Avalonia can run an application directly in the browser thanks to WebAssembly: the .NET code is downloaded and executed by the browser, and the interface is drawn inside an area of the page.

One project, several targets

dotnet new avalonia.xplat -o Quotes

The cross-platform template creates a shared project with views and ViewModels and a "startup" project for each target: desktop, browser, Android and iOS. The browser project is very small:

internal sealed partial class Program
{
    private static Task Main(string[] args) =>
        BuildAvaloniaApp().StartBrowserAppAsync("out");

    public static AppBuilder BuildAvaloniaApp() =>
        AppBuilder.Configure<App>();
}

The string "out" is the id of the HTML element in which the application is drawn.

When it makes sense

• Offering an online version of an existing desktop tool, for example a configurator or a technical calculator.
• Demos and free trials with no installation.
• Internal tools to be used even from workstations where software cannot be installed.

Limits to be aware of

• The first load downloads the runtime and the application: optimise it with trimming and compression.
• The code runs inside the browser sandbox: no free access to files and network, HTTP calls subject to CORS rules.
• For a public site indexed by search engines, server-generated HTML remains the right choice.

Used in the right place, it lets you reach users through the browser while reusing every last line of the desktop application's code.