<ListBox x:Name="listbox" ItemsSource="{Binding StringList}" SelectionMode="Multiple"/>
<Button Command="{Binding GetListItemsCommand}" CommandParameter="{Binding SelectedItems, ElementName=listbox}" Content="GetSelectedListBoxItems"/>

xmlns:i="http://schemas.microsoft.com/expression//2010/interactivity"
xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions"

<GridView x:Name="GridName">
<i:Interaction.Triggers>
   <i:EventTrigger EventName="SelectionChanged">
       <i:InvokeCommandAction Command="{Binding Datacontext.SelectionChangedCommand, ElementName=YourUserControlName}" CommandParameter="{Binding SelectedItems, ElementName=GridName}" />
    </i:EventTrigger>
</i:Interaction.Triggers>

public DelegateCommand<object> SelectionChangedCommand {get;set;}

SelectionChangedCommand = new DelegateCommand<object> (items => {
   var itemList = (items as ObservableCollection<object>).Cast<YourDto>().ToList();
}

// Your goes heres

// Your goes heres