How to make DockPanel fill available space

Bind the Width of the DockPanel to the ActualWidth of the ListBoxItem:

<DockPanel Width="{Binding ActualWidth, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}}}">...

Another option: you could just redefine the ItemContainerStyle so that the ListBoxItem is stretched horizontally:

<ListBox.ItemContainerStyle>    <Style TargetType="ListBoxItem">        <Setter Property="HorizontalContentAlignment" Value="Stretch"/>    </Style></ListBox.ItemContainerStyle>

The nice thing about dock panels is they already fill all the available space. LastChildFill is true by default (but I set it below for clarity), so just don't set the DockPanel attribute on the last child, and it will fill the available space.

<DockPanel HorizontalAlignment="Stretch" LastChildFill="true">    <TextBlock DockPanel.Dock="Left"               Text="{Binding Path=Name}"               FontSize="10"               Foreground="Black" />    <TextBlock                Text="{Binding Path=Price, StringFormat=\{0:C\}}"               FontSize="10"               Foreground="Black" /></DockPanel>

DockPanels are evil. Temptation to use StackPanel/DockPanel combination for complex layouts leads to "layout dead ends". Use a Grid:

<Grid>  <TextBlock HorizontalAlignment="Left"...  <TextBlock HorizontalAlignment="Right".../Grid>

I use Grids almost exclusively, using a separate grid for each block of elements that "belong together"