WPF binding to Listbox selectedItem

Question

Can anyone help with the following - been playing about with this but can t for the life of me get it to work   I ve got a view model which contains the following properties   public ObservableCollection lt Rule gt  Rules   get  set    public Rule SelectedRule   get  set      In my XAML I ve got    lt ListBox x Name  lbRules  ItemsSource   Binding Path Rules             SelectedItem   Binding Path SelectedRule  Mode TwoWay   gt   lt ListBox ItemTemplate gt       lt DataTemplate gt               lt StackPanel Orientation  Horizontal  gt                   lt TextBlock Text  Name     gt                   lt TextBox x Name  ruleName  gt                       lt TextBox Text gt                           lt Binding Path  Name  UpdateSourceTrigger  PropertyChanged    gt                       lt  TextBox Text gt                   lt  TextBox gt               lt  StackPanel gt       lt  DataTemplate gt   lt  ListBox ItemTemplate gt      Now the ItemsSource works fine and  I get a list of Rule objects with their names displayed in lbRules    Trouble I am having is binding the SelectedRule property to lbRules  SelectedItem  I tried binding a textblock s text property to SelectedRule but it is always null    lt TextBlock Text   Binding Path SelectedRule Name     gt    The error I m seeing in the output window is  BindingExpression path error   SelectedRule  property not found   Can anyone help me with this binding - I can t see why it shouldn t find the SelectedRule property   I then tried changing the textblock s text property as bellow  which works  Trouble is I want to use the SelectedRule in my ViewModel    lt TextBlock Text   Binding ElementName lbRules  Path SelectedItem Name     gt    Thanks very much for your help

User · Answer

For me  I usually use DataContext together in order to bind two-depth property such as this question    lt TextBlock DataContext   Binding SelectedRule   Text   Binding Name     gt   Or  I prefer to use ElementName because it achieves bindings only with view controls    lt TextBlock DataContext   Binding ElementName lbRules  Path SelectedItem   Text   Binding Name     gt

User · Answer

First off  you need to implement INotifyPropertyChanged interface in your view model and raise the PropertyChanged event in the setter of the Rule property  Otherwise no control that binds to the SelectedRule property will  know  when it has been changed   Then  your XAML   lt TextBlock Text   Binding Path SelectedRule Name     gt    is perfectly valid if this TextBlock is outside the ListBox s ItemTemplate and has the same DataContext as the ListBox

User · Answer

since you set your itemsource to your collection  your textbox is tied to each individual item in that collection  the selected item property is useful in this scenario if you were trying to do a master-detail form  having 2 listboxes  you would bind the second listbox s itemsource to the child collection of rules  in otherwords the selected item alerts outside controls that your source has changed  internal controls those inside your datatemplate already are aware of the change   and to answer your question yes in most circumstances setting the itemsource is the same as setting the datacontext of the control

User · Answer

Inside the DataTemplate you re working in the context of a Rule  that s why you cannot bind to SelectedRule Name -- there is no such property on a Rule  To bind to the original data context  which is your ViewModel  you can write    lt TextBlock Text   Binding ElementName lbRules  Path DataContext SelectedRule Name     gt    UPDATE  regarding the SelectedItem property binding  it looks perfectly valid  I tried the same on my machine and it works fine  Here is my full test app   XAML    lt Window x Class  TestWpfApplication ListBoxSelectedItem      xmlns  http   schemas microsoft com winfx 2006 xaml presentation      xmlns x  http   schemas microsoft com winfx 2006 xaml      Title  ListBoxSelectedItem  Height  300  Width  300      xmlns app  clr-namespace TestWpfApplication  gt       lt Window DataContext gt           lt app ListBoxSelectedItemViewModel  gt       lt  Window DataContext gt       lt ListBox ItemsSource   Binding Path Rules   SelectedItem   Binding Path SelectedRule  Mode TwoWay   gt           lt ListBox ItemTemplate gt               lt DataTemplate gt                   lt StackPanel Orientation  Horizontal  gt                       lt TextBlock Text  Name     gt                       lt TextBox Text   Binding Name    gt                   lt  StackPanel gt               lt  DataTemplate gt           lt  ListBox ItemTemplate gt       lt  ListBox gt   lt  Window gt    Code behind   namespace TestWpfApplication            lt summary gt          Interaction logic for ListBoxSelectedItem xaml          lt  summary gt      public partial class ListBoxSelectedItem   Window               public ListBoxSelectedItem                         InitializeComponent                          public class Rule               public string Name   get  set               public class ListBoxSelectedItemViewModel               public ListBoxSelectedItemViewModel                         Rules   new ObservableCollection lt Rule gt                                  new Rule     Name    Rule 1                    new Rule     Name    Rule 2                    new Rule     Name    Rule 3                                      public ObservableCollection lt Rule gt  Rules   get  private set             private Rule selectedRule          public Rule SelectedRule                       get   return selectedRule                set                               selectedRule   value

User · Answer

Yocoder is right   Inside the DataTemplate  your DataContext is set to the Rule its currently handling    To access the parents DataContext  you can also consider using a RelativeSource in your binding    lt TextBlock Text   Binding RelativeSource  RelativeSource FindAncestor  AncestorType  x Type     Your Parent control here        Path DataContext SelectedRule Name     gt    More info on RelativeSource can be found here   http   msdn microsoft com en-us library system windows data relativesource aspx

[wpf] WPF binding to Listbox selectedItem

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