Select the "old value" in a listbox in a edit page - silverlight

Question

In my app I have a Save / Edit page.
The current flow is the following: the user has a main page, with a list of elements. He can click on the "add" button, it goes to an "Add" page, in which he can enter information and store it. Once he does it, the information is saved and shown in the list.
If he clicks in the list, he moves to an "Edit" page, in which he can change the information.
In reality, the Add and the Edit page are the same, the second has the fields populated while the first hasn't.
I have 3 listboxes in this page, one for severities, one for categories and one for reporter. This information is selected in the listbox before saving, and in the edit phase, it should be selected automatically, so the user knows the "old" value.
To select the values automatically, I tried two approaches:
1-In my xaml:
<ListBox Height="103" Name="lbSeverities" Width="439" HorizontalAlignment="Left" Margin="20,0,0,0" SelectionMode="Single" ItemsSource="{Binding Severities}" DisplayMemberPath="Name" SelectedItem="{Binding Task.Severity}"/>
And I also override the Equals method of the Severity Class to a reasonable implementation.
2- In my xaml
<ListBox Height="103" Name="lbSeverities" Width="439" HorizontalAlignment="Left" Margin="20,0,0,0" SelectionMode="Single" ItemsSource="{Binding Severities}" DisplayMemberPath="Name" SelectedIndex="{Binding Task.Severity, Converter={StaticResource SeverityToIndexConverter}}"/>
And I created the SeverityToIndexConverter with this code:
public class SeverityToIndexConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if (value != null && value is Severity)
{
Severity currentSeverity = (Severity)value;
for (int i = 0; i < (App.Current as App).MainViewModel.Severities.Count; i++)
{
Severity sev = (App.Current as App).MainViewModel.Severities[i];
if (currentSeverity.ID == sev.ID)
return i;
}
}
return -1;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
Both of them presented the same results: sometimes the values are automatically selected, but sometimes not. It is very unstable when it is selected.
I imagined about some exception, tried to catch it, but I do not get anything.
I also tried to debug, I noticed that in case 1, the equals method is called in parallel for all members of the collection, so I tried the second approach. Debugging didn't lead me to any answer.
Has anyone faced a similar situation?
What can I do to make the Listbox value to be selected when the user enters in the "Edit" Page?
Thanks,
Oscar

Answer 1

I do the same thing. My xaml looks like your first example.
Here's what I'd do:
Create a new property called Severity. In the getter return Task.Severity. Bind the listbox SelectedItem to the new property.
It may also be a timing thing so you may have to call NotifyPropertyChanged on the new property once the listbox has loaded its list.