When we want to return null in X++ and assign it to an CLR object, it will throw out an issue that "Microsoft Dynamics anytype cannot be marshaled to CLR Object", this is because null in X++ is anytype. So if we want to return null and assign it to an CLR Object, we should know the type of the CLR Object (refer which class). Then we can use following code:

returnCLRInterop:：Null("System.xxxx");