co_await expression needs await_ready function - c++

Question

I have a Win32 programm, where I want to add some winRT calls. Among other things I want to open a file without a GUI interface.
I use the async file open call from the StorageFile class, because the next call needs an IStorageFile interface.
#include <roapi.h>
#include <winrt/Windows.Storage.h>
#include <winrt/Windows.Foundation.h>
void openFile()
{
using namespace winrt;
using namespace winrt::Windows::Foundation;
using namespace winrt::Windows::Storage;
HRESULT rtn = RoInitialize(RO_INIT_MULTITHREADED);
winrt::hstring path{ L"C:\\Users...\\mytextfile.txt"};
//wait for open the file
auto file = co_await StorageFile::GetFileFromPathAsync(path);
//IStorageFile interface needed
}
int main()
{
openFile();
return 0;
}
At the moment, the compiler complains that the co_await expression requires a suitable "await_ready" function and none was found.
I`m not sure if this is the case due to a missing header include or if "co_await" can not be used within a win32 application.
Edit:
My visual studio project setup is:
- use c++17, add cppwinrt.exe to my include directories, link against windowsapp.lib and use windows sdk version 10.0.17134.0.

Answer 1

The problem is that the openFile() function does not have the proper return type to handle co_await.
See the research and work that went into the answer I created for C++11 threads to update MFC application windows. SendMessage(), PostMessage() required? which contains a list of recommendations for various approaches to coroutines.
This question was about using C++/WinRT with MFC but the material also applies with WinAPI.
See as well synchronizing SDK with Windows 10 update and using WinRT with Standard C++ which contains a simple console application example using the Web Syndication async functionality to retrieve a list of URLs from an RSS feed. There are a number of links to documentation, some of which is a bit out dated now.
Addendum: Sample Console application
I created the following simple console application using Visual Studio 2017. I created the text file and then ran this in the debugger. I then renamed the text file and ran it again in the debugger and an exception was thrown since the file with that name no longer existed.
See also C++/WinRT, part of Windows SDK 17134 is not compatible with Visual Studio 15.8 Preview 3 which describes a compiler option you may need to change. I did.
// console_winrt.cpp : This file contains the 'main' function. Program execution begins and ends there.
//
// Requires the following changes to the project properties in the C++ compiler section.
// - C++ language standard must be set to C++17
// - Add /await to the Additional options
#include "pch.h"
#pragma comment(lib, "windowsapp")
#include <winrt/Windows.Storage.h>
#include <winrt/Windows.Foundation.h>
#include <iostream>
winrt::Windows::Foundation::IAsyncAction openMyFile()
{
winrt::hstring path{ L"D:\\Users\\rickc\\mytextfile.txt" };
//wait for open the file
auto file = co_await winrt::Windows::Storage::StorageFile::GetFileFromPathAsync(path);
//IStorageFile interface needed
auto xDate = file.DateCreated();
std::cout << "file was found " << std::endl;
}
int main()
{
// initialize the WinRT apartment.
winrt::init_apartment();
auto x = openMyFile();
// wait on the file access since that is all we are doing and we need to give it time.
x.get();
return 0;
}
I used the following properties settings.