How could this reference to a string be causing a Null Reference Exception?

Question

There is a struct:

public struct ReturnedCommands
{
    public string   key;
    public string   command;
};

...and a variable:

public static ReturnedCommands returnedCommands;

...and a Null Reference Exception occurring, returning the value "3":

public bool PendingCommandsExecute_QUERY()
{
    int i = 0;
    try
    {
        i = 1;
        cmdResults b = cmdResults.cmdFail;
        bool retVal = false;
        string command = "QUERY";

        if (returnedCommands.key != command) 
            return false;

        HashResultsAdd(command, "Started");
        i = 2;

        HashStatus.Clear();

        string sNum = PendingCommands.SiteFetchSiteNumber; 
        i = 3; // <-- last one reached (debug int displayed in NRE)
        string s = string.Empty;
        if (returnedCommands.command != null)
        {
            s =  command + "|" + sNum.Trim() + "|t_inv|SELECT|" + returnedCommands.command;
        }
        i = 4;
        HashStatusAdd(command, s);
        . . .
    } 
    catch( Exception ex )
    {
        MessageBox.Show(ex.Message + " reached debug int " + i.ToString());
    }
    return true;
} // PendingCommandsExecute_QUERY

When the NRE occurs, it shows "Exception: Null Reference Exception reached debug int 3". So, it must be imploding while referencing returnedCommands.command, but how could that be, as returnedCommands.command is simply a string? First verifying it's not null (the "if (returnedCommands.command != null)" line) should be safe, so that test should not be problematic; if it passes that test, it is just a string concatenation operation, so how could that be causing a NRE?

Instead of using some occult ints, can't you just use the debugger to step through the affected lines? — Nolonar, Jun 10 '13 at 15:52
By the way: `ex.ToString()` yields a lot more information than `ex.Message` — Nolonar, Jun 10 '13 at 15:53
@Nolonar: Unfortunately, no; it's a long story, but this is a paleographic effort using VS 2003 for a Windows CE project - no emulators available, no direct debugging is possible. If farmers worked this way, they'd be out in the fields with stone tools. — B. Clay Shannon-B. Crow Raven, Jun 10 '13 at 16:01
Probably "returnedCommands" is overwritten by another thread. — Andre, Jun 10 '13 at 16:01
@Zicore: That could be true; if so, how can I guard against the NRE? ... I wonder if there's any significance to NRE being so close to NRA...??? — B. Clay Shannon-B. Crow Raven, Jun 10 '13 at 16:03
Please use `ex.ToString()` for logging the stacktrace first, you should see the line numbers then. — Andre, Jun 10 '13 at 16:37

score 5 · Accepted Answer · answered Jun 10 '13 at 15:52

5

if PendingCommands.SiteFetchSiteNumber is null (and thus sNum is null), then this would cause a null reference exception when you call sNum.Trim()

Also if you show the full stack trace it should give you the exact line number that the error occurred on, which is a lot simpler to debug than using an incremental value to figure it out.

answered Jun 10 '13 at 15:52

Martin Ernst

5,629
2
17
14

The assignment "i = 3" is after that reference, so wouldn't the value of i in the exception be 2 in that case? – B. Clay Shannon-B. Crow Raven Jun 10 '13 at 16:32
No, the assignment i = 3 is after you assign sNum = PendingCommands.SiteFetchSiteNumber. This is valid to assign null to a variable, the problem comes in that a couple of lines later, you are trying to call a method (Trim) on a "null object" – Martin Ernst Jun 10 '13 at 16:37

Andre · Answer 2 · 2013-06-10T16:20:38.420

1

It's most likely a missing check for multi thread access of the public static member.

public static ReturnedCommands returnedCommands;

I cannot see what you exactly want to achieve there, but I consider you reviewing the architecture, because it'll most likely fail when other threads have access to this member and overwrite it with null.

Probably you can use a queue for your commands.

edited Jun 10 '13 at 16:20

answered Jun 10 '13 at 16:14

Andre

1,228
11
24

Yes, this app is a total mess, and it needs to revamped, refactored, and rearchitected, I can't argue with you there. Whether that's going to happen is not up to me, though. – B. Clay Shannon-B. Crow Raven Jun 10 '13 at 16:47
I wish I could mark both answers as correct, because doubtless this one is, too... – B. Clay Shannon-B. Crow Raven Jun 10 '13 at 17:19

How could this reference to a string be causing a Null Reference Exception?

2 Answers2