How to tell if there's no default file extension association on OS post-Windows XP?

问题

Going back to the days of Windows XP one could use the following code to tell if there's no file association existed for an extension:

TCHAR buffPath[MAX_PATH] = {0};
DWORD dwszBuffPath = MAX_PATH;

HRESULT hR = ::AssocQueryString(
    ASSOCF_NOFIXUPS | ASSOCF_VERIFY, 
    ASSOCSTR_EXECUTABLE,
    _T(".weirdassextension"),
    NULL,
    buffPath,
    &dwszBuffPath);

if(hR != S_OK &&
    hR != E_POINTER)
{
    //Association does not exist
}

But since Windows 8, the AssocQueryString API returns S_OK and buffPath is set to something like C:\WINDOWS\system32\OpenWith.exe if it doesn't find anything.

Is there a better way now to determine that file extension has no Shell association?

PS. I do not want to just compare the file name to OpenWith.exe. What if there's a legit executable called just that... There must be a better way.

回答1:

I think I got it. The trick was to use the correct flags. This seems to work from XP and up:

WCHAR wbuffPath[MAX_PATH] = {0};
DWORD dwszBuffPath = MAX_PATH;
HRESULT hR = ::AssocQueryStringW(ASSOCF_INIT_IGNOREUNKNOWN, 
    ASSOCSTR_EXECUTABLE,
    L".weirdassextension",
    NULL,
    wbuffPath,
    &dwszBuffPath);

if(hR == 0x80070483)   // HRESULT_FROM_WIN32(ERROR_NO_ASSOCIATION)
{
    //The association is missing
}

There's one other trick there, that took me some time to figure out -- DO NOT use AssocQueryStringA(). The shim for AssocQueryStringA() that converts its passed string parameters to Unicode has a bug in XP (and evidently in Vista as well) that will make that API fail on those OS. So, if you do your own ANSI-to-Unicode conversion and call AssocQueryStringW() the problem will go away (Evidently 14 years is not enough time for Microsoft to fix that bug?).

来源：https://stackoverflow.com/questions/35249903/how-to-tell-if-theres-no-default-file-extension-association-on-os-post-windows

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