Friend.jpg" Width="50"/>
  <Image Source="Images\Home.jpg" Width="150"/>
  <Image Source="Images\Moon flower.jpg" Width="100"/>
</ListBox>
 
Since the AnimatedWrapPanel derives from Panel, I can just drop it in there, and it works. The only other thing that had to be changed is the HorizontalScrollBarVisibility on the list box, and that is because for the AnimatedWrapPanel to work right, the container can't be allowed to scroll horizontally. So with that set, the ListBox ends up looking like this:

List Box with Animated Wrap Panel

Of course, the static picture doesn't really show off the animated nature of the AnimatedWrapPanel, but oh well.

So you can also set the ItemsPanel for an ItemsControl in C# code, although it is even wordier than the XAML. Below is the code to replicate the first example, making a list box list items horizontally:
FrameworkElementFactory factory = 
    new FrameworkElementFactory(typeof(VirtualizingStackPanel))

factory.SetValue(VirtualizingStackPanel.OrientationProperty, Orientation.Horizontal)

specialListBox.ItemsPanel = new ItemsPanelTemplate(factory);
 
The annoying thing here is that you end up having to create a factory - which is happening behind the scenes in the XAML code too, you just didn't have to explicitly write it out. Well, that is it for how to use an ItemsPanel in WPF.
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