Question

I have this LINQ query:

var resourcePlanningInWeek = resourcePlanning.Where(rp => rp.PlanDate >= dateFrom && rp.PlanDate <= dateTo);
var holidays = new HolidayManager().GetByPeriod(dateFrom, dateTo);
var resourcePlanningExcludedHolidays= resourcePlanningInWeek.Where(rpiw => ( holidays.Where(h => h.HolidayDate = rpiw.PlanDate).Count = 0))

When executed, I get following error:

Cannot implicitly convert type ‘DateTime’ to ‘bool’

Somewone know why?

Answer 1

1. You need to use == instead of = when you want to make a comparison. That’s relevant at two places in the last line.
2. The call to Count is missing the parentheses.

var resourcePlanningExcludedHolidays = resourcePlanningInWeek
    .Where(rpiw => holidays.Where(h => h.HolidayDate == rpiw.PlanDate)
                           .Count() == 0);

However, there is a better way of writing this:

var resourcePlanningExcludedHolidays = resourcePlanningInWeek
    .Where(rpiw => !holidays.Any(h => h.HolidayDate == rpiw.PlanDate));

This is better, because:

1. It is shorter and more readable
2. It stops enumerating holidays as soon as the condition is true the first time. Count() always enumerates the complete list.

An even better approach would be to use a HashSet<DateTime>:

var holidays
    = new HashSet<DateTime>(new HolidayManager().GetByPeriod(dateFrom, dateTo)
                                                .Select(x => x.HolidayDate));

var resourcePlanningExcludedHolidays
    = resourcePlanningInWeek.Where(rpiw => !holidays.Contains(rpiw.PlanDate));