Declaring and accessing the List[][] in c#?

Question

I have declared below list:

List<int>[][] main=new List<int>[4][];

main[0]=new List<int>[3];

I am getting error "Object reference not set to an instance of object", if I try to add element using below statement:

main[0][0].add(3);

Can you please let me know what's wrong with the statement and the equivalent correct statement ?

Element `0` of `main[0]` is probably still a null List. Instantiate it to a new list first. Please include the actual error (I suspect it's a nullRef exception) — ryanyuyu, Apr 07 '15 at 22:05
"I am getting error" - *what* error? Is it a compiler error or a runtime error? What's the error message? Please be specific in your questions. — O. R. Mapper, Apr 07 '15 at 22:06
possible duplicate of [What is a NullReferenceException and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) - after reading the basic description, please make sure to read "**Array Elements**" and "**Jagged Arrays**" sections. — quetzalcoatl, Apr 07 '15 at 22:12
'I need array of integers, but size is dynamic' that would just look like this; `List`. Based on the comment and the wackiness of `List[][]` I'm not sure if you actually want guidance in using that structure of you're actually looking for something much much simpler like a 2d array or a plain `List`. — evanmcdonnal, Apr 07 '15 at 22:13

score 5 · Accepted Answer · answered Apr 07 '15 at 22:11

This instruction main[0]=new List<int>[3];will create an object of type Array of List of size 3 and will allocate the necessary space on the heap for that object but it does not instanciate each element. Each element will have the default value equal to null

Therefore main[0][0].add(3);is an attempt to call add on a null object. You should prior to this line have something like: main[0][0] = new List<int>();

score 0 · Answer 2 · answered Apr 07 '15 at 22:15

You've left out a step to initiliaze one of the dimensions. The first one is main[0]=new List<int>[3]; but you also need main[0][0] = new List<int>();

So something like this should do it;

        List<int>[][] main = new List<int>[3][];
        main[0] = new List<int>[3];
        main[0][0] = new List<int>();

        main[0][0].Add(1000);

        Console.WriteLine(main[0][0].FirstOrDefault()); // 1000

Declaring and accessing the List[][] in c#?

2 Answers2