Question

I’m beginning to climb the walls here. I’ve filled a combo box with a list of items, and them I’m trying to find out which has been selected. I was also trying to set the combox box to auto selected the 0th item in the list. But that won’t work either, I’m getting a binding expression path error on both GroupIndex and SelectedGroup. However I can’t figure out what the problem is.

Combo Box Xaml:

<ComboBox     
Name="GroupComboBox"
SelectedIndex="{Binding Path=GroupIndex}"
SelectedItem="{Binding Path=SelectedGroup}"
DisplayMemberPath="Name"
SelectedValuePath ="Name"
ItemsSource="{Binding Path=Groups}"
Height="38"
HorizontalAlignment="Left"
Margin="159,115,0,0"
VerticalAlignment="Top"
Width="185"
FontSize="24" Text="Select a Group" IsEditable="False" IsReadOnly="False" />

Here is the code for filling the combo box.

    void webService_GroupListChanged(string response)
    {
        JavaScriptSerializer serializer = new JavaScriptSerializer();

        IList<JSONGroup> listOfGroups = new List<JSONGroup>();

        listOfGroups = serializer.Deserialize<List<JSONGroup>>(response);

        Application.Current.Dispatcher.BeginInvoke((Action)(() =>
        {
            Groups = new ObservableCollection<Group>();

            foreach(JSONGroup group in listOfGroups)
            {
                Groups.Add(new Group(group.name, group.id));
            }

        }));
    }

    private int groupIndex;
    private int GroupIndex
    {
        get { return this.groupIndex; }
        set
        {
            if (this.groupIndex != value)
            {
                this.groupIndex = value;
                this.OnPropertyChanged("GroupIndex");
            }
        }
    }


    private Group selectedGroup;
    private Group SelectedGroup
    {
        get { return this.selectedGroup; }
        set
        {
            if (this.selectedGroup != value)
            {
                this.selectedGroup = value;
                this.OnPropertyChanged("SelectedGroup");
            }
        }
    }


    private ObservableCollection<Group> groups;
    public ObservableCollection<Group> Groups
    {
        get { return this.groups; }
        set
        {
            if (this.groups != value)
            {
                this.groups = value;
                this.OnPropertyChanged("Groups");
            }
        }
    }

Answer 1

1. For use in a Binding, your GroupIndex and SelectedGroup properties must be public.
2. Don’t set SelectedItem and SelectedIndex at the same time.
3. If you prefer to set SelectedItem, that exact item (SelectedGroup) has to be contained in Groups.