c# - wpf mvvm treeview contexmenu -

i'm learning wpf/mvvm , got stuck on following... code below working

<listbox x:name="listbox" dockpanel.dock="top" itemssource="{binding items}" displaymemberpath="name" >   <listbox.contextmenu>       <contextmenu datacontext="{binding path=placementtarget, relativesource={relativesource self}}">          <menuitem header="show selected"           command="{binding path=datacontext.showselectedcommand}"          commandparameter="{binding path=selecteditems}"/>       </contextmenu>   </listbox.contextmenu> </listbox> 

but when i've replaced listbox on treeview, such

<treeview x:name="tview"  dockpanel.dock="top"  displaymemberpath="name" tag="{binding datacontext, relativesource={relativesource self}}" itemssource="{binding items}" >                 <treeview.contextmenu>                     <contextmenu  datacontext="{binding placementtarget.tag, relativesource={relativesource self}}" staysopen="true">                         <menuitem header="show selected" command="{binding datacontext.showselectedcommand}"                                   commandparameter="{binding path=selecteditems}">                              </menuitem>                          </contextmenu>                 </treeview.contextmenu>             </treeview> 

the showselectedcommand doesn't invoked. please explain what's wrong there , how can make work. lot.