Is there anyway obtaining the window handle in pages with C++/WinRT?

[sxlllslgh]

My window contains a root frame to switch pages:

<Window
    x:Class="MyApp.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MyApp"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">
    <Frame x:Name="rootFrame" />
</Window>

For a page:

<Page
    x:Class="MyApp.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MyApp"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">
    <Grid>
        <Button x:Name="readFileButton" Click="chooseFile">Select a file</Button>
    </Grid>
</Page>

Now I want to add a function for the button to open the FilePicker dialog, which need to pass the window handle to it:

IAsyncAction MainPage::chooseFile() {
    FileOpenPicker picker;

    auto initializeWithWindow{ picker.as<IInitializeWithWindow>() };
    // Here need to pass the window handle
    initializeWithWindow->Initialize(windowHandle);

    picker.FileTypeFilter().Append(L"*.jpg");
    this->selectedFile = co_await picker.PickSingleFileAsync();
}

The problem is using the follow code to retrieve the handle only works in window class:

auto windowNative{ this->try_as<IWindowNative>() };
HWND windowHandle{ 0 };
windowNative->get_WindowHandle(&windowHandle);

In the page class, the windowHandle is null. How to resolve this problem?
One way I am trying is to add a property in MIDL:

runtimeclass MainPage : Microsoft.UI.Xaml.Controls.Page {
    MainPage();
    HWND WindowHandle;
}

But there is no HWND type in MIDL, any one knows how to deal with it?

[castorix]

You can set winrt::Microsoft::UI::Xaml::Window window{ nullptr }; in App.xaml.h as public instead of private
Then you can access it globally from Application::Current() (with try_as < App >() )

[sxlllslgh]

Thank you very much for your answer! It did work!
But I still wonder why Page could not access its host Window instance, hope someone can tell me.