Binding a WPF ComboBox to a custom list

Question

I have a ComboBox that doesn t seem to update the SelectedItem SelectedValue  The ComboBox ItemsSource is bound to a property on a ViewModel class that lists a bunch of RAS phonebook entries as a CollectionView  Then I ve bound  at separate times  both the SelectedItem or SelectedValue to another property of the ViewModel  I have added a MessageBox into the save command to debug the values set by the databinding  but the SelectedItem SelectedValue binding is not being set  The ViewModel class looks something like this  public ConnectionViewModel       private readonly CollectionView  phonebookEntries      private string  phonebookeEntry       public CollectionView PhonebookEntries               get   return  phonebookEntries               public string PhonebookEntry               get   return  phonebookEntry            set                       if   phonebookEntry    value  return               phonebookEntry   value              OnPropertyChanged  quot PhonebookEntry quot                       The  phonebookEntries collection is being initialised in the constructor from a business object  The ComboBox XAML looks something like this   lt ComboBox ItemsSource  quot  Binding Path PhonebookEntries  quot      DisplayMemberPath  quot Name quot      SelectedValuePath  quot Name quot      SelectedValue  quot  Binding Path PhonebookEntry  quot    gt   I am only interested in the actual string value displayed in the ComboBox  not any other properties of the object as this is the value I need to pass across to RAS when I want to make the VPN connection  hence DisplayMemberPath and SelectedValuePath are both the Name property of the ConnectionViewModel  The ComboBox is in a DataTemplate applied to an ItemsControl on a Window whose DataContext  has been set to a ViewModel instance  The ComboBox displays the list of items correctly  and I can select one in the UI with no problem  However when I display the message box from the command  the PhonebookEntry property still has the initial value in it  not the selected value from the ComboBox  Other TextBox instances are updating fine and displaying in the MessageBox  What am I missing with databinding the ComboBox  I ve done a lot of searching and can t seem to find anything that I m doing wrong   This is the behaviour I m seeing  however it s not working for some reason in my particular context  I have a MainWindowViewModel which has a CollectionView of ConnectionViewModels  In the MainWindowView xaml file code-behind  I set the DataContext to the MainWindowViewModel  The MainWindowView xaml has an ItemsControl bound to the collection of ConnectionViewModels  I have a DataTemplate that holds the ComboBox as well as some other TextBoxes  The TextBoxes are bound directly to properties of the ConnectionViewModel using Text  quot  Binding Path ConnectionName  quot   public class ConnectionViewModel   ViewModelBase       public string Name   get  set        public string Password   get  set       public class MainWindowViewModel   ViewModelBase          List lt ConnectionViewModel gt         public CollectionView Connections   get  set       The XAML code-behind  public partial class Window1       public Window1                 InitializeComponent            DataContext   new MainWindowViewModel             Then XAML   lt DataTemplate x Key  quot listTemplate quot  gt       lt Grid gt           lt ComboBox ItemsSource  quot  Binding Path PhonebookEntries  quot              DisplayMemberPath  quot Name quot              SelectedValuePath  quot Name quot              SelectedValue  quot  Binding Path PhonebookEntry  quot    gt           lt TextBox Text  quot  Binding Path Password  quot    gt       lt  Grid gt   lt  DataTemplate gt    lt ItemsControl ItemsSource  quot  Binding Path Connections  quot      ItemTemplate  quot  StaticResource listTemplate  quot    gt   The TextBoxes all bind correctly  and data moves between them and the ViewModel with no trouble  It s only the ComboBox that isn t working  You are correct in your assumption regarding the PhonebookEntry class  The assumption I am making is that the DataContext used by my DataTemplate is automatically set through the binding hierarchy  so that I don t have to explicitly set it for each item in the ItemsControl  That would seem a bit silly to me   Here is a test implementation that demonstrates the problem  based on the example above  XAML   lt Window x Class  quot WpfApplication7 Window1 quot      xmlns  quot http   schemas microsoft com winfx 2006 xaml presentation quot      xmlns x  quot http   schemas microsoft com winfx 2006 xaml quot      Title  quot Window1 quot  Height  quot 300 quot  Width  quot 300 quot  gt       lt Window Resources gt           lt DataTemplate x Key  quot itemTemplate quot  gt               lt StackPanel Orientation  quot Horizontal quot  gt                   lt TextBox Text  quot  Binding Path Name  quot  Width  quot 50 quot    gt                   lt ComboBox ItemsSource  quot  Binding Path PhonebookEntries  quot                      DisplayMemberPath  quot Name quot                      SelectedValuePath  quot Name quot                      SelectedValue  quot  Binding Path PhonebookEntry  quot                      Width  quot 200 quot   gt               lt  StackPanel gt           lt  DataTemplate gt       lt  Window Resources gt       lt Grid gt           lt ItemsControl ItemsSource  quot  Binding Path Connections  quot              ItemTemplate  quot  StaticResource itemTemplate  quot    gt       lt  Grid gt   lt  Window gt   The code-behind  namespace WpfApplication7            lt summary gt          Interaction logic for Window1 xaml          lt  summary gt      public partial class Window1   Window               public Window1                         InitializeComponent                DataContext   new MainWindowViewModel                         public class PhoneBookEntry               public string Name   get  set            public PhoneBookEntry string name                        Name   name                       public class ConnectionViewModel   INotifyPropertyChanged                private string  name           public ConnectionViewModel string name                         name   name              IList lt PhoneBookEntry gt  list   new List lt PhoneBookEntry gt                                                                                                  new PhoneBookEntry  quot test quot                                                     new PhoneBookEntry  quot test2 quot                                                                phonebookEntries   new CollectionView list                     private readonly CollectionView  phonebookEntries          private string  phonebookEntry           public CollectionView PhonebookEntries                       get   return  phonebookEntries                       public string PhonebookEntry                       get   return  phonebookEntry                set                               if   phonebookEntry    value  return                   phonebookEntry   value                  OnPropertyChanged  quot PhonebookEntry quot                                     public string Name                       get   return  name                set                               if   name    value  return                   name   value                  OnPropertyChanged  quot Name quot                                    private void OnPropertyChanged string propertyName                        if  PropertyChanged    null                  PropertyChanged this  new PropertyChangedEventArgs propertyName                      public event PropertyChangedEventHandler PropertyChanged             public class MainWindowViewModel               private readonly CollectionView  connections           public MainWindowViewModel                         IList lt ConnectionViewModel gt  connections   new List lt ConnectionViewModel gt                                                                                                                            new ConnectionViewModel  quot First quot                                                                  new ConnectionViewModel  quot Second quot                                                                  new ConnectionViewModel  quot Third quot                                                                             connections   new CollectionView connections                      public CollectionView Connections                       get   return  connections                       If you run that example  you will get the behaviour I m talking about  The TextBox updates its binding fine when you edit it  but the ComboBox does not  Very confusing seeing as really the only thing I ve done is introduce a parent ViewModel  I am currently labouring under the impression that an item bound to the child of a DataContext has that child as its DataContext  I can t find any documentation that clears this up one way or the other  I e   Window - gt  DataContext   MainWindowViewModel    Items - gt  Bound to DataContext PhonebookEntries      Item - gt  DataContext   PhonebookEntry  implicitly associated  I don t know if that explains my assumption any better      To confirm my assumption  change the binding of the TextBox to be  lt TextBox Text  quot  Binding Mode OneWay  quot  Width  quot 50 quot    gt   And this will show the TextBox binding root  which I m comparing to the DataContext  is the ConnectionViewModel instance

User · Answer

To bind the data to ComboBox List lt ComboData gt  ListData   new List lt ComboData gt     ListData Add new ComboData   Id    quot 1 quot   Value    quot One quot      ListData Add new ComboData   Id    quot 2 quot   Value    quot Two quot      ListData Add new ComboData   Id    quot 3 quot   Value    quot Three quot      ListData Add new ComboData   Id    quot 4 quot   Value    quot Four quot      ListData Add new ComboData   Id    quot 5 quot   Value    quot Five quot       cbotest ItemsSource   ListData  cbotest DisplayMemberPath    quot Value quot   cbotest SelectedValuePath    quot Id quot    cbotest SelectedValue    quot 2 quot    ComboData looks like  public class ComboData      public int Id   get  set       public string Value   get  set         note that Id and Value have to be properties  not class fields

User · Answer

I had what at first seemed to be an identical problem  but it turned out to be due to an NHibernate WPF compatibility issue  The problem was caused by the way WPF checks for object equality  I was able to get my stuff to work by using the object ID property in the SelectedValue and SelectedValuePath properties    lt ComboBox Name  CategoryList            DisplayMemberPath  CategoryName            SelectedItem   Binding Path CategoryParent             SelectedValue   Binding Path CategoryParent ID             SelectedValuePath  ID  gt    See the blog post from Chester  The WPF ComboBox - SelectedItem  SelectedValue  and SelectedValuePath with NHibernate  for details

User · Answer

I had a similar issue where the SelectedItem never got updated   My problem was that the selected item was not the same instance as the item contained in the list  So I simply had to override the Equals   method in my MyCustomObject and compare the IDs of those two instances to tell the ComboBox that it s the same object   public override bool Equals object obj        return this Id     obj as MyCustomObject  Id

User · Answer

You set the DisplayMemberPath and the SelectedValuePath to  Name   so I assume that you have a class PhoneBookEntry with a public property Name   Have you set the DataContext to your ConnectionViewModel object   I copied you code and made some minor modifications  and it seems to work fine  I can set the viewmodels PhoneBookEnty property and the selected item in the combobox changes  and I can change the selected item in the combobox and the view models PhoneBookEntry property is set correctly   Here is my XAML content    lt Window x Class  WpfApplication6 Window1          xmlns  http   schemas microsoft com winfx 2006 xaml presentation          xmlns x  http   schemas microsoft com winfx 2006 xaml          Title  Window1  Height  300  Width  300  gt   lt Grid gt       lt StackPanel gt           lt Button Click  Button Click  gt asdf lt  Button gt           lt ComboBox ItemsSource   Binding Path PhonebookEntries                     DisplayMemberPath  Name                    SelectedValuePath  Name                    SelectedValue   Binding Path PhonebookEntry     gt       lt  StackPanel gt   lt  Grid gt   lt  Window gt    And here is my code-behind   namespace WpfApplication6             lt summary gt          Interaction logic for Window1 xaml          lt  summary gt      public partial class Window1   Window               public Window1                         InitializeComponent                ConnectionViewModel vm   new ConnectionViewModel                DataContext   vm                     private void Button Click object sender  RoutedEventArgs e                          ConnectionViewModel DataContext  PhonebookEntry    test                        public class PhoneBookEntry               public string Name   get  set             public PhoneBookEntry string name                        Name   name                     public override string ToString                         return Name                       public class ConnectionViewModel   INotifyPropertyChanged               public ConnectionViewModel                         IList lt PhoneBookEntry gt  list   new List lt PhoneBookEntry gt                 list Add new PhoneBookEntry  test                 list Add new PhoneBookEntry  test2                  phonebookEntries   new CollectionView list                      private readonly CollectionView  phonebookEntries          private string  phonebookEntry           public CollectionView PhonebookEntries                       get   return  phonebookEntries                       public string PhonebookEntry                       get   return  phonebookEntry                set                               if   phonebookEntry    value  return                   phonebookEntry   value                  OnPropertyChanged  PhonebookEntry                                     private void OnPropertyChanged string propertyName                        if  PropertyChanged    null                  PropertyChanged this  new PropertyChangedEventArgs propertyName                      public event PropertyChangedEventHandler PropertyChanged            Edit  Geoffs second example does not seem to work  which seems a bit odd to me  If I change the PhonebookEntries property on the ConnectionViewModel to be of type ReadOnlyCollection  the TwoWay binding of the SelectedValue property on the combobox works fine   Maybe there is an issue with the CollectionView  I noticed a warning in the output console      System Windows Data Warning  50   Using CollectionView directly is not fully supported  The basic features work  although with some inefficiencies  but advanced features may encounter known bugs  Consider using a derived class to avoid these problems    Edit2   NET 4 5   The content of the DropDownList can be based on ToString   and not of DisplayMemberPath  while DisplayMemberPath specifies the member for the selected and displayed item only

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