public class FancyMultiBinding : MultiBinding

{

private static Logger logger = LogManager.GetCurrentClassLogger();

private static Binding CreateFirstBinding()
{
    Binding bind = new Binding();

    bind.RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor, typeof(Window), 1);
    bind.Path = new PropertyPath("Resources[FooBar]");

    return bind;
}

public FancyMultiBinding()
{
    // If this is specified in XAML, don't make the developer re-type 
    // the converter and first binding every time. Apply them by default. 

    this.Converter = new FancyMultiConverter();
    this.Bindings.Add(CreateFirstBinding());
}
}
<TextBlock FontSize="40" Foreground="Pink">

<TextBlock.Text>

<local:FancyMultiBinding ConverterParameter="TESTING">

<Binding Path="Width" RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type Window}}"/>

<Binding Path="Height" RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type Window}}"/>

</local:FancyMultiBinding>

</TextBlock.Text>

</TextBlock>

When I set a breakpoint in FancyMultiConverter.Convert, the array 'values' has the Width and Height values, but not the string result of the FooBar resource.

When I implement the same thing in WPF, the array of 'values' in the converter has all three binding results: The one applied in the constructor, plus the two specified in XAML.

I do have a workaround for this, but it's not pretty. I apply the default first binding in the constructor, which works if it's the only binding, and in ProvideValue, I check if it's still there, and re-apply it if it was erased by the list in XAML.

public class FancyMultiBinding : MultiBinding

{

private static Logger logger = LogManager.GetCurrentClassLogger();

private static Binding CreateFirstBinding()
{
    Binding bind = new Binding();

    bind.RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor, typeof(Window), 1);
    bind.Path = new PropertyPath("Resources[FooBar]");

    return bind;
}

Binding defaultFirstBinding;

public FancyMultiBinding()
{
    // If this is specified in XAML, don't make the developer re-type 
    // the converter and first binding every time. Apply them by default. 

    this.Converter = new FancyMultiConverter();

    defaultFirstBinding = CreateFirstBinding();
    this.Bindings.Add(defaultFirstBinding);
}

public override object ProvideValue(IServiceProvider serviceProvider)
{
    if (Bindings[0] != defaultFirstBinding)
    {
        Bindings.Insert(0, defaultFirstBinding);
    }

    return base.ProvideValue(serviceProvider);
}
}