Win32 Desktop Applications Calling WinRT APIs on Windows 8: Which Contracts Cross the Boundary

Direct Answer

On Windows 8, unpackaged Win32 desktop processes can call a limited subset of WinRT APIs, but cannot participate in contract-based features like Share. The key distinction is between system-level APIs (accessible) and app-model-dependent APIs (restricted).

Callable from Win32 Desktop

• System services: Windows.System.Diagnostics, basic Windows.Storage (local folders), Windows.ApplicationModel.UserDataAccounts
• Device APIs: Windows.Devices.Enumeration for local hardware, basic Windows.Devices.Geolocation
• Networking: Windows.Networking for hostname resolution, basic connection APIs

Restricted to Packaged Apps

• Contracts: Share, Print, File Open/Save, Search, Device, and Camera contracts
• App lifecycle APIs: Windows.ApplicationModel.Activation, background task registration
• UI components: Windows.UI.Xaml, Windows.UI.ViewManagement

Share Contract Limitation

A plain Win32 executable cannot act as a Share source or target. The contract system requires:

1. Package identity registration in the Windows Store
2. App container execution context
3. Modern UI lifecycle management

Desktop apps must use traditional mechanisms (clipboard, file system) for data sharing.

Full-Screen Transition Behavior

Calling certain WinRT APIs from desktop code may trigger a partial Modern UI transition:

• File pickers: Invoke Windows.Storage.Pickers.FileOpenPicker may show picker UI
• URI launching: Windows.System.LauncherOptions may activate Charms bar
• System settings: Accessing system URIs may open Settings charm

This transition cannot be fully avoided when invoking UI-related WinRT APIs. The desktop process remains the owner, but Windows may temporarily switch to the Modern UI shell context.

Technical Implementation

To call WinRT from Win32 on Windows 8:

1. Use C++/CX with Windows.lib or C# with System.Runtime.InteropServices.WindowsRuntime
2. Initialize WinRT via RoInitialize (C++) or let the runtime auto-initialize (C#)
3. Handle RPC_E_WRONG_THREAD by marshaling to UI thread for UI APIs

Verification test: Run this from a Win32 console app on Windows 8:

#include <windows.h>
#include <winrt/Windows.Storage.h>

int main() {
    auto folder = Windows::Storage::ApplicationData::Current().LocalFolder();
    printf("Local folder path: %ws\n", folder.Path().Data());
    return 0;
}

This should succeed. Attempting to create a ShareProvider will fail with E_ACCESSDENIED.