is_valid_property_value() should also be implementable by GDScript/GDExtension. I suggest taking a look at #94047 for inspiration.

@chocola-mint
I am not familiar with GDExtension. Should I define a private _is_valid_property_value() for child class?

Something like this?

bool Resource::is_valid_property_value(const String &p_path, const Variant &p_value, String &p_error_message) const
  return _is_valid_property_value(); 
}

bool Resource::_is_valid_prooperty_value(const String &p_path, const Variant &p_value, String &p_error_message) const
  return true;
}

@xuhuisheng

I would write it like this:

bool Resource::is_valid_property_value(const String &p_path, const Variant &p_value, String &p_error_message) const {
  if (script_instance) {
    bool valid;
    String ret = script_instance->is_valid_property_value(&valid);
    if (valid) {
      return ret;
    }
  }
  if (_extension && _extension->is_valid_property_value) {
    bool ret;
    GDExtensionBool is_valid;
    _extension->is_valid_property_value(_extension_instance, &is_valid, &ret);
    if (is_valid) {
      return ret;
    }
  }
  return _is_valid_property_value(); 
}

bool Resource::_is_valid_property_value(const String &p_path, const Variant &p_value, String &p_error_message) const
  return true;
}

You'll need to declare a function pointer called GDExtensionClassIsValidPropertyValue here. Something like: typedef void (*GDExtensionClassIsValidPropertyValue)(GDExtensionClassInstancePtr p_instance, GDExtensionBool *r_is_valid, GDExtensionBool* p_out);

Then, add a placeholder function with the same signature in PlaceholderExtensionInstance. Something like this:

static void placeholder_instance_is_valid_property_value(GDExtensionClassInstancePtr p_instance, GDExtensionBool *r_is_valid, GDExtensionBool *p_out) {
  *r_is_valid = true;
}

You'll then need to add GDExtensionClassIsValidPropertyValue is_valid_property_value_func to GDExtensionClassCreationInfo4. (I think you only need to do it for the non-deprecated version but I'm not sure)

And then, add GDExtensionClassIsValidProeprtyValue is_valid_property_value to ObjectGDExtension.

You'll also need to assign is_valid_property_value here:

And also assign the placeholder &PlaceholderExtensionInstance::placeholder_instance_is_valid_property_value here.

Take a look at how to_string() is implemented (GDExtensionClassToString) and it shouldn't be too hard to figure out where to go next from there.