Binding ListPicker.SelectedIndex problem

Question

I'm trying to do a two way binding of the SelectedIndex attribute of a ListPicker in a Windows Phone 7 UserControl.

It raises the following exception when I set the DataContext: SelectedIndex must always be set to a valid value.

This is the XAML code

<Grid x:Name="LayoutRoot">
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto"/>
        <RowDefinition Height="Auto"/>
    </Grid.RowDefinitions>

    <toolkit:ListPicker
        Grid.Row="0"
        x:Name="List1"
        SelectionChanged="Picker_SelectionChanged"
        SelectedIndex="{Binding PickerSelectedIndex, Mode=TwoWay}"
        ItemTemplate="{StaticResource PickerTemplate}"
        ItemsSource="{Binding MyList}"/>
</Grid>

And the code behind in DataContext

    private ObservableCollection<MyClass> myList = null;
    public ObservableCollection<MyClass> MyList
    {
        get { return this.myList; }
        set
        {
            if (value != this.myList)
            {
                this.myList= value;
                NotifyPropertyChanged("MyList");

                this.PickerSelectedIndex = 0;
            }
        }
    }

    private int pickerSelectedIndex = 0;
    public int PickerSelectedIndex
    {
        get
        {
            return this.pickerSelectedIndex;
        }
        set
        {
            this.pickerSelectedIndex= value;
        }
    }

Putting a breakpoint in PickerSelectedIndex.get I can see that it is returned correctly (0). I am sure that the problem is SelectedIndex="{Binding PickerSelectedIndex, Mode=TwoWay}" because deleting this line solves the problem, and I can see the ListPicker correctly loaded with the data from MyList.

I can't see where is the problem...

Answer 1

Moving SelectedIndex after ItemsSource solved the problem.

This is the working snippet

<toolkit:ListPicker
    Grid.Row="0"
    x:Name="List1"
    SelectionChanged="Picker_SelectionChanged"
    ItemTemplate="{StaticResource PickerTemplate}"
    ItemsSource="{Binding MyList}"
    SelectedIndex="{Binding PickerSelectedIndex, Mode=TwoWay}"/>

Anyone have an explanation for this?

Answer 2

My guess would be that the binding is being applied with a default value of zero when created and before the items exist. It's therefore trying to select the first item (with a zero index) before that item is created.

Try making sure that the ViewModel property of PickerSelectedIndex defaults to -1.
You may also want to delay setting the binding until the items are created.