Compose LINQ-to-SQL predicates into a single predicate

Question

(An earlier question, Recursively (?) compose LINQ predicates into a single predicate, is similar to this but I actually asked the wrong question... the solution there satisfied

Answer 1

Normally you would chain invocations of .Where(...). E.g.:

var a = dataContext.Customers;
if (kwd1 != null)
    a = a.Where(t => t.Customer.Forenames.Contains(kwd1));
if (kwd2 != null)
    a = a.Where(t => t.Customer.Forenames.Contains(kwd2));
// ...
return a;

LINQ-to-SQL would weld it all back together into a single WHERE clause.

This doesn't work with OR, however. You could use unions and intersections, but I'm not sure whether LINQ-to-SQL (or SQL Server) is clever enough to fold it back to a single WHERE clause. OTOH, it won't matter if performance doesn't suffer. Anyway, it would look something like this:

 ff = null, ss = null;

foreach (k in keywords) {
    if (keywords != null) {
        var f = dataContext.Customers.Where(t => t.Customer.Forenames.Contains(k));
        ff = ff == null ? f : ff.Union(f);

        var s = dataContext.Customers.Where(t => t.Customer.Surname.Contains(k));
        ss = ss == null ? s : ss.Union(s);
    }
}
return ff.Intersect(ss);