How can a desktop app use a Windows Runtime object that infers UI context from its thread? The IInitializeWithWindow pattern

Many objects in the Windows Runtime can be used from desktop apps. For today’s example, we’ll use the File­Open­Picker. This is a rather artificial example because you could just use the IFile­Dialog interface to get equivalent functionality in a desktop app, but I just picked it for use as an example.

Start with our scratch program and make these changes:

#include <winrt/windows.storage.pickers.h>

winrt::Windows::Foundation::IAsyncAction
ShowFilePickerAsync(HWND hwnd)
{
    auto picker = winrt::Windows::Storage::Pickers::FileOpenPicker();
    picker.FileTypeFilter().Append(L".jpg");
    auto file = co_await picker.PickSingleFileAsync();
}

winrt::fire_and_forget OnChar(HWND hwnd, TCHAR ch, int cRepeat)
{
    co_await ShowFilePickerAsync(hwnd);
}

// Add to WndProc
    HANDLE_MSG(hwnd, WM_CHAR, OnChar);

Run this program and press a key. The program will crash because the File­Open­Picker looks for a Core­Window on the current thread to serve as the owner of the dialog. But we are a Win32 desktop app without a Core­Window.

The solution is to use the IInitialize­With­Window interface. Many Windows Runtime objects which infer the Core­Window from the current thread support the IInitialize­With­Window interface to allow a Win32 desktop app to specify an explicit window.

Make the following changes to the program:

#include <shobjidl.h>
#include <winrt/windows.storage.pickers.h>

winrt::Windows::Foundation::IAsyncAction
ShowFilePickerAsync(HWND hwnd)
{
    auto picker = winrt::Windows::Storage::Pickers::FileOpenPicker();
    picker.as<IInitializeWithWindow>()->Initialize(hwnd);
    picker.FileTypeFilter().Append(L".jpg");
    auto file = co_await picker.PickSingleFileAsync();
}

This time, the File Open dialog opens because we explicitly provided a window handle to use as the owner.

The IInitialize­Window­Window pattern is used mostly in the case where an object is simply constructed. There is another pattern for the case where an object is obtained by calling a method: The interop pattern, which I covered some time ago.

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