Function Pointers, Delegates, Legacy Code...and some sympathy

[Mark Olbert]

First off, the sympathy is for all you poor buggers out there who have to figure out how to marry
Managed Extensions for C++ onto your legacy code. My condolences; my brief experience with the
process leads me to believe the Microsoft hates all of its VC++ developers :)

Now, the question. I'm getting a NullReferenceEx ception in a __gc class that wraps functionality in
a legacy C/C++ app (it's actually part of the Nero burning rom api).

The legacy API expects to be given a configuration structure that contains, among other things, a
pair of function pointers which the codebase calls at various times. After much hair-pulling, I came
up with the following:

__delegate bool IdleCallbackHan dler( void* pUserData );
__delegate NeroUserDlgInOu t UserDialogCallb ackHandler( void* pUserData, NeroUserDlgInOu t type, void*
data );

[StructLayout(La yout::Sequentia l)]
__gc public struct NeroIdleCallbac k
{
public:
IdleCallbackHan dler* ncCallbackFunct ion;
void* ncUserData;
};

[StructLayout(La yout::Sequentia l)]
__gc public struct NeroUserDialogC allback
{
public:
UserDialogCallb ackHandler* ncCallbackFunct ion;
void* ncUserData;
};

[StructLayout(La yout::Sequentia l)]
__gc public struct NeroSettings
{
public:
const char *nstNeroFilesPa th;
const char *nstVendor, *nstSoftware;
const char *nstLanguageFil e;
NeroIdleCallbac k* nstIdle;
NeroUserDialogC allback* nstUserDialog;
bool nstEnableOverbu rn;
DWORD nstOverburnSize ;
};

I configure this structure, and the callback substructures, in the constructor of my wrapper class
as follows:

theSettings = new NeroSettings();
theSettings->nstNeroFilesPa th = "NeroFiles" ;
theSettings->nstLanguageFil e = "Nero.txt";

theSettings->nstEnableOverb urn = false;
theSettings->nstOverburnSiz e = 0;

theSettings->nstIdle = new NeroIdleCallbac k();
theSettings->nstIdle->ncCallbackFunc tion = new IdleCallbackHan dler(this, &Nero::FireIdle Event);
theSettings->nstIdle->ncUserData = idleData;

theSettings->nstUserDialo g = new NeroUserDialogC allback();
theSettings->nstUserDialo g->ncCallbackFunc tion = new UserDialogCallb ackHandler(this ,
&Nero::FireUser DialogEvent);
theSettings->nstUserDialo g->ncUserData = userdlgData;

theSettings->nstVendor = "ahead";
theSettings->nstSoftware = "Nero - Burning Rom";

Everything works fine during initialization, and in retrieving certain basic information from the
legacy codebase.

But when I call a legacy function that triggers the IdleCallback, I get the NullReferenceEx ception:

void AvailableDrives ( NERO_MEDIA_TYPE mtFlags )
{
// this next line blows up... but the IdleCallback gets called first
NERO_SCSI_DEVIC E_INFOS* drives = NeroGetAvailabl eDrivesEx(MEDIA _CD, NULL);
...
}

The interesting part is the part mentioned in that comment: the IdleCallback method gets called,
and returns, but then the exception gets thrown.

And what's really interesting is that setting the ncCallbackFunct ion delegate/function pointer to
NULL in the NeroIdleCallbac k structure -- which forces the legacy codebase to NOT call the
IdleCallback -- avoids the exception.

Sorry for being so long-winded.

What I'm wondering is, am I handling the function pointer concept correctly here? Is it possible
that I've done something wrong so that when the callback returns, it tries to return to a place that
doesn't exist?

- Mark

[Ernesto Bascón]

Hi Mark:

The idea is to write an inner __nogc class inside our __gc class. The inner
class will contain the callback function (as a static method).

This article was very helpful to me:

http://www.codeproject.com/managedcpp/cbwijw.asp

Best regards
Ernesto

<Mark Olbert> wrote in message
news:ck******** *************** *********@4ax.c om...

First off, the sympathy is for all you poor buggers out there who have to figure out how to marry Managed Extensions for C++ onto your legacy code. My condolences; my brief experience with the process leads me to believe the Microsoft hates all of its VC++ developers :)
Now, the question. I'm getting a NullReferenceEx ception in a __gc class that wraps functionality in a legacy C/C++ app (it's actually part of the Nero burning rom api).

The legacy API expects to be given a configuration structure that contains, among other things, a pair of function pointers which the codebase calls at various times. After much hair-pulling, I came up with the following:

__delegate bool IdleCallbackHan dler( void* pUserData );
__delegate NeroUserDlgInOu t UserDialogCallb ackHandler( void* pUserData, NeroUserDlgInOu t type, void* data );

[StructLayout(La yout::Sequentia l)]
__gc public struct NeroIdleCallbac k
{
public:
IdleCallbackHan dler* ncCallbackFunct ion;
void* ncUserData;
};

[StructLayout(La yout::Sequentia l)]
__gc public struct NeroUserDialogC allback
{
public:
UserDialogCallb ackHandler* ncCallbackFunct ion;
void* ncUserData;
};

[StructLayout(La yout::Sequentia l)]
__gc public struct NeroSettings
{
public:
const char *nstNeroFilesPa th;
const char *nstVendor, *nstSoftware;
const char *nstLanguageFil e;
NeroIdleCallbac k* nstIdle;
NeroUserDialogC allback* nstUserDialog;
bool nstEnableOverbu rn;
DWORD nstOverburnSize ;
};

I configure this structure, and the callback substructures, in the constructor of my wrapper class as follows:

theSettings = new NeroSettings();
theSettings->nstNeroFilesPa th = "NeroFiles" ;
theSettings->nstLanguageFil e = "Nero.txt";

theSettings->nstEnableOverb urn = false;
theSettings->nstOverburnSiz e = 0;

theSettings->nstIdle = new NeroIdleCallbac k();
theSettings->nstIdle->ncCallbackFunc tion = new IdleCallbackHan dler(this, &Nero::FireIdle Event); theSettings->nstIdle->ncUserData = idleData;

theSettings->nstUserDialo g = new NeroUserDialogC allback();
theSettings->nstUserDialo g->ncCallbackFunc tion = new UserDialogCallb ackHandler(this , &Nero::FireUser DialogEvent);
theSettings->nstUserDialo g->ncUserData = userdlgData;

theSettings->nstVendor = "ahead";
theSettings->nstSoftware = "Nero - Burning Rom";

Everything works fine during initialization, and in retrieving certain basic information from the legacy codebase.

But when I call a legacy function that triggers the IdleCallback, I get the NullReferenceEx ception:
void AvailableDrives ( NERO_MEDIA_TYPE mtFlags )
{
// this next line blows up... but the IdleCallback gets called first
NERO_SCSI_DEVIC E_INFOS* drives = NeroGetAvailabl eDrivesEx(MEDIA _CD, NULL);
...
}

The interesting part is the part mentioned in that comment: the IdleCallback method gets called, and returns, but then the exception gets thrown.

And what's really interesting is that setting the ncCallbackFunct ion delegate/function pointer to NULL in the NeroIdleCallbac k structure -- which forces the legacy codebase to NOT call the IdleCallback -- avoids the exception.

Sorry for being so long-winded.

What I'm wondering is, am I handling the function pointer concept correctly here? Is it possible that I've done something wrong so that when the callback returns, it tries to return to a place that doesn't exist?

- Mark

[Mark Olbert]

Thanx, Ernesto! That article was a big help!