<TreeView
        Grid.Row="0"
        Name="tvTopics"
        VerticalAlignment="Stretch"
        HorizontalAlignment="Stretch"
        MouseDoubleClick="tvTopics_MouseDoubleClick"
        ItemsSource="{Binding TierOneItems}"
        SelectedItemChanged="treeView1_SelectedItemChanged">
        <TreeView.ItemContainerStyle>
            <Style TargetType="{x:Type TreeViewItem}">
                <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
                <Setter Property="IsSelected" Value="{Binding Topic.IsSelected, Mode=TwoWay}" />
                <Setter Property="FontWeight" Value="Normal" />
                <Style.Triggers>
                    <Trigger Property="IsSelected" Value="True">
                        <Setter Property="FontWeight" Value="Bold" />
                    </Trigger>
                    <Trigger Property="IsExpanded" Value="True">
                        <Setter Property="IsSelected" Value="True" />
                    </Trigger>
                </Style.Triggers>
            </Style>
        </TreeView.ItemContainerStyle>
        <TreeView.ItemTemplate>
            <HierarchicalDataTemplate ItemsSource="{Binding Children}">
                <TextBlock Text="{Binding Name}" />
            </HierarchicalDataTemplate>
        </TreeView.ItemTemplate>
    </TreeView>

private void tvTopics_MouseDoubleClick(object sender, MouseButtonEventArgs e)
    {

        TreeView tv = sender as TreeView;

        if (tv.SelectedItem is TopicTreeItemViewModel)
        {
           Model.SelectedTopic = ((TopicTreeItemViewModel)tv.SelectedItem).Topic;

        }
     }

public class TopicTreeViewModel : NotificationObject, ITopicTreeViewModel
    {

        [ImportingConstructor]
        public TopicTreeViewModel(IGatewayService storyService, IEventAggregator eventAggregator)
        {
            this.storyService = storyService;
            this.eventAggregator = eventAggregator;

            this.AddTopicCommand = new DelegateCommand<object>(this.AddTopic);

            Helper.SubscriptionTokenList_LocationSearch.Add(this.eventAggregator.GetEvent<LocationSearchEvent>().Subscribe(OnLocationSearch, ThreadOption.UIThread));
            Helper.SubscriptionTokenList_SubjectSearch.Add(this.eventAggregator.GetEvent<SubjectSearchEvent>().Subscribe(OnSubjectSearch, ThreadOption.UIThread));

        }
        public void MouseDoubleClick(Topic topic)
        {
            if (topic != null && topic is Topic)
            {
                switch (this.searchType)
                {
                    case SearchType.Location:
                        this.eventAggregator.GetEvent<AddLocationEvent>().Publish((Topic)topic);
                        break;
                    case SearchType.Subject:
                        this.eventAggregator.GetEvent<AddSubjectEvent>().Publish((Topic)topic);
                        break;
                }
            }
        }

public interface ITopicTreeViewModel
{
    ReadOnlyCollection<TopicTreeItemViewModel> TierOneItems { get; }

    ICommand SearchCommand { get; }

    string SearchText { get; set; }

    Topic SelectedTopic { get; set; }
}