WPF MultiBinding set fallback string for a null binding

Question

If I do not set the Date, the result becomes "price=2000, date=". Can I make it "price=2000, date=unknown" instead?

<TextBlock x:Name="Test">
    <TextBlock.Text>
        <MultiBinding StringFormat="{}price={0}, date={1:d}">
            <Binding Path="Price" />
            <Binding Path="Date" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>

public partial class MainWindow : Window
{
    public int Price { get; set; }
    public DateTime? Date { get; set; }
    public MainWindow()
    {
        InitializeComponent();
        Price = 2000;
        //Date = DateTime.Now;
        Test.DataContext = this;
    }
}

Answer 1

In this case, you can use the TargetNullValue Property of your binding:

<TextBlock x:Name="Test">
    <TextBlock.Resources>
        <local:DateConverter x:Key="DateConverter" />
    </TextBlock.Resources>
        
    <TextBlock.Text>
        <MultiBinding StringFormat="{}price={0}, date={1:d}">
            <Binding Path="Price" />
            <Binding Path="Date" TargetNullValue="unknown" />
        </MultiBinding>
    </TextBlock.Text>
</TextBlock>