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SAFEARRAY* & COM Interop

 
 
=?Utf-8?B?QnJlbmRhbiBHcmFudA==?=
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      18th Sep 2006
I’ve got a C# library that I’ve built into a COM component that will be used
from a VC++ 6 application and while the creation of the COM object side of
things seem to be working fine, using the object fully is another matter.

On the C# side of things I’ve got an object defined like so:

[ClassInterface(ClassInterfaceType.None)]
public class PID : IPID
{
private string type;
private int number;
private PID[] pids;

public string Type
{
get { return type;}
set { type = value;}
}

public int Number
{
get { return number;}
set { number = value;}
}

public PID[] PIDs
{
get { return pids;}
set { pids = value;}
}

public PID(){}

public PID(string type, int number)
{
this.type = type;
this.number = number;
this.pids = new PID[0];
}

public PID(string type, int number, PID[] pids)
{
this.type = type;
this.number = number;
this.pids = pids;
}
}

I can instantiate an instance of this object in the MFC app with no
problem... the problem is when I try to set the PIDs array to... anything but
null really.

The issue is that the C# array is viewed by the MFC app as pointer to a
SAFEARRAY and all attempts to create an array of PID objects and assign them
to the PIDs property fails.

I’ve tried straight up (and cheesy) typecasting:

IPIDPtr pPID1(__uuidof(PID));
IPID *pid1 = pPID1;
....
IPID *pidArray1[2];
pidArray1[0] = pid2;
pidArray1[1] = pid3;
pid1->PIDs = (SAFEARRAY*)pidArray1;

Which causes a friendly “User breakpoint called from code at 0x7c901230” and
points to code I do not have the debug symbols for.

I also tried explicitly creating a SAFEARRAY with SafeArrayCreate:

SAFEARRAY * psa;
SAFEARRAYBOUND rgsabound[1];

rgsabound[0].lLbound = 0;
rgsabound[0].cElements = 2;

psa = SafeArrayCreate(VT_VARIANT, 1, rgsabound);

LONG index = 0;
SafeArrayPutElement(psa, &index, &pid2);
index = 1;
SafeArrayPutElement(psa, &index, &pid3);

fi->PIDs = psa;

In this example everything works fine up until we assign PIDs (but not when
we set PIDs to null)... then we get an exception which reads:

Unhandled exception at 0x7c81eb33 in MFCWSTestApp.exe: Microsoft C++
exception: _com_error @ 0x0012f8c4.

When the following line is called from the .tli wrapper:

inline void IFileInformation::PutPIDs ( SAFEARRAY * _arg1 ) {
_com_dispatch_method(this, 0x60020004, DISPATCH_PROPERTYPUT,
VT_EMPTY, NULL,
L"\x2009", _arg1);
}



Thinking that I should be a little more explicit with regards to the size of
the elements (and based on another example) I tried the following:

SAFEARRAY *params = SafeArrayCreateVector(VT_R4, 0, 2);

int paramMin = 0;

SafeArrayAccessData(params, (void**)&pid1);
memcpy(&paramMin, &pid1, sizeof(IPID));
SafeArrayUnaccessData(params);

paramMin = 1;
SafeArrayAccessData(params, (void**)&pid2);
memcpy(&paramMin, &pid2, sizeof(PID));
SafeArrayUnaccessData(params);

This test behaves similarly to the previous one in that the bulk of the code
executes without any error... but then an unhandled exception is thrown the
moment we try to assign PIDs to anything... including null:

Unhandled exception at 0x7c81eb33 in MFCWSTestApp.exe: Microsoft C++
exception: _com_error @ 0x0012f894.

Because of the far easier to use environment and debugger I have spent a
fair amount of time trying to resolve this issue within the VC7.1
environment, unfortunately both VC6 and VC7 have the exact same issues with
the code in question.

Aside from the obvious issue of trying to use VC6... does anyone see what I
might be doing wrong here or have any ideas on how I could fix it?
 
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SvenC
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Posts: n/a
 
      18th Sep 2006
Hi Brendan,

see inline

"Brendan Grant" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I've got a C# library that I've built into a COM component that will be
> used
> from a VC++ 6 application and while the creation of the COM object side of
> things seem to be working fine, using the object fully is another matter.
>
> On the C# side of things I've got an object defined like so:
>
> [ClassInterface(ClassInterfaceType.None)]
> public class PID : IPID
> {

<snip/>
> private PID[] pids;

<snip/>
> public PID[] PIDs
> {
> get { return pids;}
> set { pids = value;}
> }


What does the COM declaration in C++ code look like? Have a look at your tlh
file. Is the retval a VARIANT* or a SAFEARRAY* ?
What is the definition of the interface IPID?
What do you expect to return there anyways? You have a class PID which
exposes an array of PID instances, each exposing a PID array again?
I am astonished that you can return a PID[]? To my knowledge you cannot
expose a class as parameter type, just an interface. Is it really PID[] or
IPID[] what you are returning?

<snip/>

>
> I can instantiate an instance of this object in the MFC app with no
> problem... the problem is when I try to set the PIDs array to... anything
> but
> null really.
>
> The issue is that the C# array is viewed by the MFC app as pointer to a
> SAFEARRAY and all attempts to create an array of PID objects and assign
> them
> to the PIDs property fails.
>
> I've tried straight up (and cheesy) typecasting:
>
> IPIDPtr pPID1(__uuidof(PID));
> IPID *pid1 = pPID1;


When you take a raw interface pointer you should call AddRef and Release.
You get a little help here because pPID1 is a smartpointer so for its
lifetime pid1 is valid also. So why not you pPID1 directly and remove pid1.

> ...
> IPID *pidArray1[2];
> pidArray1[0] = pid2;
> pidArray1[1] = pid3;
> pid1->PIDs = (SAFEARRAY*)pidArray1;


What is pid1 and pid2? Have they been initialized?

> Which causes a friendly "User breakpoint called from code at 0x7c901230"
> and
> points to code I do not have the debug symbols for.
>
> I also tried explicitly creating a SAFEARRAY with SafeArrayCreate:
>
> SAFEARRAY * psa;
> SAFEARRAYBOUND rgsabound[1];
>
> rgsabound[0].lLbound = 0;
> rgsabound[0].cElements = 2;
>
> psa = SafeArrayCreate(VT_VARIANT, 1, rgsabound);
>
> LONG index = 0;
> SafeArrayPutElement(psa, &index, &pid2);
> index = 1;
> SafeArrayPutElement(psa, &index, &pid3);


This looks the most promising. But you made some small mistakes:
I guess pid2 and pid3 are of type IPID*, correct? If so you must not use &,
just pass pid2 and pid3 as they are already pointers so they are valid for
void*.
BUT: void* ist used because you have absolutely no type safety. The compiler
trusts you, and infect you lied to him
You said: I want a safearray of VARIANTs. And later on you when you should
pass a VARIANT through void* you put an IPID* in there.
I guess you should setup two VARIANTs where you set the type to VT_UNKNOWN
and set punkVal to pid2 and pid3.

VARIANT v1, v2;
v1.vt = v2.vt = VT_UNKNOWN;
v1.punkVal = pid2; // I assume pid2 is AddReffed and you no longer use pid2
later on, so you transfer ownership to v1.
v2.punkVal = pid3; // same assumption

Now you call:
SafeArrayPutElements(psa, &index, &v1); // same for v2

> fi->PIDs = psa;
>
> In this example everything works fine up until we assign PIDs (but not
> when
> we set PIDs to null)... then we get an exception which reads:
>
> Unhandled exception at 0x7c81eb33 in MFCWSTestApp.exe: Microsoft C++
> exception: _com_error @ 0x0012f8c4.
>
> When the following line is called from the .tli wrapper:
>
> inline void IFileInformation::PutPIDs ( SAFEARRAY * _arg1 ) {
> _com_dispatch_method(this, 0x60020004, DISPATCH_PROPERTYPUT,
> VT_EMPTY, NULL,
> L"\x2009", _arg1);
> }


How comes IFileInformation into play? I thought we were using IPID?

>
>
> Thinking that I should be a little more explicit with regards to the size
> of
> the elements (and based on another example) I tried the following:
>
> SAFEARRAY *params = SafeArrayCreateVector(VT_R4, 0, 2);


Why do you create an array of doubles when you want to store interface
pointers?

>
> int paramMin = 0;
>
> SafeArrayAccessData(params, (void**)&pid1);
> memcpy(&paramMin, &pid1, sizeof(IPID));
> SafeArrayUnaccessData(params);
>
> paramMin = 1;
> SafeArrayAccessData(params, (void**)&pid2);
> memcpy(&paramMin, &pid2, sizeof(PID));
> SafeArrayUnaccessData(params);


Why do you copy values 0 and 1 at the adresses of pid1 and pid2. What type
are pid1 and pid2?

> This test behaves similarly to the previous one in that the bulk of the
> code
> executes without any error... but then an unhandled exception is thrown
> the
> moment we try to assign PIDs to anything... including null:
>
> Unhandled exception at 0x7c81eb33 in MFCWSTestApp.exe: Microsoft C++
> exception: _com_error @ 0x0012f894.
>
> Because of the far easier to use environment and debugger I have spent a
> fair amount of time trying to resolve this issue within the VC7.1
> environment, unfortunately both VC6 and VC7 have the exact same issues
> with
> the code in question.
>
> Aside from the obvious issue of trying to use VC6... does anyone see what
> I
> might be doing wrong here or have any ideas on how I could fix it?


--
SvenC


 
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