NullReferenceException is only thrown sometimes?

Question

I have 2 text files, test1.txt and test2.txt. test2.txt is actually a duplicate of test1.txt.

I want to code something that allows a textbox to read a value from either of the text files so that if one of the text files is empty, it can read from the other text file for the value and vice versa.

I have tested my code by manually emptying the contents of test1.txt first, then emptying the contents of test2.txt which works perfectly as there is always a value appearing on the textbox. However, when both text files contain contents at the same time, NullReferenceException is thrown whenever I try to execute it.

Don't mind if I ask because I probably sound stupid, but is it because neither of the files is empty, that's why this exception is thrown? I have read this post about NullReferenceException, but I don't understand which category my error falls under.

This is a sample of the text files:

^ Michael Johnson^  ^ michaelj^ ^ ^ ^ ^ ^ ^ ^ ^ ^  ^ ^ ^ michaeljohnson@example.com^ ^ 123456^ ^ ^ 123456^ ^ ^ ^ ^ ^ ^

This is my code:

using System.IO;



Log = @"C:\logs\submission\test1.txt";
Log2 = @"C:\logs\submission\test2.txt";

if (!File.Exists(Log))
{
    File.Create(Log).Dispose();
}

if (!File.Exists(Log2))
{
    File.Create(Log2).Dispose();
}

FileStream fs1 = new FileStream(Log, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite);
FileStream fs2 = new FileStream(Log2, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite);

var sr1 = new StreamReader(fs1);
var sr2 = new StreamReader(fs2);

while (!sr1.EndOfStream || !sr2.EndOfStream)
{
    if (new FileInfo(Log).Length != 0)
    {
        var line1 = sr1.ReadLine();
        var values1 = line1.Split('^');

        if (values1[3].ToString().Trim() == User.Identity.Name.)
        {
            number = values1[18].ToString().Trim();
        }

        if (!(values1[3].ToString().Trim().Contains(User.Identity.Name)))
        {
            unregisteredUser = User.Identity.Name;
        }
    }

    else
    {
        if (new FileInfo(Log2).Length != 0)
        {
            var line2 = sr2.ReadLine();
            var values2 = line2.Split('^');

            if (values2[3].ToString().Trim() == User.Identity.Name)
            {
                number = values2[18].ToString().Trim();
            }

            if (!(values2[3].ToString().Trim().Contains(User.Identity.Name)))
            {
                unregisteredUser = User.Identity.Name;
            }
        }
    }
}

sr1.Close();
sr2.Close();
fs1.Close();
fs2.Close();

This is the exact location where the exception is thrown:

var values1 = line1.Split('^');

Read that post you referenced again. It is THE post on the subject. Obviously in your case `line1` is null sometimes. — John3136, Apr 21 '16 at 10:32

score 0 · Accepted Answer · answered Apr 21 '16 at 11:37

If both files have content this: if (new FileInfo(Log).Length != 0) and this if (new FileInfo(Log2).Length != 0)

are true. If one filestream ends, but the other one still has something to read this will result in a Readline on the ennded stream as well and thus will not return a string, but null. Afterwards you try to access Split on this not existing value which makes your program fail at the end.

score -1 · Answer 2 · answered Apr 21 '16 at 10:37

So you want to extract the number value from one of both files with a given user-name?

I'd use this LINQ query which is much more readable:

var numbers = File.ReadLines(Log).Concat(File.ReadLines(Log2))
 .Select(l => l.Split('^'))
 .Where(fields => fields.Length >= 19 
    && fields[3].Trim().Equals(User.Identity.Name, StringComparison.InvariantCultureIgnoreCase))
 .Select(fields => fields[18].Trim());

string number = numbers.FirstOrDefault(); 
string unregisteredUser = number == null ? User.Identity.Name : null;

NullReferenceException is only thrown sometimes?

2 Answers2