how to debug `Null reference exception occured` in linq?

Question

string search = textBoxNachname.Text;
var Liste = _db.T_Subscribers
                .Where(x => x.firstname.StartsWith(search))
                .Except(_selectedcourse.T_Coursedetail.Select(b => b.T_Subscribers))
                .Where(M => M.T_Tln_Student == null || M.T_Tln_Stud.Status.T_Status.T_Statusart == _studentEx).ToList();

I have written the above piece of code to extract a list whose name starts with the Search element in textbox...then I need to exclude the names who have already enrolled for the course, then if they are not the students of the Institution (M => M.T_Tln_Student == null) and ex-students include in the list..

But I am getting Null reference exception occurred...

Have you tried sticking a breakpoint in the expressions to see what is null? Any answer to this could only make guesses... — Adam Houldsworth
– Adam Houldsworth, Commented Oct 8, 2013 at 13:25
you can't add breakpoints inside a linq query. The error you get means that 1 of your parameters is null. — Deblaton Jean-Philippe
– Deblaton Jean-Philippe, Commented Oct 8, 2013 at 13:26
@patxy Is that a linq to sql limitation? I do it with linq to objects frequently. — Adam Houldsworth
– Adam Houldsworth, Commented Oct 8, 2013 at 13:26
@patxy Ah, I guess you mean by clicking in the side-bar, which you are correct will highlight the entire row. I am not referring to this. Have you tried right clicking the expression content itself and doing "Breakpoint -> Insert Breakpoint"? I might have just made your life a little easier... I wish people would check the facts before up-voting incorrect things. — Adam Houldsworth
– Adam Houldsworth, Commented Oct 8, 2013 at 13:29
@patxy I cannot speak for Linq to SQL as I never use it, I only use Linq to Objects, hence my initial question for clarification. Personally I would break the expressions out into method groups that can be debugged a little more clearly. — Adam Houldsworth
– Adam Houldsworth, Commented Oct 8, 2013 at 13:31

G.Y · Accepted Answer · 2013-10-08 13:32:53Z

4

This is how you can debug this:

var Liste1 = _db.T_Subscribers.Where(x => x.firstname.StartsWith(search));
var Liste2 = Liste1.Except(
               _selectedcourse.T_Coursedetail.Select(b => b.T_Subscribers));
var Liste3 = Liste2.Where(M =>
                M.T_Tln_Student == null ||
                M.T_Tln_Stud.Status.T_Status.T_Statusart == _studentEx);
var Liste = Liste3.ToList();

The focus is to use this technique to split things.

answered Oct 8, 2013 at 13:32

G.Y

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Comments

Jens Kloster · Accepted Answer · 2013-10-08 13:35:54Z

0

look at this line:

.Where(M => M.T_Tln_Student == null || 
            M.T_Tln_Stud             // might be null
                        .Status      // might be null
                        .T_Status    // might be null
                        .T_Statusart // might be null
                           == _studentEx)

I would suggest that you start your search for the NullReferenceException here

.Where(M => M.T_Tln_Student == null || 
            M.T_Tln_Stud == null ||
            M.T_Tln_Stud.Status == null||
            M.T_Tln_Stud.Status.T_Status == null ||
            M.T_Tln_Stud.Status.T_Status.T_Statusart == null ||
            M.T_Tln_Stud.Status.T_Status.T_Statusart == _studentEx)

edited Oct 8, 2013 at 13:35

answered Oct 8, 2013 at 13:30

Jens Kloster

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