How do I create a std::filesystem::exists() compatible path from a Windows::Storage::StorageFile?

I am using a FileOpenPicker to get a file (StorageFile^) in a Windows Universal App. That seems to work fine, and if I output the StorageFile->Path->Data(), it returns the expected file path (C:...\MyFile.txt).

When I try to open it and dump the contents to a string things go wrong (the string receives nothing), so as the first step, I was trying to verify the file path with std::filesystem::exists().

To snip it down to the relevant bit:

void MyClass::MyFunction(StorageFile^ InFile)
{
    std::filesystem::path FilePath = InFile->Path->Data();

    if (std::filesystem::exists(FilePath))
    {
        //Do things
    }
}

When I run this, I get an exception:

Exception thrown at 0x7780A842 in MyApp.exe: Microsoft C++ exception: std::filesystem::filesystem_error at memory location 0x039BC480.
Unhandled exception at 0x7AACF2F6 (ucrtbased.dll) in MyApp.exe: An invalid parameter was passed to a function that considers invalid parameters fatal.

Seemingly, the path I attempt to pass into std::filesystem::exists() is invalid.

Any help pointing out where I am going wrong would be very appreciated!

This question had originally been marked as a duplicate of this question however that solution does not seem to work as it requires CLI(?) whilst I think I am using WinRT (however I can not find where in my project or settings to check that beyond having winrt available in includes).

Solution

The key things to remember about StorageFile in the Windows Runtime (using C++/CX or C++/WinRT) is that (a) it's not necessarily a file on disk, and (b) even if it is a file on disk you don't necessary have permissions to open it directly.

The only 'generally safe' pattern you can use for doing traditional file I/O operations on the StorageFile instance you are given by the UWP FilePicker is to create a temp directory copy from it and then parse the temp copy:

C++/CX

#include <ppltasks.h>
using namespace concurrency;

using Windows::Storage;
using Windows::Storage::Pickers;

auto openPicker = ref new FileOpenPicker();
openPicker->ViewMode = PickerViewMode::Thumbnail; 
openPicker->SuggestedStartLocation = PickerLocationId::PicturesLibrary; 
openPicker->FileTypeFilter->Append(".dds"); 

create_task(openPicker->PickSingleFileAsync()).then([](StorageFile^ file)
{
    if (file)
    {
        auto tempFolder = Windows::Storage::ApplicationData::Current->TemporaryFolder;
        create_task(file->CopyAsync(tempFolder, file->Name, NameCollisionOption::GenerateUniqueName)).then([](StorageFile^ tempFile)
        {
            if (tempFile)
            {
                std::filesystem::path FilePath = tempFile->Path->Data();
...
            }
        });
    }
});

C++/WinRT

#include "winrt/Windows.Storage.h"
#include "winrt/Windows.Storage.Pickers.h"

using namespace winrt::Windows::Storage;
using namespace winrt::Windows::Storage::Pickers;

FileOpenPicker openPicker;
openPicker.ViewMode(PickerViewMode::Thumbnail);
openPicker.SuggestedStartLocation(PickerLocationId::PicturesLibrary);
openPicker.FileTypeFilter().Append(L".dds");

auto file = co_await openPicker.PickSingleFileAsync();
if (file)
{
    auto tempFolder = ApplicationData::Current().TemporaryFolder();
    auto tempFile = co_await file.CopyAsync(tempFolder, file.Name(), NameCollisionOption::GenerateUniqueName);
    if (tempFile)
    {
        std::filesystem::path FilePath = tempFile.Path().c_str();
...
    }
}

See Microsoft Docs