UWP Template 10 and Service Dendency Injection (MVVM) not WPF

Question

I have spent over two weeks searching google, bing, stack overflow, and msdn docs trying to figure out how to do a proper dependency injection for a mobile app that I am developing. To be clear, I do DI every day in web apps. I do not need a crash course on what, who, and why DI is important. I know it is, and am always embracing it.

What I need to understand is how this works in a mobile app world, and in particular a UWP Template 10 Mobile app.

From my past, in a .net/Asp app I can "RegisterType(new XYZ).Singleton() blah" {please forgive syntax; just an example} in App_Start.ConfigureServices. This works almost identical in .netcore, granted some syntactic changes.

My problem is now I am trying to provide my api is going to an UWP app that needs to digest my IXYZ service. By no means do I think that they should "new" up an instance every time. There has to be a way to inject this into a container on the UWP side; and I feel I am missing something very simple in the process.

Here is the code I have:

App.xaml.cs

public override async Task OnStartAsync(StartKind startKind, IActivatedEventArgs args)
    {
        // TODO: add your long-running task here

        //if (args.Kind == ActivationKind.LockScreen)
        //{

        //}
        RegisterServices();
        await NavigationService.NavigateAsync(typeof(Views.SearchCompanyPage));

    }

public static IServiceProvider Container { get; private set; }

private static void RegisterServices()
{
    var services = new ServiceCollection();
    services.AddSingleton<IXYZ, XYZ>();
    Container = services.BuildServiceProvider();
}

MainPage.xaml.cs:

public MainPage()
{
   InitializeComponent();
   NavigationCacheMode = NavigationCacheMode.Enabled;
}

MainPageViewModel:

public class MainPageViewModel : ViewModelBase
{
    private readonly IXYZ _xyz;
    public MainPageViewModel(IXYZ xyz)
    {
        //Stuff
        _xyz= xyz;
    }

}

I now get the error: XAML MainPage...ViewModel type cannot be constructed. In order to be constructed in XAML, a type cannot be abstract, interface nested generic or a struct, and must have a public default constructor.

I am willing to use any brand of IoC Container, but what I need is an example of how to properly use DI for services in a UWP app. 99.9% of questions about DI is about Views (i.e. Prism?) not just a simple DI for a service (i.e. DataRepo; aka API/DataService).

Again, I feel I am missing something obvious and need a nudge in the right direction. Can somebody show me an example project, basic code, or a base flogging on how I should not be a programmer...please don't do that (I don't know if my ego could take it).

Answer 1

You can try to Microsoft.Hosting.Extensions just like ASP.NET, there's an implementation on Xamarin.Forms by James Montemagno, as well it can be used in UWP I have tried and it works perfectly. You have to change some parts in order to get it working.

In OnLaunched Method add Startup.Init();

    public static class Startup
    {
        public static IServiceProvider ServiceProvider { get; set; }
        public static void Init()
        {
            StorageFolder LocalFolder = ApplicationData.Current.LocalFolder;
            var configFile = ExtractResource("Sales.Client.appsettings.json", LocalFolder.Path);

            var host = new HostBuilder()
                        .ConfigureHostConfiguration(c =>
                        {
                            // Tell the host configuration where to file the file (this is required for Xamarin apps)
                            c.AddCommandLine(new string[] { $"ContentRoot={LocalFolder.Path}" });

                            //read in the configuration file!
                            c.AddJsonFile(configFile);
                        })
                        .ConfigureServices((c, x) =>
                        {
                            // Configure our local services and access the host configuration
                            ConfigureServices(c, x);
                        }).
                        ConfigureLogging(l => l.AddConsole(o =>
                        {
                            //setup a console logger and disable colors since they don't have any colors in VS
                            o.DisableColors = true;
                        }))
                        .Build();

            //Save our service provider so we can use it later.
            ServiceProvider = host.Services;
        }

        static void ConfigureServices(HostBuilderContext ctx, IServiceCollection services)
        {
            //ViewModels
            services.AddTransient<HomeViewModel>();
            services.AddTransient<MainPageViewModel>();
        }

        static string ExtractResource(string filename, string location)
        {
            var a = Assembly.GetExecutingAssembly();
            
            using (var resFilestream = a.GetManifestResourceStream(filename))
            {
                if (resFilestream != null)
                {
                    var full = Path.Combine(location, filename);

                    using (var stream = File.Create(full))
                    {
                        resFilestream.CopyTo(stream);
                    }
                }
            }
            return Path.Combine(location, filename);
        }
    }

Injecting a ViewModel is possible as well which is pretty nice.