wpf-rule-virtualization-patterns

name	wpf-rule-virtualization-patterns
description	WPF UI virtualization for large collections: VirtualizingStackPanel, recycling, pixel scroll.
user-invocable	false

Virtualization Patterns

UI virtualization rules for WPF controls that display large collections.

---

ItemsControl with VirtualizingStackPanel

Plain ItemsControl does not virtualize by default — it materializes every item at load. For large collections, replace the default panel with VirtualizingStackPanel and enable container recycling.

<!-- Wrong: no virtualization, all items materialized at once -->
<ItemsControl ItemsSource="{Binding LargeCollection}">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <StackPanel />
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
</ItemsControl>

<!-- Correct: VirtualizingStackPanel with recycling -->
<ItemsControl ItemsSource="{Binding LargeCollection}"
              VirtualizingPanel.IsVirtualizing="True"
              VirtualizingPanel.VirtualizationMode="Recycling"
              VirtualizingPanel.ScrollUnit="Pixel"
              ScrollViewer.CanContentScroll="True">
    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <VirtualizingStackPanel />
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>
</ItemsControl>

---

ListView / ListBox Attached Properties

ListView and ListBox use a VirtualizingStackPanel by default, but the optimal settings must still be set explicitly via attached properties:

<ListView ItemsSource="{Binding LargeCollection}"
          VirtualizingPanel.IsVirtualizing="True"
          VirtualizingPanel.VirtualizationMode="Recycling"
          VirtualizingPanel.ScrollUnit="Pixel"
          ScrollViewer.CanContentScroll="True">
    ...
</ListView>

---

Key Settings

Attached Property	Recommended Value	Effect
VirtualizingPanel.IsVirtualizing	True	Enable UI virtualization
VirtualizingPanel.VirtualizationMode	Recycling	Reuse containers instead of re-creating
VirtualizingPanel.ScrollUnit	Pixel	Smooth pixel scrolling (vs. item-by-item)
ScrollViewer.CanContentScroll	True	Required for panel-level virtualization

---

Notes

• VirtualizationMode="Recycling" is almost always preferred over Standard — it avoids GC pressure from container creation/destruction during scroll.
• ScrollUnit="Pixel" eliminates the "jumping" effect that occurs with item-based scrolling when items have variable height.
• Virtualization is disabled if the ScrollViewer wrapping the items panel has CanContentScroll="False" — always verify this setting.
• Do not place a VirtualizingStackPanel inside an ScrollViewer with unconstrained height — it will lose virtualization because all items become "visible".