Thread: COM and IActiveDesktop

Hi all,

Just trying to write an application that plays with the desktop under windows XP. I need to use the COM interface IActiveDesktop and I have found plenty of examples, however I cannot compile them.

I keep getting this error. Searching for this error returns very little information - Anyone got any ideas?

c:\Program Files\Microsoft Visual Studio .NET\Vc7\atlmfc\include\atlcomcli.h(451): error C2787: 'IActiveDesktop' : no GUID has been associated with this object

Cheers

(By the way - that is using XP and Visual C++ Standard 2002).

Thanks - read the article but it doesn't appear to be the same problem.

I cut and pasted the code that Fordy posted as an answer some time ago to a question regarding IActiveDesktop. The code is below copied directly from the other thread.

Code:

#include <windows.h>//Essential stuff
#include <wininet.h>//This has to be here...bug!!!!!!
#include <shlobj.h>//For IActiveDesktop
#include <comdef.h>//For neat VC++ COM Classes



struct COM_INIT{//To manage COM
	COM_INIT(){CoInitialize(0);}
	~COM_INIT(){CoUninitialize();}
}COM_INIT_;

int WINAPI WinMain(HINSTANCE hInst,HINSTANCE,LPSTR,int){
	
	char szFile[MAX_PATH+1] = {0};
	OPENFILENAME ofn = {sizeof(OPENFILENAME)};
	ofn.hInstance = GetModuleHandle(0);
	ofn.lpstrFilter = "BMP\0*.BMP\0JPeg\0*.JPG\0Gif\0*.GIF\0";
	ofn.lpstrFile = szFile;
	ofn.nMaxFile = MAX_PATH+1;
	ofn.lpstrTitle = "Pick Bitmap, JPeg or Gif to replace wallpaper";

	if(!GetOpenFileName(&ofn))return 0;

	try{
		
		WALLPAPEROPT wpo = {sizeof(wpo),WPSTYLE_STRETCH};
		_com_ptr_t<_com_IIID<IActiveDesktop,
			&__uuidof(IActiveDesktop)> > lpIAD;

		lpIAD.CreateInstance(CLSID_ActiveDesktop);	

		lpIAD->SetWallpaper(_bstr_t(szFile),0);

		lpIAD->SetWallpaperOptions(&wpo,0);

		lpIAD->ApplyChanges(AD_APPLY_ALL);


		MessageBox(0,"Wallpaper changed","",MB_OK);

	}
	catch(_com_error& e){
		MessageBox(0,(e.Description().length() ? 
			e.Description() : "COM Error"),0,MB_OK);
	}

	return 0;
}

The following are the errors I get when compiling (I have also added the preprocessor directive as described in the MSDN article.)

main.cpp(29): error C2787: 'IActiveDesktop' : no GUID has been associated with this object
main.cpp(29): error C3203: '_com_IIID' : class template invalid as template argument for template parameter '_IIID', expected a real type
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\comip.h(62): error C2825: '_IIID::Interface': cannot form a qualified name
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\comip.h(62): error C2039: 'Interface' : is not a member of 'operator``global namespace'''
c:\Program Files\Microsoft Visual Studio .NET\Vc7\include\comip.h(62): fatal error C1507: previous user errors and subsequent error recovery halt further compilation

Are there any specific project settings I need to implement here?

I don't think the & should be before the uuidof(). Also try:

Code:

		_com_ptr_t<_com_IIID<IActiveDesktop,
			&IID_IActiveDesktop)> >

Other than that, I'm out of ideas. You need someone who knows what they are talking about.

EDIT: It seems to be already declared in <comdef.h> Try just:

Code:

IActiveDesktopPtr lpIAD;

The __uuidof trick worked on the version of VC++ I was using...It can be handy on a few occasions

try;

_com_ptr_t<_com_IIID<IActiveDesktop, &IID_IActiveDesktop> > lpIAD;

Damn it, from one problem to the next!

Having worked this out in a procedural manner I am attempting to work this out in a OOP fashion.

Unfortunately I keep getting an error 183: Cannot create a file when that file already exists.

Now, I have a class that represents the active desktop.

And just for the sake of testing I have put the following code in the constructor:

Code:

CWallpaper::CWallpaper()
{
	CoInitialize(0);

	HRESULT hr;
	IActiveDesktop *pActiveDesktop;

	//Create an instance of the Active Desktop
	hr = CoCreateInstance(CLSID_ActiveDesktop, NULL, CLSCTX_INPROC_SERVER,
						IID_IActiveDesktop, (void**)&pActiveDesktop);

	WALLPAPEROPT opt;

	opt.dwSize = sizeof(WALLPAPEROPT);
	opt.dwStyle = 2;

	//Insert code to call the IActiveDesktop methods
	pActiveDesktop->SetWallpaper(L"c:\\tmp\\shores.bmp", 0);
	pActiveDesktop->SetWallpaperOptions(&opt, 0);

	pActiveDesktop->ApplyChanges(AD_APPLY_ALL);

	// Call the Release method
	pActiveDesktop->Release();
}

The error is occuring just after the call to CoCreateInstance. Any Ideas, google hasn't been to helpful in this respect..

Cheers.

EDIT: I may have it working, but I am not sure why, can anyone shed any light on what the error message actually means in this context (I have been able to attribute it to many errors, but none that are relevant to this problem)?

CoCreateInstance() and other com apis do not return an error via SetLastError()/GetLastError(). If the thread error number happens to be set after a call to a com function, it does not even mean that the function has failed. You should only rely on the return HRESULT.

In fact this applies to non-com functions. You can only rely on the error number if it is documented that you can do so. (For most functions, but not all, this is only when the function returns failure).

For example a function (eg. CoCreateInstance()) may call CreateFile():

CreateFile documentation:
If the function succeeds, the return value is an open handle to the specified file. If the specified file exists before the function call and dwCreationDisposition is CREATE_ALWAYS or OPEN_ALWAYS, a call to GetLastError returns ERROR_ALREADY_EXISTS (even though the function has succeeded). If the file does not exist before the call, GetLastError returns zero.

So the error number can be set even though the outer function may have succeeded.

Use the SUCCEEDED() and FAILED() macros to check hresults.

P.S I take it you gave up on using smart pointers.

Doh!

[edit]
Actually, just reexamined that project - I actually meant it did not return S_OK - so I am not the idiot that we all thought I was (almost).