Dapper.net "where ... in" query doesn't work with PostgreSQL
PostgreSQL IN operator doesn't support array (or any other collection) as parameter, only a normal list (the one which you're generating with the ToInSql method), for PostgreSQL you need to use ANY operator, like this:
SELECT manufacturer, model, year, AVG(price) as averageprice, AVG(miles) as averagemiles, COUNT(*) as countFROM productsWHERE manufacturer = ANY(@manufacturers)AND model = ANY(@models)AND year = ANY(@years)GROUP BY manufacturer, model, year