How do builder.Services.AddSingleton and DependencyService.Register differ for dependency injection in .NET MAUI?

The two registration approaches use entirely different dependency injection containers:

• builder.Services.AddSingleton<ILocalize, Localize>();
  This registers the service with the Maui app's built-in dependency injection container (Microsoft.Extensions.DependencyInjection). When you resolve services (usually via constructor injection), the Maui DI container provides the instance. However, calling DependencyService.Get() won’t see this registration because it isn’t using that container.

• dependencyService.Register<ILocalize, Localize>();
  This registers the service with the DependencyService container (a legacy container originally from Xamarin.Forms). When you call DependencyService.Get(), it will resolve the instance using this container.

In your situation, since you're attempting to resolve the instance using DependencyService.Get(), only the registration made through dependencyService.Register works. The two containers are separate, so adding a service to the Maui services container won’t populate DependencyService.

If you want to retrieve services via DependencyService.Get, you should register with DependencyService.Register. Alternatively, the recommended approach in .NET MAUI is to use the Maui DI container (via builder.Services) and use constructor injection or another method that resolves services from that container instead of using DependencyService.