R – Silverlight MVVM ListBoxItem IsSelected

Bind IsSelected to a property in your data class. When the property is changed, execute the logic to update the IsSelected property in other data objects:

<ListBox>
    <ListBox.ItemContainerStyle>
        <Style TargetType="ListBoxItem">
            <Setter Property="IsSelected" Value="{Binding IsSelected}"/>
        </Style>
    </ListBox.ItemContainerStyle>
</ListBox>

Then in your data class you can have something like this:

public bool IsSelected
{
    get { return _isSelected; }
    set
    {
        if (_isSelected != value)
        {
            _isSelected = value;
            OnPropertyChanged("IsSelected");

            UpdateOtherItems();
        }
    }
}

Or you could have the data item raise an IsSelectedChanged event and have the owning class manage the interdependencies of the selection.

It looks like when the binding havent executed when execution reached ItemsSource="{Binding Path=Contacts, Mode=TwoWay}". Can you verify whether you instantiating the collection in the constructor of the ViewModel, because I feel at the binding time the Contacts instance not set(null)