When does ‘trotz’ govern the dative case?

Question

It is said that trotz historically went well with both the dative and the genitive cases, while in contemporary usage the genitive becomes the norm and the dative survives only in idiomatic ‘frozen’ expressions. My difficulty is with those special cases when the dative case is used instead of the genitive. I have no idea when the dative must be used.

It seems to me that the so-called idiomatic expressions are a bunch of combinations of pronouns and trotz, such as trotzdem, trotz allem/alledem.
Could I conclude that when trotz goes with a pronoun, the dative case should be used instead of the genitive while with a noun, the genitive should be used?

If my claim is wrong, could anyone provide some tip on when the dative case must be used instead of the genitive with trotz?

Answer 1

Canoo.net sums it up nicely. The general rule is: Use genitive. Then the following exceptions apply:

• Since genitive plural is unmarked, always use dative plural if there is no article and there are no adjectives;
• Always use (uninflected) dative singular for single nouns without adjectives or article;
• You may use dative if the noun has no article, only adjectives;
• You may always use dative colloquially;
• The forms trotzdem, trotz Allem and trotz alledem require their dative case.

In the end, it turns out that there seem to be more exceptions than rules, however, it does still seem to be more concise writing it this way rather than reversed (use dative unless …).