首先，使用SelectMany展平列表，然后使用FirstOrDefault进行过滤:

First, use SelectMany to flatten the list, then FirstOrDefault to filter:

CountryList.SelectMany(c => c.stateList).FirstOrDefault(d => d.Name == passedInValue);

请注意，由于FirstOrDefault可以使用谓词，因此实际上不需要Where子句.

Note that because FirstOrDefault can take a predicate, you don't actually need the Where clause.