Xamarin.UWP Unhandled Exception during launch

Question

My objective is to extend an existing Xamarin project with UWP .

I have two shared projects , X. mobile.shared and X. application.shared , so i added reference for the Uwp project to this two project.

After following the steps to add uwp from the documentation , I have also installed all the Nuget packages available in these shared projects as it's noted in the answer of these issue .

Now when I debug the UWP project, I don't get any error but an unhandled exception , I tried to pin point the ligne that trigger that exception and I found that uncommenting this line:

 rootFrame.Navigate(typeof(MainPage), e.Arguments);

Triggers the unhandled exception:

The message contains,as you see in the image:

    Message "Object reference not set to an instance of an object." 

which means that there is a referencing to a null object !

What am I doing wrong ?

Update

With a BreakPoint on the ligne triggering the error , i tried to see the value of the method is arguments , i found that DeclaringMethod called by typeofMain() is returning " SystemInvalidOpertaionException " .

what that means ?

Update 2

Getting the advice of @Martin Zikmund , i checked the Stack trace and i found that the problem occurs at :

  Acme.PhoneBookDemo.Core.Dependency.DependencyResolver.Resolve[T]()\r\n at Acme.PhoneBookDemo.App.OnStart()

back to that class, this is the code of OnStart() method:

 protected override async void OnStart()
    {
        base.OnStart();

        if (Device.RuntimePlatform == Device.iOS)
        {
            SetInitialScreenForIos();
            await UserConfigurationManager.GetIfNeedsAsync();
        }

        await DependencyResolver.Resolve<INavigationService>().InitializeAsync();

        OnResume();
    }

the problem is triggred when the debugger arrive at this line:

 await DependencyResolver.Resolve<INavigationService>().InitializeAsync();

Answer 1

According to the exception it occurs during DependencyResolver.Resolve<T> call. DependencyResolver is a class in Xamarin.Forms which acts as a "service locator".

I don't have access to the source code, but this means there is a service that requires a platform-specific implementation which is not provided in the UWP project.

Check the Stack Trace for the Exception and you should be able to see where the dependency was missing and then be able to tell what needs to be implemented.I suggest you should look especially in the ViewModel of the first view that is loaded as it seems the service is required there (app crashes immediately on launch).

For details on how Dependency injection works in Xamarin.Forms see the Docs .

Answer 2

I got a response from xamarin-docs 's github , telling that in order to extend your existing UWP project with UWP all the nuget packages you use in your solution should have a support to UWP (not only the ones that exists in .shared ).

In my solution i have many packages that dosen't support UWP, example :

ACRSupport supports only android and ios

In conclusion, aspnetzero 's project can't be extended to use UWP.

UPDATE OF ANSWER:

A co-worker in my organisation found a solution by coping the implementation of *.droid project.

This answer is useful for aspBoilerplate user, in particular aspnetzero user, who want to extend his project with UWP .

This are the steps , to make it work :

1. First ,follow this tutorial

2. If you run you project after the first step , you may be surprised by an " unhandled exception = Object reference not set to an instance of an object. ", this happens because the Dependancy Injection Container(aka service locator) can't find the registration of your uwp abpmodule , So :

   2.1. Define a Module for your add Uwp project (Add>NewItem>PhoneBookDemoXamarinUwpModule.cs):

     using Abp.Modules; using Abp.Reflection.Extensions; namespace Acme.PhoneBookDemo { [DependsOn(typeof(PhoneBookDemoXamarinSharedModule))] public class PhoneBookDemoXamarinUwpModule : AbpModule { public override void Initialize() { IocManager.RegisterAssemblyByConvention(typeof(PhoneBookDemoXamarinUwpModule).GetAssembly()); } } } 

   In the initialize method above we register dependencies , IocManager is used to register the module class.

3. in App.xaml.cs 's OnLaunched() method , add these lines :

    //rootFrame.NavigationFailed += OnNavigationFailed; ApplicationBootstrapper.InitializeIfNeeds<PhoneBookDemoXamarinUwpModule>(); await StartUpApplicationAsync(); //Xamarin.Forms.Forms.Init(e); // requires the `e` parameter 

   then add this methods:

     private async Task StartUpApplicationAsync() { await UserConfigurationManager.GetIfNeedsAsync(); } private static void ConfigureFlurlHttp() { FlurlHttp.Configure(c => { c.HttpClientFactory = new ModernHttpClientFactory(); }); } 

4. Add these Nuget Packages to your Uwp Project:

   Xamarin.forms (ensure it's the same version with your *.Mobile.shared project)

   Win2D.uwp

5. Finally from your uwp project, add a reference to Acme.PhoneBook.Mobile.shared , Acme.PhoneBook.Application.shared & Acme.PhoneBook.Application.Core

And that is it , it would run for you , as it did to me