LINQ Left Join Error

Question

I have 2 Generic Lists called Students and Entries They are in 1 to 0..1 relationship. Students 1:0..1 Entries

I want to get the list of Student Name, ID and English, Maths detail from Entries if exists.

My LINQ query is as below and I got the null reference error at se.English. Could you pls help? Thanks.

var query = from s in students
            join t in entries
            on s.StudentDetailID equals t.StudentDetailID into StudentEntries
            from se in StudentEntries.DefaultIfEmpty()                                    
            select new { s.StudentDetailID, s.LastName, s.FirstName,
            se.English};

Maryam Arshi · Accepted Answer · 2013-04-24 13:09:54Z

1

As you said you have a 1-0..1 relationship so it is possible that se.english will be null(because you used an left outer join), you should handle it in your query, I assumed it it would be String and put "" if it is null, You can handle it as you wish.

var query = from s in students
            join t in entries
            on s.StudentDetailID equals t.StudentDetailID into StudentEntries
            from se in StudentEntries.DefaultIfEmpty()                                    
            select new { s.StudentDetailID, s.LastName, s.FirstName,
            English = se.English?? ""};

edited Apr 24, 2013 at 13:09

answered Apr 24, 2013 at 12:57

Maryam Arshi

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Cyril Gandon · Accepted Answer · 2013-04-24 12:56:29Z

0

se could be null, because you assigned it StudentEntries.DefaultIfEmpty(). And Default of a class is null. So you can't call se.English without testing it for null before:

var query = from s in students
        join t in entries
        on s.StudentDetailID equals t.StudentDetailID into StudentEntries
        from se in StudentEntries.DefaultIfEmpty()                                    
        select new { s.StudentDetailID, s.LastName, s.FirstName,
        se == null ? null : se.English};

answered Apr 24, 2013 at 12:56

Cyril Gandon

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