NullReferenceException was unhandled in c# array

Question

I am getting the error - NullReferenceException was unhandled, in the following code. I want to extract characters from string pt. However I am getting correct value outside the for loops, but not the same inside it.

ArrayList list = read();
int N = Values.N;
string pt = Values.PlainText;
MessageBox.Show(""+pt.Length+" "+pt[0]);
int count = 0;
char[][][] array = new char[6][][];
for(int i=0;i<6;i++)
{
    for(int j=0;j<N;j++)
    {
        for(int k=0;k<N;k++)
        {
            if (count < pt.Length)
            {
                array[i][j][k] = 'r';
                //array[i][j][k] = pt[count];
                //count++;
            }
            else
            {
                array[i][j][k] = 'x';
            }
        }
    }
}

You could read this article to get into multidimensional arrays a bit: msdn.microsoft.com/en-us/library/2yd9wwz4%28v=vs.71%29.aspx — Alexander Efimov
– Alexander Efimov, Commented Jul 19, 2012 at 13:14
I have already read that, I want dynamic values to be entered, but in this link all the examples are given with the help of preinitialised values for arrays — Rahul
– Rahul, Commented Jul 19, 2012 at 13:16

David M · Accepted Answer · 2012-07-19 13:11:33Z

7

You have to initialise the second and third levels of the arrays, you can't just assign elements. So:

ArrayList list = read();
int N = Values.N;
string pt = Values.PlainText;
MessageBox.Show(""+pt.Length+" "+pt[0]);
int count = 0;
char[][][] array = new char[6][][];
for(int i=0;i<6;i++)
{
    for(int j=0;j<N;j++)
    {
        array[i] = new char[N][]; // <---- Note
        for(int k=0;k<N;k++)
        {
            array[i][j] = new char[N]; // <---- Note
            if (count < pt.Length)
            {
                array[i][j][k] = 'r';
                //array[i][j][k] = pt[count];
                //count++;
            }
            else
            {
                array[i][j][k] = 'x';
            }
        }
    }
}

answered Jul 19, 2012 at 13:11

David M

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6 Comments

Rahul Over a year ago

In my application, N will be entered by user i.e. second and 3rd levels of Array, so how should I tackle this problem?

Rup Over a year ago

What problem? You know N at the time you're looping, so by creating the dimensions just before you enter the loop it'll just work won't it? Though if you need an arbitrary length collection you can use List.

David M Over a year ago

Just the same way - you have the value in a variable presumably, just use that when you initialise the next two levels of the array?

Rahul Over a year ago

N is unknown till runtime, so I cant initialize the array, this is the problem

David M Over a year ago

But at the point where you execute that line of code, you know it. It's the upper limit of your loop. So just use it there...

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