Null reference exception on navigation property

Question

Can anybody suggest the cause of the following problem? My code is breaking on the line

integerList.Integers.Add(integer);

with the error message

Object reference not set to an instance of an object.

IntegerList.Integers is null which I suspect is the cause but what is the solution? Do I have to set IntegerList.Integers values when I initialize the variable so it is never null?

Models

public class IntegerList { public int IntegerListID { get; set; } public string Direction { get; set; } public long Performance { get; set; } public virtual ICollection<Integer> Integers { get; set; } } public class Integer { public int IntegerID { get; set; } public int IntegerValue { get; set; } public int IntegerListID { get; set; } public virtual IntegerList IntegerList { get; set; } }

ViewModel

public class IntegerViewModel { [UIHint("Integers")] public IntegerValueViewModel IntegerValues { get; set; } public string Direction { get; set; } } public class IntegerValueViewModel { public ICollection<int> IntegerValue { get; set; } }

Controller

 [HttpPost] public ActionResult Index(IntegerViewModel integerViewModel) { if (ModelState.IsValid) { var integerList = new IntegerList { Direction = integerViewModel.Direction }; foreach (var item in integerViewModel.IntegerValues.IntegerValue) { var integer = new Integer { IntegerValue = item }; integerList.Integers.Add(integer); }

Almost all cases of NullReferenceException are the same. Please see "What is a NullReferenceException in .NET?" for some hints. — John Saunders
– John Saunders, Commented Oct 5, 2013 at 22:17

Shyju · Accepted Answer · 2013-10-05 20:46:36Z

Because the Integers property is not initialized and it is null. You were trying to call the Add method on a null thing which gave you an error

Solution : Initialize the property in the IntegerList class constructor

public class IntegerList { public int IntegerListID { get; set; } public string Direction { get; set; } public long Performance { get; set; } public virtual ICollection<Integer> Integers { get; set; } public IntegerList() { Integers =new List<Integer>(); } }

Be careful of instantiating children objects as new List<Integer>. If you initialize the Integers child collection as a new List<Integer> in the constructor then when you want to do a .Where clause on the child property Integers you will get all the child Integers out of the Db for that parent record and then the .Where clause will be applied. If the child list is large then this affects performance dramatically.

J.W. · Accepted Answer · 2013-10-05 20:46:49Z

When you do

var integerList = new IntegerList { Direction = integerViewModel.Direction };

the associated Integers is not initialized, you can initialize it in IntegerList contsctructor. or do the following

var integerList = new IntegerList { Direction = integerViewModel.Direction, Integers=new List<Integer>() };

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