handle null query c# when equals null, Object reference not set to an instance of an object

Question

I have a problem when converting sql to query c# about left join here is sql query

select mTeacher.Id as Id, mTeacher.Name as Name, mStudents.Name as Addresses
from Teachers mTeacher
 left join Students mStudents
    on mStudents.TeacherId=mTeacher.Id
    where mStudents.Name = 'some of word'

here is the image

and here is i converted to sql to query c#

 var zzz= from mTeacher in repo.Teachers
                      join mStudents in repo.mStudents on mTeacher.Id equals mStudents.TeacherId into a
                      from y1 in a.DefaultIfEmpty()
                      where mTeacher.Name.Equals("someofword") or mStudent.Name.Equals("somofword")
                      select new { mTeacher.Id,mTeacher.Name};

there will be student will have null value mStudent.Name.Equals("somofword") i got something like this how to handle this

Possible duplicate of [LEFT OUTER JOIN in LINQ](https://stackoverflow.com/questions/3404975/left-outer-join-in-linq) — peeyush singh, Oct 07 '19 at 05:24
not duplicate i think my case is different, i use where in my case but the another no — Dzarrah Developer, Oct 07 '19 at 05:37
You can try your queries without where clause, that may help uncover the problem. — peeyush singh, Oct 07 '19 at 05:44
Maybe you should explain what you are trying to do, the above query should work fine (except that in C# case you are not checking for nulls in the left join), which is explained in the linked answer. Also please do not try to explain your problem as a part of comment, it is confusing whether you just typed something random or is this another issue you are having. — peeyush singh, Oct 07 '19 at 06:43
sorry it's my fault the result is same actually now i want to make this to be query c# where mStudents.Name = 'some of word' as you know that is null value then it will return object not reference instance if it's null — Dzarrah Developer, Oct 07 '19 at 06:56

score 0 · Accepted Answer · answered Oct 07 '19 at 08:11

If I go by the SQL query, your where clause is only looking up Student.Name.

This has to be not null (as per you where query). So I would suggest that you use simply an inner join. This will solve your C# conversion issue as well.

If you still want to retain the check, then you can modify your where clause to:

where mTeacher.Name.Equals("someofword") or (mStudent != null && mStudent.Name.Equals("somofword"))

The null check will make sure that you do not run into the object is null error (remember in case of left join the student value can be null).

Also add a similar null check in your select clause, if you are referring to values from student table.

 select new { mTeacher.Id,mTeacher.Name, StudentName = ( y1 == null ) ? "No Student" : y1.Name};

handle null query c# when equals null, Object reference not set to an instance of an object

1 Answers1