NullReference Exception in multithread program for reading data

Question

In my C# win form project, I read some data by web service. It takes a few time, so I have a thread for reading like this:

System.Threading.ThreadPool.QueueUserWorkItem((o) =>
{
    SMSCenter_RecievedMessages = (new SMSCenterGetMessages()).getMessages());

    this.BeginInvoke(new Action(() =>
    {
        if (SMSCenter_RecievedMessages != null && SMSCenter_RecievedMessages.Length != 0)
        {
            // some code
        }
    }));
});

after 2 days running, some time I get this exception:

Null Reference Exception was unhandled : Object Reference Not set to an instance of an object.

Is it possible that before creating a new of object, getMessage() function is accured? Because I did it by thread. what is the wrong by this code?

Thanks for any helping?

At which line you are getting the exception ? coz I don't find Nullreference possibility in your current code. . . — Rohit Prakash
– Rohit Prakash, Commented Feb 21, 2015 at 10:45
@RohitPrakash I got it in SMSCenter_RecievedMessages = (new SMSCenterGetMessages()).getMessages()); that will accure few times — Elahe
– Elahe, Commented Feb 21, 2015 at 10:46
In that case, off course, SMSCenterGetMessages object is not being created at the time. Try using Parallel stacks to figure out the exact case. — Rohit Prakash
– Rohit Prakash, Commented Feb 21, 2015 at 10:49
@RohitPrakash how is it possible that new does not instantiate a new object and results to a null instead? — zerkms
– zerkms, Commented Feb 21, 2015 at 10:50

Douglas · Accepted Answer · 2015-02-21 11:09:39Z

2

One possible cause of your error is that SMSCenter_RecievedMessages is presumably declared as a class field. If your code snippet can be executed multiple times in succession (e.g. from a click event handler), then this could lead to race conditions.

Suppose that an event handler causes your code snippet to start getting executed. The background thread populates SMSCenter_RecievedMessages with a non-null value, and dispatches control to the UI thread. The UI thread evaluates SMSCenter_RecievedMessages != null to true.

In the meantime, another event handler had caused your code snippet to start getting executed again (concurrently). This time, getMessages() returns null; this is assigned to the same SMSCenter_RecievedMessages field and overrides the previous value.

In the meantime, the UI thread has continued execution, and attempts to evaluate SMSCenter_RecievedMessages.Length != 0. Since the field has now been set to null, a NullReferenceException ensues.

edited Feb 21, 2015 at 11:09

answered Feb 21, 2015 at 10:53

Douglas

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5 Comments

zerkms Over a year ago

OP stated that SMSCenter_RecievedMessages = (new SMSCenterGetMessages()).getMessages()); is the line that causes a null reference exception.

Douglas Over a year ago

@zerkms: The clarification was stated in a comment after the original question had been posted. In the absence of further information, the race condition on the SMSCenter_RecievedMessages field is still the likeliest explanation.

zerkms Over a year ago

I agree that for the original question it would be the most possible explanation (if only we ignore the last lines, where OP also refers to getMessages() explicitly), but OP stated it's the (new SMSCenterGetMessages()).getMessages()) expression that fails. Which I personally don't believe, but we have what we have :-)

Elahe Over a year ago

Thanks for your good idea, but I used Lock() for whole this code, that I think it solves this race condition. but the unhandled exception is accure yet.

Douglas Over a year ago

@Elahe: I would suggest making SMSCenter_RecievedMessages a local variable instead, so that different executions of your code would not conflict with each other. If you don't use the field elsewhere, then this should be sufficient. If you do use the field elsewhere, then the lock needs to be applied for all uses, which might lead to UI freezes or deadlocks.

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