NullReferenceException when trying to add to a list of dictionaries

Question

When executing this code, I get a NullReferenceException on these lines:

List<Dictionary<Slot, string>> slots = new List<Dictionary<Slot, string>>();
                Dictionary<Slot, string> somedict = new Dictionary<Slot, string>();
                somedict.Add(new Slot(), "s");
                this.slots.Add(somedict);

I cant figure out what is going on. I created a dict with the right items, but when I try to add it to the list, I just get a NullReferenceException....

I've been looking around MSDN and this website for about 2 hours, but no luck. Can anyone help me out? I'm just trying to store a Dictionary, into a list.

namespace hashtable
{
    class Slot
    {
        string key;
        string value;

        public Slot()
        {
            this.key = null;
            this.value = null;
        }
    }

    class Bucket
    {
        public int count;
        public int overflow;
        public List<Dictionary<Slot, string>> slots;
        Dictionary<Slot, string> somedict;

        public Bucket()
        {
            this.count = 0;
            this.overflow = -1;
            List<Dictionary<Slot, string>> slots = new List<Dictionary<Slot, string>>();
            Dictionary<Slot, string> somedict = new Dictionary<Slot, string>();
            somedict.Add(new Slot(), "s");
            this.slots.Add(somedict);
            for (int i = 0; i < 3; ++i)
            {
            }
        }
    }
}

possible duplicate of [What is a NullReferenceException in .NET?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-in-net) — John Saunders, Feb 06 '13 at 01:15

score 7 · Accepted Answer · answered Feb 06 '13 at 01:05

Your Bucket constructor is creating a local variable slots but you are trying to add somedict to the (uninitialized) Bucket member slots.

Replace

List<Dictionary<Slot, string>> slots = new List<Dictionary<Slot, string>>();

with

this.slots = new List<Dictionary<Slot, string>>();

(which is the same as)

slots = new List<Dictionary<Slot, string>>();

You will have the same issue with somedict. If you do not mean it to be a class member in Bucket, do not declare it there. If you do, do not declare it as a local variable in the Bucket constructor.

Thanks, that was it. I "know" what a NullReference means, just didn't remember the proper C# syntax. — avoliva, Feb 06 '13 at 10:31

score 1 · Answer 2 · edited Dec 31 '16 at 14:58

Of course, if you use the more compact syntax of declaring local variables with var the problem is obvious...

var slots = new List<Dictionary<Slot, string>>();
var somedict = new Dictionary<Slot, string>();
somedict.Add(new Slot(), "s");
this.slots.Add(somedict);

As DocMax pointed out, you haven't initialized this.slots and probably meant...

this.slots = new List<Dictionary<Slot, string>>();
var somedict = new Dictionary<Slot, string>();
somedict.Add(new Slot(), "s");
this.slots.Add(somedict);

I suspect the declaration of the Bucket.somedict field is probably redundant as you are creating a local somedict and then adding it to the list where it can be retrieved later.

NullReferenceException when trying to add to a list of dictionaries

2 Answers2