Question

I’m using a Silverlight 4.0 project that utilizes MVVM and we have a combobox that resides on a view and has its values and selected value bound to an observable collection of Organizations and a SelectedOrganization respectively (both values exist on the viewmodel). With our project the page that this control is on can be hidden or shown. The first load everything looks great but when you go to a different control (hide the tab with the control and then go back to it) the value that is currently selected in the combo box looks like it’s blank, but when I debug, the selected value is still there.

The visual tree is getting recreated, but I have no idea why the combobox loses the text that should be in the box when the parent page is hidden and then re-shown. All other controls on the page behave correctly (autocompletetextbox, textblocks, textboxes, all of which have data bound to the viewmodel the same way).

Here’s how the combobox is declared:

<ComboBox 
    SelectedItem="{Binding SelectedOrganization, Mode=TwoWay}"
    ItemsSource="{Binding Organizations}"
    DisplayMemberPath="Name"
    Margin="5,0" 
    MinWidth="100" />

the Class for the organization is here:

[DataContract]
public class Organization
{
    [DataMember]
    public Guid OrganizationID { get; set; }

    [DataMember]
    public string Name { get; set; }
}

and the viewmodel has the following code for the bindings:

public Organization SelectedOrganization
{
    get { return (Organization)GetValue("SelectedOrganization"); }
    set 
    { 
        SetValue("SelectedOrganization", value); 
    }
}

public ObservableCollection<Organization> Organizations
{
    get { return (ObservableCollection<Organization>)GetValue("Organizations"); }
    set { SetValue("Organizations", value); }
} 

What do I need to do to keep the selected value when I switch parent pages?

Answer 1

The problem is that I declared the ItemsSource AFTER the SelectedItem. Apparently this is a bug in Silverlight 3 and 4. The answer was discussed here Silverlight Combobox and SelectedItem.