Multi selection TreeView in WPF... the easy way.

<Style TargetType="{x:Type TreeViewItem}">
<Setter Property="BorderThickness" Value="1"></Setter>
<Style.Triggers>
<Trigger  Property="c:TreeViewExtensions.IsSelected"  Value="True" >
 <Setter Property="BorderBrush" Value="Gray"></Setter>
</Trigger>
</Style.Triggers>
</Style>

...

<TreeView c:TreeViewExtensions.EnableMultiSelect="true"
c:TreeViewExtensions.SelectedItems="{Binding SelectedItems}"
ItemsSource="{Binding Items}"
... />