With LINQ to Entities, it'd look like this (the
context
variable would be an instance of your entity model):
var cars = (
from car in context.vehicles
where
car.make == makeComboBox.SelectedItem &&
car.model == modelComboBox.SelectedItem &&
car.model_year == yearComboBox.SelectedItem &&
car.engine_type == engineComboBox.SelectedItem
select new
{
transmission_type = car.transmission_type
}).Distinct().ToList();
I've not played with LINQ to SQL, but I imagine it's very similar to LINQ to Entities.