ComboBox twoway binding with XElement

[cooksOrer]

I want to bind a ComboBox to a XElement from a XDocument. The twoway-Binding works on a Textbox but not on the ComboBox:xaml:\[code\]<Window x:Class="ComboBoxBindingTest.MainWindow" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="MainWindow" Height="350" Width="525"> <Grid> <ComboBox Height="23" HorizontalAlignment="Left" Margin="31,38,0,0" Name="comboBox1" VerticalAlignment="Top" Width="120" SelectedValue="http://stackoverflow.com/questions/11417684/{Binding Path=Element[step].Element[schema].Value}" /> <TextBox Height="23" HorizontalAlignment="Left" Margin="31,117,0,0" Name="textBox1" VerticalAlignment="Top" Width="120" Text="{Binding Path=Element[step].Element[schema].Value}"/> <TextBox Height="23" HorizontalAlignment="Left" Margin="31,214,0,0" Name="textBox2" VerticalAlignment="Top" Width="120" /> </Grid></Window>\[/code\]cs:\[code\]public partial class MainWindow : Window{ public MainWindow() { InitializeComponent(); comboBox1.Items.Add(new Tuple<string, string>("1", "Wert 1")); comboBox1.Items.Add(new Tuple<string, string>("2", "Wert 2")); comboBox1.Items.Add(new Tuple<string, string>("3", "Wert 3")); comboBox1.Items.Add(new Tuple<string, string>("4", "Wert 4")); comboBox1.Items.Add(new Tuple<string, string>("5", "Wert 5")); comboBox1.Items.Add(new Tuple<string, string>("6", "Wert 6")); comboBox1.Items.Add(new Tuple<string, string>("7", "Wert 7")); comboBox1.SelectedValuePath = "Item1"; comboBox1.DisplayMemberPath = "Item2"; XDocument doc = XDocument.Parse("<dir><step><schema>2</schema></step></dir>"); DataContext = doc.Element("dir"); }}\[/code\]Changing the text in the textbox to another number from 1 to 7 results in an update of the comboBox, but not vice versa. The Value in the XML is not changed when the comboBox selection is changed. What is wrong in the ComboBox Binding?Thanks