Error : Index out of bounds of array

Question

 string[] lines = System.IO.File.ReadAllLines(@"C:\Users\myname\Desktop\test.txt");
        int cnt = lines.Count();

        int arraynum1 = 0;
        int arraynum2 = 1;
        int arraynum3 = 2;

        try
        {
            for (int x = 0; x < cnt; x++)
            {
                mc[0] = lines[arraynum1];
                mc[1] = lines[arraynum2];
                mc[2] = lines[arraynum3];

                arraynum1 = arraynum3 + 1;
                arraynum2 = arraynum3 + 1;
                arraynum3 = arraynum3 + 1;

                ListViewItem item = new ListViewItem(new[] { mc[0], mc[1], mc[2] });
                listView1.Items.Add(item);

            }

        }
        catch(Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }

Whenever I use my variable cnt in the for loop to specify which line to stop at, I receive the error listed in the title of this question, but whenever I set an integer in place of cnt in the loop it works flawlessly.

Answer 1

Shouldn't it be

    arraynum1 = arraynum1 + 1;
    arraynum2 = arraynum2 + 1;
    arraynum3 = arraynum3 + 1;

instead of

    arraynum1 = arraynum3 + 1;
    arraynum2 = arraynum3 + 1;
    arraynum3 = arraynum3 + 1;

Answer 2

Try this:

arraynum1 = arraynum1 + 1;
arraynum2 = arraynum2 + 1;
arraynum3 = arraynum3 + 1;

as you are currently referring to the same array arraynum3 in all the three cases.

arraynum1 = arraynum3 + 1;
arraynum2 = arraynum3 + 1;
arraynum3 = arraynum3 + 1;

Answer 3

Assuming your file contains 6 lines, this would be the values your mc[0]..mc[2] would hold inside your for loop:

Iteration 1 (x=0):

mc[0] = lines[0]
mc[1] = lines[1]
mc[2] = lines[2]

Iteration 2 (x=1):

mc[0] = lines[3]
mc[1] = lines[3]
mc[2] = lines[3]

Iteration 3 (x=2):

mc[0] = lines[4]
mc[1] = lines[4]
mc[2] = lines[4]

Iteration 4 (x=3):

mc[0] = lines[5]
mc[1] = lines[5]
mc[2] = lines[5]

Iteration 5 (x=4):

The exception you stated will be thrown here
mc[0] = lines[6]
mc[1] = lines[6]
mc[2] = lines[6]

Iteration 6 (x=5): Will never be reached

So instead of increasing your loop var x by 1 increase it by 3 like:

        for (int x = 0; x+2 < cnt ; x+=3)
        {
            mc[0] = lines[x];
            mc[1] = lines[x+1];
            mc[2] = lines[x+2];

            ListViewItem item = new ListViewItem(new[] { mc[0], mc[1], mc[2] });
            listView1.Items.Add(item);

        }

This approach will also consider the possibility that your file won't contain number of lines which isn't divideable by 3.