Dynamic MultiBinding?

In WPF I have a Canvas with child elements bound to data that render
as shapes.

I want the width of the Canvas bound to the maximum width of the
shapes inside it so that that it automatically resizes as items are
added or removed from the bound collection. This is one example but I
have other examples of where I want to bind to an aggregate of a
collection.

I tried using a multibinding with a converter that returns the maximum
width of the items in its Bindings collection but when I remove an
item it throws an exception saying that a multibinding can not be
changed once it is used.

Is there a recommended way of doing this?

Sorry. I got a bit carried away with MultiBindings there. I solved it
by creating a single binding to another ObservableCollection<Double>
and updating that. Obvious now