Getting the value of OpenSavePidlMRU in the registry - c++

Question

I'm trying to get the last opened directory by an open file dialog, and it seems that we can get it by first retrieving the key's name that contains the path using the first BYTE of the MRUListEx key, then the path can be obtained by reading the value of this key's name.
MRUListEx key can be find at: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32\LastVisitedPidlMRU.
The problem is that I don't know how to properly fill the "SHITEMID" structure. (it doesn't let me access the index, and it throws a Memory access violation). I don't even know if the code below is valid in any points.
Sorry, the code is very dirty for the moment but I'll revamps it when I finally find what causes these errors.
void MyClass::OnDocumentSave(bool showSaveDlg)
{
// Do something that is not relevant here...
try {
HKEY hKey = NULL;
std::wstring regPath = LR"(Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32\LastVisitedPidlMRU\)";
LSTATUS statusCode = RegOpenKeyExW(HKEY_CURRENT_USER, regPath.data(), 0, KEY_QUERY_VALUE, &hKey);
if (statusCode != ERROR_SUCCESS)
throw std::exception(std::string("Unable to open the specified registry key, sys err code: ") + std::to_string(statusCode));
BYTE data[MAX_PATH];
DWORD bufferSize = MAX_PATH;
statusCode = RegGetValueW(hKey, L"", L"MRUListEx", RRF_RT_REG_BINARY, NULL, &data, &bufferSize);
if (statusCode != ERROR_SUCCESS)
throw std::runtime_error(std::string("Failed at RegGetValue() Sys error code: ") + std::to_string(statusCode));
// Please note that the buffer has intentionally a fixed size here and everything is
// simplified for readability, but it uses dynamic memory allocation in the real code to
// handle errors such as ERROR_MORE_DATA
BYTE* pathData[512];
bufferSize = 512;
DWORD type = 0; // In case it matters, the returned value is 3
statusCode = RegGetValueW(hKey, L"", std::to_wstring(data[0]).c_str(), RRF_RT_REG_BINARY, &type, &pathData, &bufferSize);
if (statusCode != ERROR_SUCCESS)
throw std::runtime_error(std::string("Failed at RegGetValue() Sys error code: ") + std::to_string(statusCode));
// I don't know how to fill this structure, the documentation is very minimal,
// and I don't understand it.
int offset = sizeof(APPNAME);
SHITEMID shellIDList[2]{
// Throw a memory access violation at 0x*****, the debugger can't seems to
// get anything in pathData.
{ sizeof(USHORT) + sizeof(pathData), *pathData[0 + offset] },
{ 0, 0 } };
ITEMIDLIST idl{ shellIDList[0] };
// This is supposed give me the last path that was opened by a File Picker.
SHGetPathFromIDListW(&idl, initialDir.data());
}
catch (std::exception& e) {
// Silently set the initial directory to a hard-coded path instead of getting the registry value.
}
}

Answer 1

You don't seem to understand how types and simple arrays work! BYTE* pathData[512]; is not a 512 byte buffer. Use BYTE pathData[512];.
After reading into pathData, call SHGetPathFromIDList((PCIDLIST_ABSOLUTE) pathData, ...);.
That being said, that ComDlg32 key is undocumented and I don't think it stores a pidl so even if your code is corrected it is not going to work.
EnumMRUListW is a documented function you can call but it is not going to help you decode the data.
It looks to me like the location might be prefixed with the name of the .exe so as a minimum you would have to skip (lstrlenW(pathData)+1)*2 bytes before you find the real data...

Answer 2

If you're going for undocumented stuff, here is a code that can dump an MRU list like the LastVisitedPidlMRU one:
// the Shell object that can read MRU lists
static GUID CLSID_MruLongList = { 0x53bd6b4e,0x3780,0x4693,{0xaf,0xc3,0x71,0x61,0xc2,0xf3,0xee,0x9c} };
typedef enum MRULISTF
{
MRULISTF_USE_MEMCMP = 0x0,
MRULISTF_USE_STRCMPIW = 0x1,
MRULISTF_USE_STRCMPW = 0x2,
MRULISTF_USE_ILISEQUAL = 0x3,
};
typedef int (__stdcall *MRUDATALISTCOMPARE)(const BYTE*, const BYTE*, int);
MIDL_INTERFACE("00000000-0000-0000-0000-000000000000") // unknown guid but we don't care
IMruDataCompare : public IUnknown
{
public:
virtual HRESULT CompareItems(const BYTE*, int, const BYTE*, int) = 0;
};
MIDL_INTERFACE("d2c22919-91f5-4284-8807-58a2d64e561c")
IMruDataList2 : public IUnknown
{
public:
virtual HRESULT InitData(UINT uMax, MRULISTF flags, HKEY hKey, LPCWSTR pszSubKey, MRUDATALISTCOMPARE pfnCompare) = 0;
virtual HRESULT AddData(const BYTE* pData, DWORD cbData, DWORD* pdwSlot) = 0;
virtual HRESULT InsertData(const BYTE*, DWORD cbData, int* piIndex, DWORD* pdwSlot) = 0;
virtual HRESULT FindData(const BYTE* pData, DWORD cbData, int* piIndex) = 0;
virtual HRESULT GetData(int iIndex, BYTE* pData, DWORD cbData) = 0;
virtual HRESULT QueryInfo(int iIndex, DWORD* pdwSlot, DWORD* pcbData) = 0;
virtual HRESULT Delete(int iIndex) = 0;
virtual HRESULT InitData2(UINT uMax, MRULISTF flags, HKEY hKey, LPCWSTR pszSubKey, IMruDataCompare* pfnCompare) = 0;
};
int main()
{
CoInitialize(NULL);
{
CComPtr<IMruDataList2> mru;
if (SUCCEEDED(mru.CoCreateInstance(CLSID_MruLongList)))
{
const int max = 100; // get max 100 entries
mru->InitData(max, MRULISTF_USE_MEMCMP, HKEY_CURRENT_USER, L"Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\ComDlg32\\LastVisitedPidlMRU\\", nullptr);
for (auto i = 0; i < max; i++)
{
DWORD slot;
DWORD size;
// get size
if (FAILED(mru->QueryInfo(i, &slot, &size)))
continue;
// get data
// note beginning is a LPWSTR containing exe data
auto data = (LPBYTE)_alloca(size);
if (FAILED(mru->GetData(i, data, size)))
continue;
// the rest is a PIDL
auto pidl = (LPCITEMIDLIST)(data + (lstrlen((LPWSTR)data) + 1) * 2);
// get the shell item
CComPtr<IShellItem> item;
if (SUCCEEDED(SHCreateItemFromIDList(pidl, IID_PPV_ARGS(&item))))
{
// get its path
CComHeapPtr<WCHAR> path;
item->GetDisplayName(SIGDN::SIGDN_DESKTOPABSOLUTEPARSING, &path);
wprintf(L"Executable: %s LastVisited: %s\n", data, path);
}
}
}
}
CoUninitialize();
}
PS: I'm using ATL's smart classes

Answer 3

I finally figured out how to create an ITEMIDLIST structure, and it all works well.
Explanations:
After some test, I've concluded that this value is not necessarily updated. The application that uses a file picker needs to write to the registry to reflect changes.
For example, VS Community 2022 / VS Code updates this value, but the new Microsoft Notepad does not (not sure for the old one). Apparently, .NET applications seem to be updated automatically, though.
First, we obviously must open the key at Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32\LastVisitedPidlMRU.
The stored data value we need is on one of the values, which has a number as a name.
The first byte of the MRUListEx value lies the last path.
HKEY hKey = NULL;
std::wstring regMRUPath = LR"(Software\Microsoft\Windows\CurrentVersion\Explorer\ComDlg32\LastVisitedPidlMRU\)";
LSTATUS statusCode = RegOpenKeyExW(HKEY_CURRENT_USER, regMRUPath.data(), 0, KEY_QUERY_VALUE, &hKey);
if (statusCode != ERROR_SUCCESS)
// Handle error
BYTE data[MAX_PATH];
DWORD bufferSize = MAX_PATH;
statusCode = RegGetValueW(hKey, L"", L"MRUListEx", RRF_RT_REG_BINARY, NULL, &data, &bufferSize);
if (statusCode != ERROR_SUCCESS)
// Handle error
std::wstring keyName = std::to_wstring(data[0]);
Now that we have the value, we can get its data.
DWORD reqBuffSize = 0;
statusCode = RegQueryValueExW(hKey, keyName.data(), NULL, NULL, NULL, &reqBuffSize);
if (statusCode != ERROR_SUCCESS)
// maybe the specified key is invalid or missing.
BYTE pathData[reqBuffSize];
statusCode = RegGetValueW(hKey, L"", keyName.data(), RRF_RT_REG_BINARY, NULL, &pathData, &reqBuffSize);
if (statusCode != ERROR_SUCCESS)
// maybe the specified key is invalid or missing.
We have another problem: The path is not directly accessible. The executable name is prefixed from the real path, so we must remove it.
Actually, it's very easy. The delimiter between the executable name and the real path is \0\0\0.
const size_t offset = (lstrlenW(reinterpret_cast<LPCWSTR>(pathData)) * sizeof(wchar_t))
+ (2 * sizeof(wchar_t));
The obtained value is not human-readable, but it actually stores a PIDL, so we can get it with SHGetPathFromIDList. The ITEMIDList is not very well documented on the Microsoft's website, but here's a way to create one of these things:
IMalloc* pMalloc;
if (FAILED(SHGetMalloc(&pMalloc)))
// handle error, failed to get a pointer to the IMalloc interface.
PIDLIST_ABSOLUTE pIdl = (PIDLIST_ABSOLUTE)pMalloc->Alloc((reqBuffSize - offset) + 1);
pIdl->mkid.cb = (sizeof(USHORT) + (reqBuffSize - offset));
memcpy_s(pIdl->mkid.abID, (reqBuffSize - offset) + 1, pathData + offset, (reqBuffSize - offset) + 1);
PIDLIST_ABSOLUTE pNext = ILGetNext(pIdl);
if (!pNext)
// handle error, failed to get a pointer to the next item
pNext->mkid.cb = 0;
initialDir.reserve(MAX_PATH * 2);
The ITEMIDList structure has two members: cb and abID. cb is the size of a USHORT + the size of the data, and abID is a pointer to the data itself.
The last ITEMIDList struct must have cb set to zero to mark the end. ILGetNext gives us a pointer to the next element.
// Finally, return the path!
std::wstring retValue;
retValue.reserve(MAX_PATH);
if (FALSE == SHGetPathFromIDList(pIdl, initialDir.data())) {
// oh no! free the allocated memory and throw an exception
}
pMalloc->Free(pIdl);
It's not as hard as it seems to be at the beginning, but if you want to get this value for use with a file picker, it's recommended to use COM instead since it's easier and less error-prone. Thank tou all for tour response and have a good day!
If you want to get the path from a specified extension, see the OpenSavePidlMRU key. It works the same, but it requires some little adjustments.