Treeview selected item

Hi, I have a TreeView (C_Tree) with a 2 levels of items (TVIs collection of TVI). TVI is a class with Selected, which allow me to find selected item in a tree. Situation is I can read this but I cant set it. When I brows my TreeView and click it or use a keyboard, I can identify selected item, but, as soon as I set it in a code like TVIs(0).Selected=True nothing happens. Why?

Dim TVIs As New ObservableCollection(Of TVI)
C_Tree.ItemsSource = TVIs


Public Class TVI
Implements INotifyPropertyChanged

Private tvi_selected As Boolean
Public Property Selected() As Boolean
Get
Return tvi_selected
End Get
Set(value As Boolean)
tvi_selected = value
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs("tvi_selected"))
End Set
End Property

Public Event PropertyChanged(ByVal sender As Object, ByVal e As System.ComponentModel.PropertyChangedEventArgs) Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged
End Class
<Style TargetType="TreeViewItem">
<Setter Property="IsSelected" Value="{Binding Selected}"/>
</Style>

Aleksey