Improve performance of loading images into ComboBox - wpf

Question

I am using WPF ComboBox to populate a list of countries as following:
Inside ComboBox I have a VirtualizingStackPanel that contains an image and a TextBlock:
<ComboBox.ItemTemplate>
<DataTemplate>
<VirtualizingStackPanel Orientation="Horizontal">
<Image Width="30" Height="30" Margin="0" Source="{Binding code, Converter={StaticResource ImageComboBoxConverter}}" VerticalAlignment="Center"/>
<TextBlock Margin="5" Text="{Binding country_Text}"/>
</VirtualizingStackPanel>
</DataTemplate>
</ComboBox.ItemTemplate>
I am using a simple converter for DataBinding to retrieve my flag files.
The problem is, the list takes around good 3-4 seconds to populate. Can I apply some kind of buffering or caching technique for images to load faster?

Answer 1

You need to virtualize the ComboBox.ItemsPanel, not the .ItemTemplate. The ItemsPanel is the panel used to display all controls, while the ItemTemplate is used to display each control.
All the Virtualized StackPanel does is not render items which are not visible. Instead, it only renders visible items (plus a few extras for a scroll buffer), and will replace the data behind each item as you scroll. So using it to display each individual item is useless, and you need to instead use it as the panel to display all items.
Your code should probably look something like this :
<ComboBox ItemsSource="{Binding Countries}">
<ComboBox.ItemsPanel>
<ItemsPanelTemplate>
<VirtualizingStackPanel />
</ItemsPanelTemplate>
</ComboBox.ItemsPanel>
<ComboBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<Image Width="30" Height="30" Margin="0" Source="{Binding code, Converter={StaticResource ImageComboBoxConverter}}" VerticalAlignment="Center"/>
<TextBlock Margin="5" Text="{Binding country_Text}"/>
</StackPanel >
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
You may also need to add one or all of these properties to the <ComboBox> tag too.
<ComboBox VirtualizingStackPanel.IsVirtualizing="True"
VirtualizingStackPanel.VirtualizationMode="Recycling"
ScrollViewer.CanContentScroll="True" ...>