How can I get multibinding to work in XAML ListBox?

Question

The following shows me 3x "MultiTest.Model.Customers" in the ListBox (one for each record it should display).

What do I need to change in order for it to output the contents of the fields instead?

<Window.Resources>
    <Style TargetType="{x:Type ListBoxItem}">
        <Setter Property="ContentTemplate" >
            <Setter.Value>
                <MultiBinding StringFormat="{}{1}, {0} ">
                    <Binding Path="FirstName" />
                    <Binding Path="LastName"/>
                </MultiBinding>
            </Setter.Value>
        </Setter>
    </Style>
</Window.Resources>
<Grid>
    <ListBox x:Name="theCustomers"/>
</Grid>

binding in code-behind with ADO.NET Entity Framework:

MainEntities db = new MainEntities();
var customers = from c in db.CustomersSet
                select c;
theCustomers.ItemsSource = customers;

ANSWER:

Thanks, Steve, here is your answer in my Window.Resources format:

<Window.Resources>
    <Style TargetType="{x:Type ListBoxItem}">
        <Setter Property="ContentTemplate" >
            <Setter.Value>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal">
                        <TextBlock>
                            <TextBlock.Text>
                                <MultiBinding  StringFormat="{}{1}, {0} ({2})">
                                    <Binding Path="FirstName"/>
                                    <Binding Path="LastName"/>
                                    <Binding Path="ID"/>
                                </MultiBinding>
                            </TextBlock.Text>
                        </TextBlock>
                    </StackPanel>
                </DataTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</Window.Resources>
<Grid>
    <ListBox x:Name="theCustomers"/>
</Grid>

Answer 1

I've never used MultiBinding before. I have used, however, DataTemplates:

<ListBox.ItemTemplate>
    <DataTemplate>
        <StackPanel Orientation="Horizontal">
            <TextBlock Text="{Binding Name}"></TextBlock>
            <TextBlock Text=" - "/>
            <TextBlock Text="{Binding Email}" Margin="5,0"></TextBlock>
        </StackPanel>
    </DataTemplate>
</ListBox.ItemTemplate>

Hope this helps!

Answer 2

If you particularly want to use MultiBinding you should be able to use a DataTemplate with StringFormat.. something like:

<ListBox.ItemTemplate>
    <DataTemplate>
        <StackPanel Orientation="Horizontal">
            <TextBlock>
                <TextBlock.Text>
                    <MultiBinding  StringFormat="{}{1}, {0}">
                        <Binding Path="FirstName"/>
                        <Binding Path="LastName"/>
                    </MultiBinding>
                </TextBlock.Text>
            </TextBlock>
       </StackPanel>
    </DataTemplate>
</ListBox.ItemTemplate>

Or for something more complicated you could use a ValueConverter (or the multiple binding variant).