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 NullReferenceException 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 IdleCallbackHandler( void* pUserData );
__delegate NeroUserDlgInOut UserDialogCallbackHandler( void* pUserData, NeroUserDlgInOut type, void*
data );
[StructLayout(Layout::Sequential)]
__gc public struct NeroIdleCallback
{
public:
IdleCallbackHandler* ncCallbackFunction;
void* ncUserData;
};
[StructLayout(Layout::Sequential)]
__gc public struct NeroUserDialogCallback
{
public:
UserDialogCallbackHandler* ncCallbackFunction;
void* ncUserData;
};
[StructLayout(Layout::Sequential)]
__gc public struct NeroSettings
{
public:
const char *nstNeroFilesPath;
const char *nstVendor, *nstSoftware;
const char *nstLanguageFile;
NeroIdleCallback* nstIdle;
NeroUserDialogCallback* nstUserDialog;
bool nstEnableOverburn;
DWORD nstOverburnSize;
};
I configure this structure, and the callback substructures, in the constructor of my wrapper class
as follows:
theSettings = new NeroSettings();
theSettings->nstNeroFilesPath = "NeroFiles";
theSettings->nstLanguageFile = "Nero.txt";
theSettings->nstEnableOverburn = false;
theSettings->nstOverburnSize = 0;
theSettings->nstIdle = new NeroIdleCallback();
theSettings->nstIdle->ncCallbackFunction = new IdleCallbackHandler(this, &Nero::FireIdleEvent);
theSettings->nstIdle->ncUserData = idleData;
theSettings->nstUserDialog = new NeroUserDialogCallback();
theSettings->nstUserDialog->ncCallbackFunction = new UserDialogCallbackHandler(this,
&Nero::FireUserDialogEvent);
theSettings->nstUserDialog->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 NullReferenceException:
void AvailableDrives( NERO_MEDIA_TYPE mtFlags )
{
// this next line blows up... but the IdleCallback gets called first
NERO_SCSI_DEVICE_INFOS* drives = NeroGetAvailableDrivesEx(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 ncCallbackFunction delegate/function pointer to
NULL in the NeroIdleCallback 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 2 1623
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.com... 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 NullReferenceException 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 IdleCallbackHandler( void* pUserData ); __delegate NeroUserDlgInOut UserDialogCallbackHandler( void* pUserData,
NeroUserDlgInOut type, void* data );
[StructLayout(Layout::Sequential)] __gc public struct NeroIdleCallback { public: IdleCallbackHandler* ncCallbackFunction; void* ncUserData; };
[StructLayout(Layout::Sequential)] __gc public struct NeroUserDialogCallback { public: UserDialogCallbackHandler* ncCallbackFunction; void* ncUserData; };
[StructLayout(Layout::Sequential)] __gc public struct NeroSettings { public: const char *nstNeroFilesPath; const char *nstVendor, *nstSoftware; const char *nstLanguageFile; NeroIdleCallback* nstIdle; NeroUserDialogCallback* nstUserDialog; bool nstEnableOverburn; DWORD nstOverburnSize; };
I configure this structure, and the callback substructures, in the
constructor of my wrapper class as follows:
theSettings = new NeroSettings(); theSettings->nstNeroFilesPath = "NeroFiles"; theSettings->nstLanguageFile = "Nero.txt";
theSettings->nstEnableOverburn = false; theSettings->nstOverburnSize = 0;
theSettings->nstIdle = new NeroIdleCallback(); theSettings->nstIdle->ncCallbackFunction = new IdleCallbackHandler(this,
&Nero::FireIdleEvent); theSettings->nstIdle->ncUserData = idleData;
theSettings->nstUserDialog = new NeroUserDialogCallback(); theSettings->nstUserDialog->ncCallbackFunction = new
UserDialogCallbackHandler(this, &Nero::FireUserDialogEvent); theSettings->nstUserDialog->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 NullReferenceException: void AvailableDrives( NERO_MEDIA_TYPE mtFlags ) { // this next line blows up... but the IdleCallback gets called first NERO_SCSI_DEVICE_INFOS* drives = NeroGetAvailableDrivesEx(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 ncCallbackFunction
delegate/function pointer to NULL in the NeroIdleCallback 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
Thanx, Ernesto! That article was a big help! This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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