Null Object Reference Exception

Question

I have these codes that displays the first student record and first group record in my database in the Index page:

var studentToAdd = db.Students.FirstOrDefault();
var selectedGroup = db.Groups.FirstOrDefault();
ViewBag.Name = studentToAdd.Firstname;
ViewBag.Group = selectedGroup.GroupName;

It works and it displays "Richard" and "Group1" in my index page. But when I add this code that should add "Richard" to "Group1" I get a null object exception :

selectedGroup.Students.Add(studentToAdd);

How do i fix this? thanks

Nikhil Agrawal · Accepted Answer · 2013-06-09 11:27:47Z

3

When you try to add, at that point selectedGroup.Students property is null.

Do this

if (selectedGroup.Students == null)
    selectedGroup.Students = new List<Student>(); // If its a List

selectedGroup.Students.Add(studentToAdd);

answered Jun 9, 2013 at 11:27

Nikhil Agrawal

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CodeCaster · Accepted Answer · 2013-06-09 11:28:06Z

1

Your query var selectedGroup = db.Groups.FirstOrDefault() returns a Group object whose Students property is null, I guess. You can find the variable that holds the null reference by setting a breakpoint and debugging your code.

The solution depends on what techniques you use and what your Groups class looks like (amongst others lazy loading on virtual properties). The constructor of the Groups class can also initialize an empty Students collection, to which you can then add entities with .Add().

answered Jun 9, 2013 at 11:28

CodeCaster

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4 Comments

ta.speot.is Over a year ago

You can find the variable that holds the null reference by ... debugging your code OP debugging their code? What a novel idea..

CodeCaster Over a year ago

@ta.speot.is I agree, it's much easier to put the code online and let others do that for you.

WannaCSharp Over a year ago

Thanks. But believe it or not I tried my best debugging my code before I posted this question. Maybe I'm just not debugging the right way.

CodeCaster Over a year ago

@WannaCSharp how did you resolve it? And no problem, I gave you a hint how to debug this the next time. :-)

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