Order by null first and then in descending order

Question

I have LINQ Script where group by key possibly null. I got the result correctly and now I want to order by first null and then in descending order. because the group by key can be null so I cannot check as .hasValue!

In following approach I am getting null exception as it seems I am not handling null in order correctly?

var v25 = (from transaction in filteredTransactions
       join schedule in schedules on
       new { siteId = transaction.LoginSiteID, startDate = transaction.LoginDateTime.Date, payrollNumber = transaction.PayrollNumber } equals
       new { siteId = schedule.SiteId, startDate = schedule.StartTime.Value.Date, payrollNumber = schedule.PayrollNumber }
          into ezi_s_t
       from scheduleTransactions in ezi_s_t.DefaultIfEmpty()
       group transaction by scheduleTransactions into groupedScheduleTransactions
       select new
       {
           Schedule = groupedScheduleTransactions.Key,
           Transactions = groupedScheduleTransactions.ToList()
       }
     )
     .OrderBy(x=> x.Schedule == null? null : x.Schedule.EziScheduleId)
     .ToList();

Is [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/q/4660142/10352740) helpful? — nalka, Mar 22 '21 at 11:08
To prevent comparing `null` with other types, simply assign Key some small value of same type as `EziScheduleId` so it will be first when ordering by it — Tomas Chabada, Mar 22 '21 at 11:10

Ashkan Mobayen Khiabani · Accepted Answer · 2021-03-22T11:25:44.743

4

You need to OrderByDescending() when it is null and then use ThenByDescending()

.OrderByDescending(x=> x.Schedule == null)
.ThenByDescending(x => x.Schedule?.EziScheduleId);

And about your comment, the reason you are getting System.NullReferenceException error is because if Schedule is null, you cannot access its properties (as there is none) so you need to change x.Schedule.EziScheduleId to x.Schedule?.EziScheduleId

Live Demo

edited Mar 22 '21 at 11:25

answered Mar 22 '21 at 11:08

Ashkan Mobayen Khiabani

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Suggested edit: EziScheduleId should be used to search in `Ascending` order, so `ThenBy` should be used – Tomas Chabada Mar 22 '21 at 11:12
@TomasChabada but the OP says "**and then in descending order**", however I was just presenting the solution, that one OP can decide – Ashkan Mobayen Khiabani Mar 22 '21 at 11:15
you are right, I got confused by reading OP's code – Tomas Chabada Mar 22 '21 at 11:19
Yes you are right. actually after you said it, I took a look at the code, and changed my answer, and then I remembered it was mentioned in the question. – Ashkan Mobayen Khiabani Mar 22 '21 at 11:21
I am still getting error System.NullReferenceException: 'Object reference not set to an instance of an object.' <>f__AnonymousType22<j__TPar, j__TPar>.Schedule.get returned null. – K.Z Mar 22 '21 at 11:22

Order by null first and then in descending order

1 Answers1