Hi,

This code is a big hack that we don't like either, but it had no problems when we initially designed it for Unity, because it was only used when assemblies were unloaded and reloaded again.

I think the first thing to do to improve that code is avoid calling the static constructor if the type doesn't contain any DependencyProperty field.
And as you suggested in the Github issue, directly ignore all BCL types.

Keeping track of previously defined DependencyProperties is not an option because the code of the classes could have changed and new dependency properties added or removed.

What do you think?

avoid calling the static constructor if the type doesn't contain any DependencyProperty field.
It's definitely a better approach (if no other solutions are available) but there is still a chance that it may cause unexpected side effects.

because the code of the classes could have changed and new dependency properties added or removed.
Do you refer to Unity assembly hot-reloading? Does it actually able to expand existing Type objects with new properties? Or it creates new Type objects as they're coming from a new (rebuilt) assembly?

Yes, Unity unloads current assembly, compiles a new one with any changes you have made to your classes, and loads the new assembly.
So although the types have the same namespace+name, they are in fact new type objects on a new assembly (and old type objects don't exist anymore).

Ok, then It's the same as what we have for CryoFall live reloading—assemblies are just rebuilt and the data is re-serialized.

Then my original suggestion should work fine:

perhaps you should cache all the dependency properties in a Dictionary<Type, RegisteredDependencyProperty[]>. When RunClassConstructor notices that the type was once constructed, it can just foreach the RegisteredDependencyProperty[] array and force their registration.

As the old types are no longer used their instances will just lie down forever in the dictionary. New types will be registered as new entries in the dictionary, and have their static constructor called only once. And if you need to rebuild the type in RunClassConstructor, you will need to call re-registering of the previously registered dependency properties instead of force calling the static constructor. The only challenge here is to wrap all the dependency properties (when they're registered for the first time), so you can call their registration again when necessary for the specific type.

Further discussion in this ticket https://www.noesisengine.com/bugs/view.php?id=1897#bugnotes