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Constructor Injection in Xamarin Froms .xaml.cs file classes

I want to take advantage of dependency injection in my Xamarin project but can't get constructor injection to work in C# classes behind XAML views. Is there any way to do it ?
I've seen guides how to setup dependency injections in View Models, to later use them as repositories but that doesn't work for me.
So far I tried Ninject and Unity.
Code:
This is the service I want to use inside of my PCL project:
public class MyService : IMyService
{
public void Add(string myNote)
{
//Add Note logic
}
}
Interface:
public interface IMyService
{
void Add(string myNote);
}
Unity setup in App.Xaml:
public App ()
{
InitializeComponent();
var unityContainer = new UnityContainer();
unityContainer.RegisterType<IMyService, MyService>();
var unityServiceLocator = new UnityServiceLocator(unityContainer);
ServiceLocator.SetLocatorProvider(() => unityServiceLocator);
MainPage = new MainMasterMenu(); //<-- feel that I'm missing something here as I shouldn't be creating class instances with DI, right ?
}
Usage that I'd like to see. This is .CS file behind a XAML starting page:
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class MainMasterMenu : MasterDetailPage
{
private IMyService _myService;
public MainMasterMenu(IMyService myService)
{
_myService = myService
}
private void SomeFormControlClickEvent(object sender, ItemChangedEventArgs e)
{
_myService.Add("hi");
}
}

For that simple example creating the MainMasterMenu directly would be no issue, but you would have to pass the reference to your service
MainPage = new MainMasterMenu(unityContainer.Resolve<IMyService>());
But this would mean that you'll have to change that line every time the constructor of MainMasterMenu changes. You could circumvent this by registering the MainMasterMenu, too.
unityContainer.RegisterType<MainMasterMenu>();
...
MainPage = unityContainer.Resolve<MainMasterPage>();
Anyway, anytime you want to navigate to another page, which needs any dependency registered with unity, you'll have to make sure to resolve its dependencies properly, which requires (at least indirect) access to the unity container. You could pass a wrapper that encapsules the access to unity
interface IPageResolver
{
T ResolvePage<T>()
where T : Page;
}
and then implement that resolver with unity
public class UnityPageResolver
{
private IUnityContainer unityContainer;
public UnityPageResolver(IUnityContainer unityContainer)
{
this.unityContainer = unityContainer;
}
public T ResolvePage<T>()
where T : Page // do we need this restriction here?
{
return unityContainer.Resolve<T>();
}
}
This gets registered with unity
unityContainer.RegisterInstance<IUnityContainer>(this);
unityContainer.RegisterType<IPageResolver, UnityPageResolver>();
But you should have a look at the Prism library (see here) that solves many of the issues (e.g. it provides an INavigationService that lets you navigate to other pages without caring about the dependencies and it provides facilities to resolve viewmodels automatically, including dependencies).

Alert Dialog in ViewModel - MVVMCross

In the ViewModel, I have Save method where I check isValid property.
If isValid is false, then I want to display an error message.
Since AlertDialog is platform specific, I wonder how do you handle that situation in the ViewModel?
public void Save()
{
if (isValid)
{
OnExit(this, null);
}
else
{
//issue an alert dialog here
}
}
Update
I have used the following plugin and added the following line of code as follows, but it throws an error.
else
{
Mvx.Resolve<IUserInteraction>().Alert("it is not valid");
}
Update 2
Chance.MvvmCross.Plugins.UserInteraction is a namespace but it is used as a type error.
Update 3
I have added Acr.UserDialogs plugin and called as follows, but I have got the same error.
Mvx.Resolve<IUserDialogs>().Alert("it is not valid");

Using ACR User Dialogs is the simplest approach.
In your App.cs (Core/PCL) you will need to register the interface:
public class App : MvxApplication
{
public override void Initialize()
{
// Example Other registrations
CreatableTypes()
.EndingWith("Service")
.AsInterfaces()
.RegisterAsLazySingleton();
Mvx.RegisterSingleton<IUserDialogs>(() => UserDialogs.Instance);
}
}
Then you can call your alert form your ViewModel.
Mvx.Resolve<IUserDialogs>().Alert("it is not valid");
Note for Android Platform support
Then if you are supporting Android you will need to initialize UserDialog with an instance of the activity context. This will have to be done in each activity that you will be making use of UserDialogs or if you have a shared base activity you can do it there.
[Activity]
public class MainActivity : MvxActivity
{
protected override void OnCreate(Bundle bundle)
{
base.OnCreate(bundle);
SetContentView(Resource.Layout.activity_main);
// Initialize Acr UserDialogs
UserDialogs.Init(this);
}
}
Alternatively
You can follow the Mvvmcross document on using platform specific implementations of an interface if you need a more custom modal implementation.

This is how I handle the Alert messages in the viewmodel. Try this.
await App.Current.MainPage.DisplayAlert("Active subscription required", "You do not have an active subscription for Part 2 exams", "OK");

There is an existing MvvmCross plugin called User Interaction that allows displaying alerts and collecting inputs from ViewModels.
From the author BrianChance:
Really simple, easy, beautiful ways to show a message box or to collect user input from your ViewModels
Check it out here and NuGet Link Here.
To install the plugin, make sure you override LoadPlugins in your SetUp Class on iOS and Android (and windows phone) like so:
public override void LoadPlugins(MvvmCross.Platform.Plugins.IMvxPluginManager pluginManager)
{
base.LoadPlugins(pluginManager);
pluginManager.EnsurePluginLoaded<Chance.MvvmCross.Plugins.UserInteraction>();
}

My approach is that i use an event for this scenario. My base class for my view models has a EventHandler OnUserNotification, where the views can kinda subscribe to. The UserNotificationType is just an enum and i let the view kinda decide how it reacts to the situation.
The property:
public EventHandler<UserNotificationType> OnUserNotification { get; set; }
The call:
if (OnUserNotification != null)
{
OnUserNotification.Invoke(this, UserNotificationType.ENetworkError);
}
In the view:
private void onUserNotification(object sender, UserNotificationType userNotificationType)
{
// Do Something like showing a Snackbar, AlertDialog, etc...
}
Of course you can make the eventtype more complex if needed.
Didnt try the plugin which got suggested by wishmaster though, so that might be a smoother implementation?

Use Acr.UserDialogs. There is a great examples on github
You can grab it on nuget
It works well with dependency injection or a static singleton UserDialogs.Instance

How do I Configure Unity to use constructor injection

In my MVC 5 app I am using Unity. Constructor injection is working fine as expected. Suppose I have a class which is not a controller and I have this setup for constructor injection, how can I configure Unity (code not xml) to tell it to use constructor injection when I new up an instance of my class. Is this possible?
My class is below:
public class Thing
{
private readonly IAuthorisationService _authorisationService;
public Thing()
{
}
public Thing(IAuthorisationService authorisationService)
{
_authorisationService = authorisationService;
}
public void Work()
{
var result = _authorisationService.IsAdmin(Guid.NewGuid());
}
}
So I am missing the bit where I tell unity to take over and inject when I new up a Thing.
Any ideas?

Assuming default Unity configuration all classes auto-registered with Unity, so the only thing you need is to call Resolve for your class on container.
var theThing = container.Resolve<Thing>();
Note that usually you get it happen magically by having dependency on Thing in your other classes (i.e. controllers).
If you need to create multiple instances of Thing - take dependency on Func<Thing> instead (which is also auto-registered and provides way to create instances resolved by container in the way you'd use new).
public MyController(Func<Thing> thingCreator)
{
var manyThings = Enumerable.Range(0, 9001).Select(()=> thingCreator());
...
}

View is not locating the Viewmodel with default VML in MVVMCross

I'm trying to create a Portable Class such that I can use that across the platforms. It is working fine in Windows Phone 8.1 App. But when it comes to Android, then it is showing the Viewmodel as null and DataContext as Null in debugger which breaks the application debugger. When I create another viewmodel and view to test the app, its working fine on android too. What can be the possible reasons.
EDIT : It is crashing due to the constructor , in which I am passing the business Logic instance. So , Constructor is necessary i think but in that case it is crashing.I am not trying to resolve the ViewModel , i am trying to resolve the Service instance in ViewModel and for the purpose of MVVM, I am keeping the Service out from Droid Project so base.OnCreate(bundle) does not come into scene anyways.
public BookViewModel(ILogic _logic)
{
logic = _logic;
//var ss= Mvx.Resolve<ILogic>();
//var x = Mvx.CanResolve<ILogic>();
_details = logic.Read();
}
Below is the Logic Code :
public class Logic : ILogic
{
#region Attributes
List<Detail.Detail> _details = new List<Detail.Detail>();
DataLayer.DataLayer dl = new DataLayer.DataLayer();
#endregion
#region .ctor
public Logic()
{
populateList();
}
#endregion
#region Methods
private void populateList()
{
_details = dl.Access();
}
Below is the App.cs in ViewModel in which CanResolve is giving False
public class App : Cirrious.MvvmCross.ViewModels.MvxApplication
{
#region Methods
public override void Initialize()
{
Mvx.RegisterType<ILogic, Logic>();
var ss = Mvx.CanResolve<ILogic>();
RegisterAppStart<ViewModels.BookViewModel>();
}
#endregion
}

There are a few questions and answers around similar to this - e.g. similar to MVVMCross ViewModel construction failure notifications
The basic answer is that MvvmCross cannot resolve the ViewModel during the constructor - you have to wait until after the base.OnCreate(bundle) call - at this point the ViewModel will be resolved.
There's also a bit more about when ViewModel's are located in Who should create view model instances in MvvmCross and CoreDispatcher.HasThreadAccess "breaking change" (and probably a few other places too)

Ninject + ASP.NET Web Forms Not Working

I've successfully implemented Ninject in an MVC3 application, but am running into some trouble doing the same thing with ASP.NET Web Forms. I'm getting null references every time I try to access an injected property in my business layer. After setting breakpoints within the CreateKernel method, as well as several places within the ServiceLocator class, it looks like none of them are ever getting hit, so it's not even loading.
I'm sure I'm just approaching this wrong, but there is very little documentation or info out there for wiring up Ninject in a Web Forms application.
Basically here's what I have so far:
code behind
public class ReviewManager
{
[Inject] private IReviewRepository _reviewRepository { get; set; }
public ReviewManager() { }
public ReviewManager(IReviewRepository reviewRepository)
{
_reviewRepository = reviewRepository;
}
public Review GetById(int id)
{
if (id <= 0) throw new ArgumentException("ID must be greater than zero");
**I get a null reference exception on the next line. _reviewRepository is null**
return _reviewRepository.GetById(id);
}
}
global.asax.cs
public class Global : NinjectHttpApplication
{
protected override IKernel CreateKernel()
{
return ServiceLocator.Kernel;
}
// deleted for brevity
}
ServiceLocator.cs (edited for brevity, the relevant parts are here)
public static class ServiceLocator
{
public static IKernel Kernel { get; set; }
public static ILogger Logger { get; set; }
static ServiceLocator()
{
Kernel = new StandardKernel(new INinjectModule[] {
new LoggerBindings(),
new DataBindings()
});
if (Logger == null)
Logger = Kernel.Get<ILogger>();
}
}
public class LoggerBindings : NinjectModule
{
public override void Load()
{
Bind<ILogger>().To<NLogLogger>();
}
}
public class DataBindings : NinjectModule
{
public override void Load()
{
Bind<IReviewRepository>().To<ReviewRepository>();
}
}

ASP.Net via WebForms does not allow you to manage the lifecycle of all object instances (like MVC does). For example, the framework instantiates page objects. This means you probably can't implement DI in quite the same way as you would in MVC/WPF/Silverlight (the same problem is present in WinForms IIRC). You will likely have to initiate the dependency graph directly in each of your code behinds.
Translation: you will want to call ServiceLocator.Kernel.Get<IReviewRepository> when your page loads (or as lazy-init on the property).

The cool thing about MVC ist that it can run side a side of ASP.NET WebForm pages in the same application. In my opinion the best way to extend ASP.NET WebForms websites is to create new pages using MVC3 and to refactor every page that needs major changes to MVC3.
If this is no option go and use the Ninject.Web extension. It contains a IHttpModule that property injects all web pages and controlls after they are initialized. That way you can property inject the services als have them created by Ninject.

A potential workaround, by changing your DataBindings class as follows:
public class DataBindings : NinjectModule
{
public override void Load()
{
Bind<IReviewRepository>().To<ReviewRepository>();
Bind<ReviewManager>().ToSelf();
}
}
And within your caller, instead of
var rm = new ReviewManager();
Try using
var rm = ServiceLocator.Kernel.Get<ReviewManager>();
I havent tested this code, but i think it'll solve your null reference problem.

I use property injection for pages, masterpages and usercontrols. All my pages, for example, inherit from a base class that overrides RequestActivation method with the following code:
''' <summary>
''' Asks the kernel to inject this instance.
''' </summary>
Protected Overridable Sub RequestActivation()
ServiceLocator.Kernel.Inject(Me)
End Sub
And in each page I declare injectable properties:
<Inject()>
Property repo As IMyRepository