Showcases how to get the handle (HWND) of an UWP app window

AppWindowHwnd.cpp

__interface __declspec(uuid("b74ea3bc-43c1-521f-9c75-e5c15054d78c")) IApplicationWindow_HwndInterop : IInspectable
{
	// <summary>
	// Gets the window handle of this AppWindow instance as a bit-blitted WindowId.
	// </summary>
	// <remarks>To get the actual ID, use [CoreAppWindowPreview::GetIdFromWindow](https://docs.microsoft.com/en-us/uwp/api/windows.ui.core.preview.coreappwindowpreview.getidfromwindow?view=winrt-22621). This method returns the handle (HWND) of the window.
	HRESULT __stdcall get_WindowHandle(Windows::UI::WindowId* windowId) noexcept;
};

using IsWindow = int(__stdcall*)(HWND hwnd);

// Assumes that the variable appWindow represents an handle to Windows::UI::WindowManagement::AppWindow (https://docs.microsoft.com/en-us/uwp/api/windows.ui.windowmanagement.appwindow?view=winrt-22000).
Windows::UI::WindowId windowId;
IApplicationWindow_HwndInterop* appWindowInterop;
__abi_ThrowIfFailed(reinterpret_cast<IUnknown*>(appWindow)->QueryInterface<IApplicationWindow_HwndInterop>(&appWindowInterop));
__abi_ThrowIfFailed(appWindowInterop->get_WindowHandle(&windowId));

// Check if the returned window handle is valid.
auto module = LoadLibraryA("User32.dll");
auto func = reinterpret_cast<IsWindow>(GetProcAddress(module, "IsWindow"));
bool isValid = func(reinterpret_cast<HWND>(windowId.Value)) != false;
FreeLibrary(module);
appWindowInterop->Release();

Special thanks to @ahmed605 for helping out.

C#?

[ComImport, Guid("B74EA3BC-43C1-521F-9C75-E5C15054D78C"), InterfaceType(ComInterfaceType.InterfaceIsIInspectable)]
public interface IApplicationWindow_HwndInterop
{
  Windows.UI.WindowId WindowHandle { get; }
}

C#?

[ComImport, Guid("B74EA3BC-43C1-521F-9C75-E5C15054D78C"), InterfaceType(ComInterfaceType.InterfaceIsIInspectable)]
public interface IApplicationWindow_HwndInterop
{
  Windows.UI.WindowId WindowHandle { get; }
}

That looks right.

Consider also https://docs.microsoft.com/en-us/windows/win32/api/corewindow/nn-corewindow-icorewindowinterop which is fully supported.

As a caution - Windows::UI::WindowId may not always be a bit-blitted HWND - WinAppSDK has methods to convert from HWND to WindowId, like https://docs.microsoft.com/en-us/windows/windows-app-sdk/api/win32/microsoft.ui.interop/nf-microsoft-ui-interop-getwindowidfromwindow for ABI consumers and https://docs.microsoft.com/en-us/windows/apps/api-reference/cs-interop-apis/microsoft.ui/microsoft.ui.win32interop for C#.

Consider also https://docs.microsoft.com/en-us/windows/win32/api/corewindow/nn-corewindow-icorewindowinterop which is fully supported.

As a caution - Windows::UI::WindowId may not always be a bit-blitted HWND - WinAppSDK has methods to convert from HWND to WindowId, like https://docs.microsoft.com/en-us/windows/windows-app-sdk/api/win32/microsoft.ui.interop/nf-microsoft-ui-interop-getwindowidfromwindow for ABI consumers and https://docs.microsoft.com/en-us/windows/apps/api-reference/cs-interop-apis/microsoft.ui/microsoft.ui.win32interop for C#.

#1: This is for AppWindow, not CoreWindow
#2: Windows App SDK doesn't support UWP.

@jonwis My bad, I confused the two, it looks like this interface is an undocumented one to get the handle (not ID) of an UWP AppWindow but for some reason, it seems to return the HWND as a bit-blitted Windows.UI.WindowId (maybe because it's a WinRT interface, not classic COM so HWND type isn't available?). I searched a bit and turned out CoreAppWindowPreview::GetIdFromWindow but as an integer. I'll update the description and comments in the snippet.