Question

I have a ComboBox which content might change. The issue is when the content is changed, 1) i lose the previous selectedIndex, and 2) SelectedIndex is set to -1.
I would like an MVVM solution, not some solution involving code-behind.
I can have the SelectedIndex set to 0 on content change, but -1 is not a valid value for my model.

What i did as a ‘trick’ was to have the setter of the property bound to SelectedIndex to set the value to 0 when the value is set to -1. So now i don’t have incorrect value in my model. But the ComboBox does not display the “0”th item, rather an empty box.

What could i do to have my ComboBox display the first item (index 0) when its content changes ?

  <ComboBox Name="WeeksComboBox"      IsEditable="False"
                                      ItemsSource="{Binding Weeks}"
                                      SelectedIndex="{Binding WeekNumber}"
                                      >

Weeks is a Notifying property that is a list of strings. It is changed when i change the Year property. WeekNumber is a notifying property of type integer. Now when it is set to -1, i set the corresponding private member to 0 to avoid wrong value in my model.

Answer 1

I always do that in the code-behind when I change the source

For example,

Weeks = GetNewWeeksCollection();
WeekNumber = 0;

An alternative is to use the PropertyChange notification

void MyViewModel_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == "Weeks")
        WeekNumber = 0;
}

Of course, that only works if you actually set Weeks to a new collection. If you simply clear it and reset it, you’ll have to use the CollectionChanged event

void Weeks_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
    if (e.Action == NotifyCollectionChangedAction.Reset)
        WeekNumber = 0;
}