How to make an arbitrary type assignable directly in XAML…

[ZzZombo]

…like DateTime is? For instance, NodaTime requires to use a dedicated type to parse strings into its types, so I'd need to hook something up for them.

[maxkatz6]

You mean, directly assignable from a string?
You need to either implement Parse(string) (has to be done on NodaTime types directly, can be combined with IParsable interface) or add [TypeConverter] on the type or property of that type.

[ZzZombo]

So I'm so out of luck then.

[ZzZombo]

Oh, maybe I can create a custom type that can be implicitly converted to the target type and implements the correct members? Still, would make a huge refactoring.

[maxkatz6]

If you use these types in properties you own, you can set TypeConverter on them:

[TypeConverter(typeof(LocalTimeTypeConverter)]
public LocalTime Time { get; set; } // get/set omitted

And implement LocalTimeTypeConverter with runtime parsing from string, as you prefer.

XAML will pick up these converters and apply in code like:

<!-- string value will be parsed as LocalTime -->
<MyTimePicker Time="4:20" />

It likely won't work with bindings, as bindings generally expect exact types.