Avoiding null exception in EF that is thrown by your query

Question

I have a query like this :

        result =
            firstIdeaRepository.FindBy(
                i => i.FirstIdeaState == FirstIdeaState && i.Date >= start && i.Date <= end)
                               .AsEnumerable()
                               .Select(j => new RptListOfCompanyBasedOnFirstIdeaState()
                                   {
                                       Name =
                                           companyRepository.FindBy(i => i.UserId == j.UserId)
                                                            .FirstOrDefault()
                                       DateOfMeeting =
                                           calenderRepository.ConvertToPersianToShow(
                                               meetingReposiotry.FindBy(s => s.FirstIdeaId == j.Id)
                                                                .FirstOrDefault()
                                                                .Date),
                                       DateOfExit =
                                           calenderRepository.ConvertToPersianToShow(j.DateOfExit.Value),
                                       ReasonOfExit = j.ReasonOfExit,
                                   }).ToList();

    return result;

As you can see i use FirstOrDefault() and j.DateOfExit.Value and sometimes my Date doesn't have any values or sometime my other variables are null too because i use firstordefaut() like

companyRepository.FindBy(i => i.UserId == j.UserId).FirstOrDefault().

So my query throws a null exception and the result can't be created ,how can i handle this exception and for example if the .NET detects the null value ignores it by default or uses a default values for that ?

Best regards.

You would use .DefaultIfEmpty() in the call-chain before .FirstOrDefault(). — Martin Costello
– Martin Costello, Commented Oct 9, 2014 at 16:04
@martin_costello: .DefaultIfEmpty() won't help for reference types, the default for which is null. — Mrchief
– Mrchief, Commented Oct 9, 2014 at 16:05
Where exactly are the NullReferenceExceptions being thrown? — Andrew Whitaker
– Andrew Whitaker, Commented Oct 9, 2014 at 16:07
meetingReposiotry.FindBy(s => s.FirstIdeaId == j.Id) and date.value — Ehsan Akbar
– Ehsan Akbar, Commented Oct 9, 2014 at 16:07

Andrew Whitaker · Accepted Answer · 2014-10-09 16:15:29Z

I would make the following changes:

result =
    firstIdeaRepository.FindBy(
        i => i.FirstIdeaState == FirstIdeaState && i.Date >= start && i.Date <= end)
            .AsEnumerable()
            .Select(j => new RptListOfCompanyBasedOnFirstIdeaState()
            {
                Name =
                    companyRepository.FindBy(i => i.UserId == j.UserId)
                        .FirstOrDefault()
                DateOfMeeting =
                    callenderRepository.ConvertToPersianToShow(
                        meetingReposiotry.FindBy(s => s.FirstIdeaId == j.Id)
                            // project a new sequence first, before calling `FirstOrDefault`:
                            .Select(s => s.Date)
                            .FirstOrDefault(),
                DateOfExit =
                    j.DateOfExit.HasValue ? 
                        callenderRepository.ConvertToPersianToShow(j.DateOfExit.Value) :
                        null,                       
                   ReasonOfExit = j.ReasonOfExit,
               }).ToList();

When you use FirstOrDefault, there's a possibility that you'll get null back (in the case of reference types), and so you need to plan for that in your code.

For example, when assigning DateOfMeeting, you could project the results (using .Select) before using .FirstOrDefault, so that you're not ever accessing the Date property on what could be a null value.

As for DateOfExit, I've used the conditional operator to determine whether to call the calendarRepository's method at all. This assumes that DateOfExit is nullable.

Unrelated: "Calendar" is spelled with one "l" and not two.

Mrchief · Accepted Answer · 2014-10-09 16:14:19Z

1

Since you're using a nullable date, you can try filtering by values that have date, something like:

.FindBy(s => s.FirstIdeaId == j.Id && s.Date.HasValue)

This will ensure that you don't get any records with null date.

As I mentioned in comments, other cases need to be handled on case-by-case basis. Judging by the code you've shown, maybe you can handle Name as:

 Name = companyRepository.FindBy(i => i.UserId == j.UserId).FirstOrDefault() ?? "anonymous";

and so on.

Another example:

If you do want to get the record even if DateOfMeeting is null, then add a check for HasValue in subsequent part or default it to some date:

 DateOfExit = j.DateOfExit.HasValue ? 
              callenderRepository.ConvertToPersianToShow(j.DateOfExit.Value)
              : (DateTime)null,   // you need to make `DateOfExit` nullable and then handle that downstream

// or (default with current date)
 DateOfExit = j.DateOfExit.HasValue ?
              callenderRepository.ConvertToPersianToShow(j.DateOfExit.Value) 
              : callenderRepository.ConvertToPersianToShow(DateTime.Now),

// or (default with empty date)
 DateOfExit = j.DateOfExit.HasValue ?
              callenderRepository.ConvertToPersianToShow(j.DateOfExit.Value) 
              : callenderRepository.ConvertToPersianToShow(new DateTime()),

Moral of the story: figure out what the default value should be in case of null and then substitute that accordingly in the query when calling FirstOrDefault().

edited Oct 9, 2014 at 16:14

answered Oct 9, 2014 at 16:02

Mrchief

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2 Comments

Ehsan Akbar Over a year ago

So what about other null exception ? like companyRepository.FindBy(i => i.UserId == j.UserId) .FirstOrDefault()

Mrchief Over a year ago

I was going to add that to my answer. So basically the others have to be taken up on a case-by-case basis. you have to decide how you want to handle null values in those cases. E.g., for Name, you substitute it with a default value (empty string or Noname) so that you don't get null reference exceptions when you access them.

psaxton · Accepted Answer · 2014-10-09 16:18:48Z

0

The broadest solution would be to use the idea of an null object with the DefaultIfEmpty<T>(T DefaultValue) method in your query. An example would be:

var defaultMeeting = new Meeting() { Date = new DateTime() };

var dateOfMeeting = meetingRepository.FindBy(s => s.FirstIdeaId == j.Id)
        .DefaultIfEmpty(defaultMeeting)
        .FirstOrDefault()
        .Date;

edited Oct 9, 2014 at 16:18

answered Oct 9, 2014 at 16:11

psaxton

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