Re: Cringeworthy Cast in ATL?? - COM Discussion Boards

Cringeworthy Cast in ATL??

24-May-02 7:16

--->>???? Set TPinger = New Atutils.VPinger4 !!!!
Will not work

soptest

20-May-02 23:23

I have created an ATL COM object in its interface I have a method that receives another interface of the same type and wish to modify some of its properties, however I do not want these properties to be exposed by its interface i.e. equivalent of protected member in C++ class.

I have the following code:

in header:

//this class ultimately derives from IMyComObject so
// I cast a IMyComObject* to CMyComObject*
class ATL_NO_VTABLE CMyComObject :
public CComObjectRootEx<ccomsinglethreadmodel>,
public CComCoClass<cmycomobject, &clsid_mycomobject="">,
public IDispatchImpl<imycomobject, &iid_imycomobject,="" &libid_mylib="">
{

....

protected:
IMyComObjectPtr m_pConnectedTo;

}

in method implementation:

STDMETHODIMP CMyComObject::ConnectTo(IMyComObject *pMyComObject)
{
AFX_MANAGE_STATE(AfxGetStaticModuleState())

IMyComObjectPtr pMyComObjectPtr = pMyComObject;

if ( m_pConnectedTo )
Disconnect();

if ( pMyComObjectPtr->IsConnected )
pMyComObjectPtr->Disconnect();

//////////this is the 'nasty' part it does work but I dont like it!!
//////////is there a more elegant way to achieve this or is what I am doing OK?
CMyComObject* pMyBodgeCast = dynamic_cast<cmycomobject*>(pMyComObject);
pMyBodgeCast->m_pConnectedTo = this;
//////////////////////////////////////////////////////////////////////////

m_pConnectedTo = pMyComObject;

return S_OK;
}

If anyone has any comments I would really like to hear them. Thanks.

Vivek Rajan21-May-02 6:50

21-May-02 6:50

Anonymous wrote:
//////////this is the 'nasty' part it does work but I dont like it!!
//////////is there a more elegant way to achieve this or is what I am doing OK?
CMyComObject* pMyBodgeCast = dynamic_cast(pMyComObject);
pMyBodgeCast->m_pConnectedTo = this;
//////////////////////////////////////////////////////////////////////////

Hi -

What you are doing is clearly bad !! I think casting an interface to an implementation goes against the very princple of COM (separation of impl and interface).
It works for you because
(a) you have an INPROC DLL, so there is no marshalling
(b) you guarantee that the only implementaton of IMYComObject is CMyCOMObject.

I had a similar problem when I wanted prvate interfaces. I think this is a major limitation of COM interfaces. You can use the [hidden] or [restricted] keywords in IDL. But that wouldnt prevent C++ users from using it. If you dont include it in the typelib,or register it, then it wont be marshalled across apartments. The solution I used is kludgy, have a private token or key as a parameter for each of the interface methods. So, the private interface would be visible, but not usable by others because they dont know the secret key.

Here is how you might do this.
1) Specify the [hidden] or [restricted] keyword in the IDL for the IMyComObjectPrivateUse, so VB cant use it.
2) Create a private interface IMyComObjectPrivateUse, add the method SetConnectedObject(DWORD token,IMyComObject *)
3) Implement IMyComObjectPrivateUse in CMyComObject, the method looks like

// IMyComObjectPrivateUse (For Private Use Only)
// Depending on your need for security you can have a 
// fancy authentication scheme if you wish
STDMETHODIMP CMyComObject::SetConnectedObject(DWORD token,IMyComObject *pObj)
{
   if(token!=0xfeedf00d) {  // 0xfeedf00d is your secret key
     return E_NOTIMPL;
   }
   pObj->AddRef();
   m_pConnectedTo=pObj;
   return S_OK;
}

4) In your ConnectTo implementation, just QI for IMyComObjectPrivateUse and call the set method with the correct key

IMyComObjectPrivateUse * pPriv;
pMyComObject->QI(IID_IMYComObjcetPrivateUse,(LPVOID*)&pPriv)
pPriv->SetConnected(0xfeedf00d,pMyComObject);

I would love to hear anyone who has other ideas. I am currently using the above mechanism.

Hope that helps -
Vivek

soptest21-May-02 7:41

21-May-02 7:41

Just declare implemented interface IMyInterface *_pi;
then assign incomming to _pi
_pi = pIn;
If incomming interface is different then you need just typecast incomming interface. (COM object must implement all of those interfaces)

And that is it. You do not need to do QueryInterface because vtlb has all pointers to all methods of all implemented interfaces.

P.S. Typecasting interface to class which implements this interface is bad idea (because the class may have other functions implemented which are not exposed by implemented interface/s, and data members), never do this.

soptest

Vivek Rajan21-May-02 9:08

21-May-02 9:08

soptest wrote:
then assign incomming to _pi
_pi = pIn;
If incomming interface is different then you need just typecast incomming interface. (COM object must implement all of those interfaces)

This approach is not recommended because, it wouldnt work for aggregated & tearoff interfaces.

Thanks-
Vivek

soptest21-May-02 10:32

21-May-02 10:32

It has nothing to do with it. COM object does not implement them through vtlb, so it is the same thing as this example:

struct _tearoffObj{};
struct _A
{
    void GetObj(void** pObj)
    {
        *pObj = new _tearoffObj();
    }
}

_A does not implement _tearoffObj.

So the statement "_pi = pIn;" is 100% correct.

soptest

Vivek Rajan21-May-02 10:53

21-May-02 10:53

soptest wrote:

So the statement "_pi = pIn;" is 100% correct.

Sorry, that wont work.

If CoClass A implements 20 interfaces(10 interface via vtbl and 10 via tear off), you cannot get to the tear offs by assignment, you must use a QI. Same thing for aggregation.

In the world of COM, getting from one interface to another requires you to use QI. It is possible to ignore that rule and typecast, but that is asking for trouble and very poor COM practice.

soptest21-May-02 12:10

21-May-02 12:10

Blah blah blah.
When your COM object provides tearoff interface/s that object is not based on that interface.
Vtbl is dynamicaly created for that tearoff object.

*** STOP: 0x0000001E (0xC0000005,0x8016A950,0x00000001,0x00000086)
KMODE_EXCEPTION_NOT_HANDLED*** Address 8016a950 has base at 80100000 - ntoskrnl.exe

CPUID:x86 5.6.2 irql:1f SYSVER 0xf0000565

Dll Base DateStmp - Name Dll Base DateStmp - Name
80100000 337546bf - ntoskrnl.exe 80010000 33247f88 - hal.dll
80001000 334d3a53 - atapi.sys 80007000 33248043 - SCSIPORT.SYS
801d7000 336016a2 - Disk.sys 801db000 336015af - CLASS2.SYS
801df000 3356d637 - Ntfs.sys 80237000 344eeb44 - Siwvid.sys
8056e000 344eebdc - NTice.sys f1f48000 31ec6c8d - Floppy.SYS
f1f58000 31ec6ca1 - Cdrom.SYS f228c000 31ec6c99 - Null.SYS
f208c000 31ed868b - KSecDD.SYS f2290000 335e60cf - Beep.SYS
f1f88000 335bc82a - i8042prt.sys f2094000 3324806f - mouclass.sys
f209c000 31ec6c94 - kbdclass.sys f229e000 3373c39d - ctrl2cap.SYS
f1fa0000 33248011 - VIDEOPRT.SYS fe1a4000 349a9c93 - mga64.sys
f20cc000 31ec6c6d - vga.sys f1eb0000 332480dd - Msfs.SYS
f1d50000 332480d0 - Npfs.SYS fe164000 3356da41 - NDIS.SYS
f2124000 3593d4f4 - bluesave.SYS fe141000 335bd30e - Fastfat.SYS
a0000000 336157ac - win32k.sys fe0c2000 349a9cdd - mga64.dll
f1ce0000 332483b0 - Cdfs.SYS fdca2000 31ec6e6c - TDI.SYS
fdc59000 31ed0754 - nbf.sys fdc35000 337390ef - tcpip.sys
fdc18000 3362a53a - netbt.sys f1f68000 33644efb - ibmfent.sys
f1d70000 334d3add - afd.sys f2008000 33248371 - netbios.sys
f207c000 31ec6c9b - Parport.SYS fdc14000 31ec6c9b - Parallel.SYS
f2136000 31ec6c9d - ParVdm.SYS f1dd0000 332480ab - Serial.SYS
fdbaf000 3339777c - rdr.sys fdb9e000 332483b5 - mup.sys
fdaec000 3360f103 - srv.sys

Address dword dump Build [1381] - Name
f206fba4 8016a950 8016a950 00000001 00000086 00000086 00000086 - ntoskrnl.exe
f206fbd0 80115d86 80115d86 00000000 ff87a400 ff87a3f0 ff810408 - ntoskrnl.exe
f206fbec 8019d98f 8019d98f ffffffff 80139a54 80143378 00000000 - ntoskrnl.exe
f206fbf4 80139a54 80139a54 80143378 00000000 230edb30 3c8ca3b0 - ntoskrnl.exe
f206fbf8 80143378 80143378 00000000 230edb30 3c8ca3b0 26448761 - ntoskrnl.exe
f206fc10 8011bd6a 8011bd6a ff676980 ffffffff 34f9ad10 00000000 - ntoskrnl.exe
f206fc38 8011bcce 8011bcce ff676980 ffffffff 34f9ad10 00000000 - ntoskrnl.exe
f206fc60 8016a94e 8016a94e f206fee8 00000000 00000086 f206fe70 - ntoskrnl.exe
f206fc74 8016a950 8016a950 00000008 00000346 f206fe48 00000010 - ntoskrnl.exe
f206fc88 801163e3 801163e3 ff876f58 ff676980 ffffffff 00000000 - ntoskrnl.exe
f206fca4 f1f88825 f1f88825 ff876f58 ff676980 ffffffff ff876f38 - i8042prt.sys
f206fcc8 80117f93 80117f93 ff6c7328 ff876bf0 00000000 801126af - ntoskrnl.exe
f206fcd8 801126af 801126af 00000246 80112717 ff6c7328 00000103 - ntoskrnl.exe

soptest

NotYourAverageGuy6-Jun-02 9:13

NotYourAverageGuy

6-Jun-02 9:13

Cute, you frigging moron...

21-May-02 22:15

Thanks for the suggestion. I had thought of doing something similar myself, I am fairly new to ATL / COM programming and I find it all very slow going and the lack of good examples (that I could find anyway) make things even harder, all of the books I have read give only very simple example code....

moan
moan

Confused | :confused:

Vivek Rajan23-May-02 16:33

23-May-02 16:33

Another tip, if you can be sure that all access is within the same apartment (usually this means no inter-thread or inter-process access), add the [local] attribute to your IDL.

I know how you are feeling, the trick is to not give up. The road to COM enlightenment is mainly via hard-knocks Frown | :-(

HTH
Vivek

COM+ deployment help

Annesa20-May-02 2:25

Annesa

20-May-02 2:25

i have deployed a component in COM+ under windows 2000 server. I have created application proxy to run it from the client PC. The application Proxy works fine if the OS is Windows 2000 professional. Though the installation of application proxy succeeds, in Windows 98 and Windows NT work station, i get an error "ActiveX component cannot create object". Kindly help. Please to ganesh@curans.co.in

Re: COM+ deployment help

soptest20-May-02 9:40

drag and drop destination

20-May-02 9:40

what is the HRESULT?
Check access permissions, use dcomcnfg.exe utility

soptest

20-May-02 0:39

Hi,

Does anyone know how to return the destination path of a file after a drag-drop operation?

I am writing a shell extension using C++/COM. From within the shell extension I load all removable media drives and can view files on the media. When I drag a file from the shell extension to say the desktop, I need to get this destination path? Any ideas....?

Thanks very much

Anonymous Smile | :)

Re: drag and drop destination

soptest20-May-02 9:31

20-May-02 9:31

implement this method in your Drag-and-Drop handler:

IShellExtInit::Initialize(LPSHELLEXTINIT psxi,
		LPCITEMIDLIST pidlFolder,
		LPDATAOBJECT pdtobj,
		HKEY hkeyProgID)
{
    char szTargetFolder[MAX_PATH];
    if (pidlFolder) 
    {
	SHGetPathFromIDList(pidlFolder, szTargetFolder);
    }
}

soptest

Cannot expose interface

Hans Ruck19-May-02 21:51

Hans Ruck

19-May-02 21:51

I try to expose an interface using its own coclass in an MFC application with added ATL support.
Unfortunately, it doesn't seem to work in VB. The interface is visible, but it cannot be instantiate.

Do you know what is to be done?

rechi

soptest20-May-02 9:36

20-May-02 9:36

what do you mean "interface cannot be instantiate"?

P.S. You must use types recognized by VB. What is the error? Show the IDL of that interface.

soptest

Hans Ruck20-May-02 22:44

Hans Ruck

20-May-02 22:44

The problem is now half-solved. It was about registering the interface in the registry. To do this, i've added _Module.RegisterServer(TRUE) in the InitInstance method. It works now, but when creating the object, a new instance of the MFC application is started to act as server, even if another one is already running.
Do you know how to make the running one be the server also?

rechi

Vi221-May-02 1:06

Vi2

21-May-02 1:06

On ATL ActiveX EXE-server, such strategy (when creating the object, a new instance of the application is started) is realized by

<br />
        hRes = _Module.RegisterClassObjects(CLSCTX_LOCAL_SERVER, REGCLS_SINGLEUSE);<br />

instead of

<br />
        hRes = _Module.RegisterClassObjects(CLSCTX_LOCAL_SERVER, REGCLS_MULTIPLEUSE);<br />

I do not know a MFC-server, but something like this take a place.

With best wishes,
Vita