WPF: SelectedValue always null

Question

In the following example SelectedValue of TabControl is always null. Why?

<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:s="clr-namespace:System;assembly=mscorlib">
<DockPanel>
 <TextBlock Text="{Binding SelectedValue, ElementName=Tabs}" DockPanel.Dock="Bottom"/>
 <TabControl x:Name="Tabs" SelectedValuePath="Content.SelectedItem">
  <TabItem Header="TabOne">
   <ListView>
    <ListView.View>
     <GridView>
      <GridViewColumn/>
     </GridView>
    </ListView.View>
    <s:String>ItemOne</s:String>
    <s:String>ItemTwo</s:String>
   </ListView>
  </TabItem>
  <TabItem Header="TabTwo">
   <ListView>
    <ListView.View>
     <GridView>
      <GridViewColumn/>
     </GridView>
    </ListView.View>
    <s:String>ItemOne</s:String>
    <s:String>ItemTwo</s:String>
   </ListView>
  </TabItem>
 </TabControl>
</DockPanel>
</Window>

Answer 1

Not sure what your trying to do, but:

Assuming that you want the name of the selected TabItem to show up in the TextBlock, it's because your SelectedValuePath is incorrect. Try changing your TabControl tag to:

<TabControl x:Name="Tabs" SelectedValuePath="Header">

Assuming that you're trying to get the string contents inside of the ListView, try changing your TextBox binding to:

<TextBlock Text="{Binding SelectedItem.Content.SelectedItem, ElementName=Tabs}" DockPanel.Dock="Bottom"/>

Answer 2

As micahtan points out in a comment, SelectedValue does update when you switch tabs. This means that TabControl does not monitor properties in SelectedValuePath for changes, only polls them every time its SelectedItem changes.