WPF: ComboBox SelectedIndex = -1? - wpf

Question

I am using a MVVM Wizard with several pages. When I set a value in the combobox and go to the next page and switch back I want to reset the value I set before.
But all whats happening is that the combobox is empty at top and the index is -1 ?
What do I wrong?
<ComboBox ItemsSource="{Binding Path=LessonNumbers}" SelectedIndex="{Binding SelectedLessonNumber}" />
private ReadOnlyCollection<int> _lessonNumbers;
public ReadOnlyCollection<int> LessonNumbers
{
get
{
if (_lessonNumbers == null)
this.CreateLessonNumbers();
return _lessonNumbers;
}
}
private void CreateLessonNumbers()
{
var list = new List<int>();
for (int i = 1; i < 24; i++)
{
list.Add(i);
}
_lessonNumbers = new ReadOnlyCollection<int>(list);
}
private int _selectedLessonNumber;
public int SelectedLessonNumber
{
get { return _selectedLessonNumber; }
set
{
if (_selectedLessonNumber == value)
return;
_selectedLessonNumber = value;
this.OnPropertyChanged("SelectedLessonNumber");
}
}
UPDATE:
<ComboBox
SelectedIndex="0"
SelectedItem="{Binding SelectedWeeklyRotationNumber}"
ItemsSource="{Binding Path=WeeklyRotationNumbers}"
Height="23"
HorizontalAlignment="Left"
Margin="336,212,0,0"
VerticalAlignment="Top"
Width="121"
MaxDropDownHeight="100"
IsReadOnly="True"
IsTextSearchEnabled="False"
/>
private ReadOnlyCollection _weeklyRotationNumbers;
public ReadOnlyCollection WeeklyRotationNumbers
{
get
{
if (_weeklyRotationNumbers == null)
this.CreateWeeklyRotationNumbers();
return _weeklyRotationNumbers;
}
}
private void CreateWeeklyRotationNumbers()
{
var list = new List<string>();
list.Add("No rotation");
for (int i = 1; i < 16; i++)
list.Add(i.ToString());
_weeklyRotationNumbers = new ReadOnlyCollection<string>(list);
}
private string _selectedWeeklyRotationNumber;
public string SelectedWeeklyRotationNumber
{
get { return _selectedWeeklyRotationNumber; }
set
{
if (_selectedWeeklyRotationNumber == value)
return;
_selectedWeeklyRotationNumber = value;
this.RaisePropertyChanged("SelectedWeeklyRotationNumber");
Messenger.Default.Send<string>(value);
}
}
Again, what do I wrong or what is wrong with the string property?

Answer 1

Change XAML SelectedIndex to SelectedItem:
<ComboBox ItemsSource="{Binding Path=LessonNumbers}"
SelectedItem="{Binding SelectedLessonNumber}" />
UPDATE:
Somewhere you must set the DataContext of your Window to reference the collection from your XAML.
In my case I typically do that in the constructor of my view.
// this my class containing WeeklyRotationNumbers
private MainViewModel _mvm;
public MainView()
{
InitializeComponent();
_mvm = new MainViewModel();
DataContext = _mvm;
}
I added string to the read only collections:
private ReadOnlyCollection<string> _weeklyRotationNumbers;
public ReadOnlyCollection<string> WeeklyRotationNumbers
I also implemented the interface INotifyPropertyChanged which I think you did, but you are likely using
a different base class to handle the PropertyChanged event.
Everthing else I cut and paste from your code.