Calling unmanaged code from managed MFC worker thread

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Klaus - 01 Aug 2007 20:34 GMT

Hi,

I have an existing VC 6 MFC application which communicates asynchronly with
a VC 2005 managed code dll.
I use an unmanaged base class with virtual functions to access methods in
the MFC application.
Furthermore, I use a pointer to an unmanaged function to jump back into the
managed dll.

The managed part is basically a remoting enhancement which asynchronly
initiates a call from the remoting thread to the MFC application. The
information is collected within the main thread of the MFC application. The
managed code dll is invoked via a function pointer. From the managed code
additional calls to the unmanaged MFC application will be done via a pointer
to an object in the MFC. When this code is executed within the main thread
of the MFC application everything works fine.

However, if I switch to a MFC worker thread, calls via the base class
pointer results in System.AccessViolationException!

Scenario, which works:
1. Remoting thread imvoked
2. Managed DLL -> MFC App asynchron, no result
3. Switch thread to App main thread
4. MFC App -> Managed DLL
5. Managed DLL -> MFC App (synchron)

Scenario, which does not work:
1. Remoting thread imvoked
2. Managed DLL -> MFC App (asynchron, no result)
3. Switch thread to MFC worker thread!!!
4. MFC App -> Managed DLL
5. Managed DLL -> MFC App (synchron): System.AccessViolationException !!

How can I access pointer to unmanaged classes from managed code within a
worker thread?

Best regards,
Klaus

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Pixel.to.life - 01 Aug 2007 21:27 GMT

> Hi,
>
[quoted text clipped - 35 lines]
> Best regards,
> Klaus

Hi, Klaus,

I assume you are already using delegates, and marshaling of delegate
method pointers to access unmanaged functions from managed code,
correct?

http://msdn2.microsoft.com/en-us/library/bfeyhsdy.aspx

Reply to this Message

Klaus - 02 Aug 2007 10:14 GMT

Thanks for your help!

Here is the code:
myHeader.h, shared in both projects, MFC 6.0 App and VC 2005 DLL:
class CMyBase
{
public:
CMyBase() {};
virtual ~CMyBase() {};
virtual bool MyExample() = 0;
};

In MFC App this class will be derived:
class CMyBaseDerived : public CMyBase
{
public:
CMyBaseDerived();
virtual ~CMyBaseDerived();
virtual bool MyExample();
};

In VC2005, I have a pointer to the instance of CMyBaseDerived:
CMyBase *PMyBase;
This pointer will be initialized by calling a function in the DLL from the
App:
void MyInit(CMyBase *p)
{
PMyBase = p;
}

When I use this pointer in the DLL from the MFC worker thread an
AccessViolationException will be thrown:
PMyBase->MyExample();

Using this pointer within the main thread no problem occurs.

Best regards,
Klaus

>> Hi,
>>
[quoted text clipped - 48 lines]
>
> http://msdn2.microsoft.com/en-us/library/bfeyhsdy.aspx

Reply to this Message

Pixel.to.life - 03 Aug 2007 00:59 GMT

> Thanks for your help!
>
[quoted text clipped - 92 lines]
>
> - Show quoted text -

Klaus,

First of all, it doesnt not seem like your MFC app is a managed
aplpication. Do you compile it with /clr command line option?
If you are using only MFC, why do you need to build a managed
application?

Secondly, do you have the method MyExample() overridden in
CMyBaseDerived?

Thirdly, assuming you have MyExample() overridden in CMyBaseDerived,
dont you have to cast PMyBase to a pointer to the derived object?
Virtual method references are resolved at run time, so unless the
machine knows the object is of derived type, it would look for the
method's definition ion the base class (
http://www.parashift.com/c++-faq-lite/virtual-functions.html#faq-20.3
).. and if you dont have an implementation of MyExample() in the base
class, you will get an access violation.. no matter in managed/
unmanaged code.

Pardon me if I missed something.

Reply to this Message

Klaus - 03 Aug 2007 11:23 GMT

Hi,

The MFC App is pure unmanaged, built with VC 6.0!
MyExample() must be overriden in CMyBaseDerived, otherwise it could not be
instantiated.
From my understanding, I do not have to cast the pointer to the derived
class in order to call MyExample; this is exactly why the concept of virtual
methods was introduced; your web reference states nothing else.

Don't forget, everything works fine in the main thread!
So, from my point of view it is more a "security issue" in managed code
preventing me from calling unsecure code?

Best regards,
Klaus

Reply to this Message

Pixel.to.life - 03 Aug 2007 17:29 GMT

> Hi,
>
[quoted text clipped - 11 lines]
> Best regards,
> Klaus

Klaus,

Then I did not understand your problem properly, my apologies.

Good luck with it.

Pixel.To.Life.

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Calling unmanaged code from managed MFC worker thread

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