Annoying: Access Violation / Null Reference

Hi Experts,
I have a C# application that calls a DLL written by me in C++.
GPGUI.exe calls gpmain() in simplegpdll.dll with the following line:

if (simplegpdll.gpmain(...,...,...) == 1) {... return true;}

Most of the time it runs with no problems - however, sometimes (and I can't spot a pattern), it crashes with the following error:

   First-chance exception at 0x77f5b2ab in GPGUI.exe: 0xC0000005: Access violation reading location 0x00000034.
   An unhandled exception of type 'System.NullReferenceException' occurred in simplegpdll.dll

   Additional information: Object reference not set to an instance of an object.

The DLL reads a text file, and writes three (once each per operation).  All reads and writes look like this:
FILE *solstream;
fprintf(solstream,"Gen,Best Program\n%d,%d\n", generation, best_prog);

Any suggestions on what's at 0x00...34 (or how to find out), and also what is throwing the Null Ref?

Thanks in advance, J
Who is Participating?
vascovConnect With a Mentor Commented:
Hi Jazzle,

Compile your C++ DLL with debuging info. That may help you showing more info when an AV occurs.

Regarding your code, check to see that solstream is NOT null after opening it. You might be experiencing issues with file sharing, or something like that.
The NullRef might be a consequence of the other error... (the access violation)


JazzleAuthor Commented:
I have compiled it with the debuging info (it was anyway) but unfortunately it doesn't give me any extra info. :(
also, I know that solstream can't be null because of the way it's created.

I did wonder about file sharing, but i have found found that it gives a different error if the file is in use.

thanks for looking.


PS - 'hth'?
vascovConnect With a Mentor Commented:
Launch your application using the debugger.

Please, do check if solstream is NOT null anyway. It wont hurt, and it might surprise you the situations that can occur. (i'm not saying with 100% sure that the problem is in this area, but since this is the only code you're showing i definetely recommend for you to test it...)

That exception can also be happening in some marshalling between managed / unmanaged.

hope this helps (hth),

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JazzleAuthor Commented:
Okay, so I have now found that the exception is caused by the fprintf() command - so i suppose it's refering to the file i want to write to.  all the files written to are closed afterwards - so could it be a matter of threads confusing each other? (I haven't specifed any thread usage)

************* Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at fprintf(_iobuf* , SByte* )
   at gpmain(Int32 POPULATION_SIZ.....
JazzleAuthor Commented:
You were right it seems the fopen() command is sometimes returning NULL - but I can't see why - any ideas anyone?

vascovConnect With a Mentor Commented:
Use GetLastError()


JazzleAuthor Commented:
thanks - will look into GetLastError().
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