How do use setters in App.xaml to bind values to a custom class?

I have a custom class with custom properties that need to be retrieved. This works in the app.xaml:

<cols:List x:Key="SchemaModifications" x:TypeArguments="SAL:SchemaModification">
    <SAL:SchemaModification ApplicationVersion="7.7"  />
</cols:List>

but no matter what I try I can't seem to use a setter. Any tips?