C / C++ / MFC Discussion Boards

thesad13-Feb-07 6:20

13-Feb-07 6:20

I'm learning c++ and using visual studio c++ 2005 express edition. I had written a lot of program's code and try to run, and when it should be started I got an error : An unhandled exception of type 'System.NullReferenceException' occurred in System.Windows.Forms.dll Additional information: Object reference not set to an instance of an object.

What must I do, that this error disapear ???

Re: some strange error

Mark Salsbery13-Feb-07 6:56

Mark Salsbery

13-Feb-07 6:56

You must be new if you're a stranger to the NullReferenceException!

For .NET-related question there's at least two other message boards here that will be more
helpful:

(Managed) C++/CLI[^]

.NET Framework[^]

Re: some strange error

thesad13-Feb-07 7:33

thesad

13-Feb-07 7:33

I looked (Managed) C++/CLI and .NET Framework, I used search and found nothing about NullReferenceException! Do you know something about it??

Re: some strange error

Mark Salsbery13-Feb-07 7:42

Mark Salsbery

13-Feb-07 7:42

I meant ask your question on one of those message boards - C++/CLI is probably the most
appropriate since this isn't a framework question Smile | :)

The NullReferenceException class' name says it all - it's the exception thrown when you try to
dereference a null reference.

NullReferenceException Class[^]

It should be clear in the debugger which line of your code is using a reference that
hasn't been initialized/allocated yet. You may have to view the call stack to find the line in
your code (when the exception occurs).

Re: some strange error

thesad13-Feb-07 7:53

thesad

13-Feb-07 7:53

debugger shows, taht tihs error comes at this code :

this->b_em.pct->Load("b_em.jpg");<br />
this->b_em.pct->Location = System::Drawing::Point(13, 64);<br />
this->b_em.pct->Size = System::Drawing::Size(5, 5);<br />
this->b_em.pct->TabIndex = 3;<br />
this->b_em.pct->TabStop = false;<br />
this->b_em.pct->Visible = false;

so where is probelm ? Smile | :)

Re: some strange error

Mark Salsbery13-Feb-07 8:03

Mark Salsbery

13-Feb-07 8:03

Unfortunately I have never used forms with .NET. There's many helpful people on the .NET boards
that have though Smile | :)

I have no idea what class your "this->b_em.pct" object is so, based on what I see, the only
guess I can make is the Load() method call is failing - is the b_em.jpg file in the app exe folder
or in the system path?

Mark

Re: some strange error

thesad13-Feb-07 8:05

thesad

13-Feb-07 8:05

b_em is from structure

ref struct tank {<br />
  int step_x, step_y;  <br />
  System::Windows::Forms::PictureBox^ pct;<br />
};

b_em.jpg is at app exe folder

that code is at void InitializeComponent(void) function

Re: some strange error

Hamid_RT13-Feb-07 8:18

Hamid_RT

13-Feb-07 8:18

Dear thesad
if you ask on the correct forum(Managed C++/CLI) you can find your answer early

WhiteSky

Re: some strange error

Mark Salsbery13-Feb-07 8:27

Mark Salsbery

13-Feb-07 8:27

Ok. I'll take another wild guess - pct is NULL.

Maybe try

//System::Windows::Forms::PictureBox^ pct = gcnew System::Windows::Forms::PictureBox();

b_em.pct = gcnew System::Windows::Forms::PictureBox();
b_em.pct->Load(...

You can save yourself lots of typing by omitting the implied "this->" Smile | :)

Re: some strange error

thesad13-Feb-07 8:39

thesad

13-Feb-07 8:39

many thanks Mark, it really help Smile | :)

Re: some strange error

Mark Salsbery13-Feb-07 8:50

Mark Salsbery

13-Feb-07 8:50

You are welcome Smile | :)

Really, consider the C++/CLI board for future CLI-related questions. I get so confused trying to
think in .NET when I'm on the C++ board.

Cheers!
Mark "flailing in the dark" Salsbery Wink | ;)

Re: some strange error

Christian Graus13-Feb-07 8:42

Christian Graus

13-Feb-07 8:42

Your first step should be to set a breakpoint with the debugger ( F9 ) and examine your code to see which line blows up, then check to see what variable is null, causing the error. Everything else, is flailing in the dark.

Christian Graus - Microsoft MVP - C++
Metal Musings - Rex and my new metal blog

MFC tips

Alessandra7713-Feb-07 6:18

Alessandra77

13-Feb-07 6:18

Hy, since most of you have probably been programming in MFC for a very long time, I was wondering if you could give me some tips Smile | :)

on how to improve my programming skills in C++. Thanks Wink | ;)

Alessandra

Re: MFC tips

Maximilien13-Feb-07 6:38

Maximilien

13-Feb-07 6:38

first tip :

MFC != C++

second tip :

initialize all variables.

third tip :

check that all pointers are valid before using them.

...

Maximilien Lincourt
Your Head A Splode - Strong Bad

Re: MFC tips

Stephen Hewitt13-Feb-07 11:13

Stephen Hewitt

13-Feb-07 11:13

Maximilien wrote:
check that all pointers are valid before using them.

I'm of the opinion that that is a bad idea. For example, consider code that follows the following pattern throughout:

if (p)
{
    p->SomeFunction();
}

Many people (most people, in fact) get into the habit of doing this even when it is a bug for p to be NULL. Assuming p should never be NULL the extra check turns a crash at the point of the problem into some obscure problem later down the track as the function SomeFunction was never called. I’ve seen this many, many times. A better approach is as follows:

assert(p);
p->SomeFunction();

In a debug build we assert and in a release build you get a crash dump (because naturally everyone collects crash dumps from MS…) which points you at the problem.

As for IsBadReadPointer and friends, see here[^].

Steve

Re: MFC tips

Maximilien13-Feb-07 14:09

Maximilien

13-Feb-07 14:09

I did not offer anything else than "check the pointers" either you use asserts, if or some exceptions, you need to do it anyway.

Stephen Hewitt wrote:
As for IsBadReadPointer and friends, see here[^].

I never use or used those functions.

Maximilien Lincourt
Your Head A Splode - Strong Bad

Re: MFC tips

Mark Salsbery13-Feb-07 6:53

Mark Salsbery

13-Feb-07 6:53

Alessandra77 wrote:
...tips on how to improve my programming skills in C++

Fourth tip:

live it, breathe it, love it.

Re: MFC tips

toxcct13-Feb-07 7:05

toxcct

13-Feb-07 7:05

fifth tip:
go get a break (and use that time to buy a book)

[VisualCalc][Flags Beginner's Guide] | [Forums Guidelines][My Best Advice]

Re: MFC tips

Hamid_RT13-Feb-07 7:15

Hamid_RT

13-Feb-07 7:15

Maybe you think its not necessary but always provide comments for your code ,use of become names for variables,your program must check exception

WhiteSky

Re: MFC tips

Alessandra7713-Feb-07 8:58

Alessandra77

13-Feb-07 8:58

Thanks to you all!
Smile | :)

Alessandra

Re: MFC tips

Hamid_RT13-Feb-07 17:23

Hamid_RT

13-Feb-07 17:23

You're welcome Wink | ;)

WhiteSky

what if the server has a unix os

prithaa13-Feb-07 5:40

prithaa

13-Feb-07 5:40

Hello,

I think if I have an object of CDatabase class,to which if a proper connection string is given then it enables me to access a database from a server whose OS is windows xp.

But if the OS is Unix or linux then what changes I have to bring about in my program written in VC++ for windows.
I have a different OS only on the server .

Will the code wriiten on the server using CSocket class work ?

Where am I going wrong.Which topics should I cover?

Thanks
Prithaa

Re: what if the server has a unix os

toxcct13-Feb-07 5:57

toxcct

13-Feb-07 5:57

it doesn't matter what OS runs on the remote dataabase server because Sockets are made to communicate through IP protocol so fully independant of the implementation

[VisualCalc][Flags Beginner's Guide] | [Forums Guidelines][My Best Advice]

Re: what if the server has a unix os

prithaa13-Feb-07 18:21

prithaa

13-Feb-07 18:21

Hello,

Thanks

Which topics should I cover to understand databases and remote access of data

Prithaa

64 bit compile problems [modified]

Mike Curry - Canada13-Feb-07 5:08

Mike Curry - Canada

13-Feb-07 5:08

I am a new poster, and a long time reader... that sounds so familiar... and cheesy. Wink | ;)

Anyhow, I am experiencing some strange errors while compiling my application for 64 bit, and have tried google without much luck.

One of the more consfusing problems I am running into is with the following line that compiles fine in 32 bit, but not 64 bit, and am looking for advice or direction (even a smack in the head - if its obvious).

Here is the line, and the compile error, as well as the definition of the function being called...

definitions

WINMMAPI MMRESULT WINAPI timeSetEvent( IN UINT uDelay, IN UINT uResolution,<br />
    IN LPTIMECALLBACK fptc, IN DWORD_PTR dwUser, IN UINT fuEvent);<br />
void TSstringSocket::SelectTimer(UINT wTimerID, UINT msg, ULONG_PTR dwUser, DWORD dw1, DWORD dw2) {<br />
...<br />
}<br />

Code

UINT m_timerID;<br />
m_timerID = timeSetEvent( 100, wTimerRes, SelectTimer, (ULONG_PTR)this, TIME_PERIODIC );<br />

Error

Error	162	error C2664: 'timeSetEvent' : cannot convert parameter 3 from 'void (__cdecl *)(UINT,UINT,ULONG_PTR,DWORD,DWORD)' to 'LPTIMECALLBACK'	c:\Documents and Settings\mikec\Desktop\tix64\Backend\BaseObjects\TSstringSocket.cpp	217	<br />

Any ideas?

Mike Curry

-- modified at 11:17 Tuesday 13th February, 2007

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