XAML.io v0.7 now exports C#+XAML projects as native Windows .exe, macOS .app, and Linux executable files. The whole pipeline runs in the browser via WebAssembly, including packaging, file generation, and even Apple Mach-O ad-hoc code-signing. No server. No backend compile step.
New version 0.6 of the free browser-based .NET IDE adds NuGet package support and code sharing via URL. Share a link to a running C# project and anyone can view, run, edit, and fork it instantly.