Bdtjtk

I might write no sense on the next lines:

I tried to derive a struct named mainpage from winrt::Windows::UI::Xaml::Controls::Page and pass it to winrt::xaml_typename<>(), in "<>" is mainpage.

It will be understandable when you look at the code:
Point number 1 "(1)" is the mainpage.h file, is very short.
Point number 2 "(2)" is the App.cpp file, it only has the necessary.

//(1) mainpage.h



#pragma once



#include "pch.h"



struct mainpage : winrt::Windows::UI::Xaml::Controls::Page

{

    mainpage() {



        winrt::Windows::UI::Xaml::Controls::Button thebutton = winrt::Windows::UI::Xaml::Controls::Button();

        thebutton.Click([&](const IInspectable& sender, const winrt::Windows::UI::Xaml::RoutedEventArgs& event_arg)

        {

            thebutton.Content(winrt::box_value(L"Clicked"));

        });

    }

};



//(2) App.cpp



#include "pch.h"

#include "mainpage.h"



struct App : winrt::Windows::UI::Xaml::ApplicationT<App>

{

    void OnLaunched(const winrt::Windows::ApplicationModel::Activation::LaunchActivatedEventArgs& event_arg)

    {

        winrt::Windows::UI::Xaml::Controls::Frame root_frame{ nullptr };

        winrt::Windows::UI::Xaml::UIElement content = winrt::Windows::UI::Xaml::Window::Current().Content();

        if (content)

        {

            root_frame = content.try_as<winrt::Windows::UI::Xaml::Controls::Frame>();

        }



        // Do not repeat app initialization when the Window already has content,

        // just ensure that the window is active

        if (root_frame == nullptr)

        {

            // Create a Frame to act as the navigation context and associate it with

            // a SuspensionManager key

            root_frame = winrt::Windows::UI::Xaml::Controls::Frame();



            root_frame.NavigationFailed({ this, &App::OnNavigationFailed });



            if (event_arg.PreviousExecutionState() == winrt::Windows::ApplicationModel::Activation::ApplicationExecutionState::Terminated)

            {

                // Restore the saved session state only when appropriate, scheduling the

                // final launch steps after the restore is complete

            }



            if (event_arg.PrelaunchActivated() == false)

            {

                if (root_frame.Content() == nullptr)

                {

                    // When the navigation stack isn't restored navigate to the first page,

                    // configuring the new page by passing required information as a navigation

                    // parameter

                    root_frame.Navigate(winrt::xaml_typename<mainpage>(), box_value(event_arg.Arguments()));

                }

                // Place the frame in the current Window

                winrt::Windows::UI::Xaml::Window::Current().Content(root_frame);

                // Ensure the current window is active

                winrt::Windows::UI::Xaml::Window::Current().Activate();

            }

        }

        else

        {

            if (event_arg.PrelaunchActivated() == false)

            {

                if (root_frame.Content() == nullptr)

                {

                    // When the navigation stack isn't restored navigate to the first page,

                    // configuring the new page by passing required information as a navigation

                    // parameter

                    root_frame.Navigate(winrt::xaml_typename<mainpage>(), box_value(event_arg.Arguments()));

                }

                // Ensure the current window is active

                winrt::Windows::UI::Xaml::Window::Current().Activate();

            }

        }

    }

    void App::OnNavigationFailed(const IInspectable&, const winrt::Windows::UI::Xaml::Navigation::NavigationFailedEventArgs& event_arg)

    {

        throw winrt::hresult_error(E_FAIL, winrt::hstring(L"Failed to load Page ") + event_arg.SourcePageType().Name);

    }

};



int __stdcall wWinMain(HINSTANCE, HINSTANCE, PWSTR, int) {

    winrt::Windows::UI::Xaml::Application::Start((auto &&) { winrt::make<App>(); });

}

If it is not possible, how can I properly create a page?

It is simple if you have installed the CppWinRT VSIX. Then create a "View model" and edit the files. You shouldn't derive the struct from the winrt::Windows::UI::Xaml::Controls::Page class because winrt::xaml_typename<> needs a WinRT class in <>.