如何对“列表”框中的项目求和

我有两个ListBox, Source和Destination。目标ListBox有用户选择的Items。

如何对目标ListBox中的项目进行求和?

目的ListBox有两种数据类型String (Description)和decimal (Cost)。我只想总结一下成本。下面是XAML代码

<ListBox Height="237" HorizontalAlignment="Left" Margin="44,191,0,0" Name="lstDestination" VerticalAlignment="Top" Width="264" Grid.ColumnSpan="2" ItemsSource="{Binding Source={StaticResource myItemList}}">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <StackPanel Orientation="Vertical">
                <DockPanel >
                   <TextBlock FontWeight="Bold" Text="Item:"  DockPanel.Dock="Left" Margin="5,0,10,0"/>
                   <TextBlock Text="{Binding Path=Resource}" Foreground="Green" FontWeight="Bold" />
                   <TextBlock FontWeight="Bold" Text="Cost:"  DockPanel.Dock="Left" Margin="5,0,10,0"/>
                  <TextBlock Text="{Binding Path=Cost}" FontWeight="Bold" />
                </DockPanel>
            </StackPanel>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

这是我试图总结的代码:

private decimal SumDLstBox()
{
    decimal totalCost;
    foreach (var i in lstDestination.SelectedItems)
    {
        totalCost+=i.Cost; //Error is thrown here             
    }
    return totalCost;
}
//Data Source to List Box Source
 var items = from r in dc.IResources select r;
        lstSource.ItemsSource = items;

用户选择了Items s/他需要的，我的挑战是获得所选项目的总数(已移动到ListBox目的地)

private decimal SumDLstBox()
{
    return lstDestination.SelectedItems.Cast<Foo>().Sum(f => f.Cost);
}