WPF ComboBox databinding ItemsSource item.ToString() to property setter. Why

comboboxdata-bindingmvvmwpf

For some reason when I bind a ComboBox to a list of POCOs, the property which is bound to SelectedValue is set twice:

With value = POCO.ToString()
With value = POCO.Key property, which is the intended behaviour

I have the following ComboBox that is bound to properties in my ViewModel:

<ComboBox ItemsSource="{Binding Path=AllowedClassifications}" 
          DisplayMemberPath="Value" 
          SelectedValue="{Binding Path=TargetGroup.Classification}" 
          SelectedValuePath="Key" />

The properties in ViewModel are defined as:

//ICollection is implemented by ObservableCollection<T>
//DataBaseFieldValue has two public properties: string Key, string Value
public ICollection<DatabaseFieldValue> AllowedClassifications
{
    get { return _allowedClassifications; }
    private set { _allowedClassifications = value; }
}

public Model.TargetGroup TargetGroup
{
    get { return _targetGroup; }
    private set { _targetGroup = value; OnPropertyChanged("TargetGroup"); }
}

TargetGroup.Classification is defined as:

public string Classification
{
    get { return _classification; }
    set 
    {
        System.Diagnostics.Debug.WriteLine("Classification: " + value);
        _classification = value; 
        OnPropertyChanged("Classification"); 
    }
}

Debug output:

Classification:
MyNamespace.DatabaseFieldValue

Classification: 2

What's happening here? Am I doing this completely wrong?

Best Answer

Everything looks OK in your code, except for the fact that according to your XAML the property which is bound to SelectedValue should be set to the POCO.Key value rather than POCO.Value (as you wrote you expected). I have just created a test project with similar setup and everything works.

Alternatively, you could try using SelectedItem property of combobox in combination with ItemTemplate:

<ComboBox ItemsSource="{Binding Path=AllowedClassifications}"    
          SelectedItem="{Binding Path=TargetGroup.Classification}" >
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Value}"/>
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

In this case the TargetGroup.Classification property must be of type DatabaseFieldValue.

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C# – Binding a WPF ComboBox to a custom list

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:

<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">
<Grid>
    <StackPanel>
        <Button Click="Button_Click">asdf</Button>
        <ComboBox ItemsSource="{Binding Path=PhonebookEntries}"
                  DisplayMemberPath="Name"
                  SelectedValuePath="Name"
                  SelectedValue="{Binding Path=PhonebookEntry}" />
    </StackPanel>
</Grid>
</Window>

And here is my code-behind:

namespace WpfApplication6
{

    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    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<PhoneBookEntry> list = new List<PhoneBookEntry>();
            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.

Best Answer

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