Platform entry points

Your App class is shared. Each platform adds one small file that constructs it and hands it to a runner. That is the entire platform surface of a typical application — six files, none longer than a screen.

What a runner does

WindowsApp.Run and its siblings create the window or surface, bring up the graphics backend, call DeviceServices.Initialize with that platform's service implementations, and finish by invoking BaseApp.Init — which is what eventually calls your Create. Your code never constructs a device service and never touches a swap chain.

The six files

Windows — Platforms/Windows/App.xaml.cs

using Season.Platforms.Windows;

public partial class App : Microsoft.UI.Xaml.Application
{
    protected override void OnLaunched(LaunchActivatedEventArgs args)
    {
        WindowsApp.Run(new Hello.App());
    }
}

Linux — Platforms/Linux/Linux.cs

using Season.Platforms.Linux;

internal class Program
{
    static void Main(string[] args) => LinuxApp.Run(new App());
}

Android — Platforms/Android/MainActivity.cs

using Season.Platforms.Android;

[Activity(Theme = "@style/Maui.SplashTheme", MainLauncher = true,
    ScreenOrientation = ScreenOrientation.FullSensor)]
internal class MainActivity : BaseActivity
{
    protected override void OnCreate(Bundle savedInstanceState)
    {
        if (!AndroidApp.IsInitialized)
            AndroidApp.Run(new App());

        base.OnCreate(savedInstanceState);
    }
}

Deriving from BaseActivity is what wires the Android lifecycle and touch events into the engine. The AndroidApp.IsInitialized guard matters because Android recreates activities: a configuration change must not start a second engine.

iOS and Mac Catalyst — Platforms/iOS/Program.cs

using Season.Platforms.iOS;

internal class Program
{
    static void Main(string[] args) => iOSApp.Run(new App());
}

Web — from a Blazor WebAssembly page, after the canvas exists

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (!firstRender || _started) return;
    _started = true;

    await WebApp.Run(new App(), JSRuntime, HttpClient, CanvasId, "");
}

Asking which platform you are on

Occasionally you need to. DeviceServices.Core.Platform returns a Platform value — Windows, Linux, MacCatalyst, Android, iOS or Web — and the reference application uses it to keep features off the platforms that cannot host them.

if (DeviceServices.Core.Platform is Platform.Web)
{
    // No window capture, no local model inference.
}

// Services that are not implemented everywhere are null, not stubs.
if (DeviceServices.Recorder != null)
    await DeviceServices.Recorder.Start();

The null check is the convention, not a defensive habit: a service with no implementation on a platform is left null deliberately so that the gap is visible at the call site rather than failing silently at runtime. The full list is in cross-platform support.