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Property / method injection using Autofac in filter attributes

Trying to use autofac for dependency injection by property.
The instance is always null and there is no dependency injected.
Below is class where the property needs to be injected.
public class UserAccount
{
public IAccountService AccountService { get; set; }
public string Message()
{
return AccountService.Message();
}
}
I have tried three different ways to inject the property but none was successful
Method 1 :
builder.Register(c => {
var result = new UserAccount();
var dep = c.Resolve<IAccountService>();
result.SetDependency(dep);
return result;
});
Method 2 :
builder.RegisterType<UserAccount>().PropertiesAutowired();
Method 3 :
builder.Register(c => new UserAccount { AccountService = c.Resolve<IAccountService>()});
PS : Method injection of above is welcomed.

You should prevent letting your container create data-centric objects, such as your UserAccount entity. This leads to complicated scenarios, such as the one you are in now.
In general, your DI Container should resolve only components—those are the classes in your system that contain the application's behavior, without having any interesting state. Those types of classes are typically long lived, or at least, longer lived than data-centric objects.
Data-centric objects, like entities, can best be created by hand. Not doing so would either lead to entities with big constructors, which easily causes the constructor over-injection code smell. As remedy, you might fall back on using Property Injection, but this causes a code smell of its own, caused Temporal Coupling.
Instead, a better solution is to:
Create entities by hand, opposed to using a DI Container
Supply dependencies to the entity using Method Injection, opposed to using Property Injection
With Method Injection, your UserAccount would as follows:
// This answer assumes that this class is an domain entity.
public class UserAccount
{
public Guid Id { get; set; }
public byte[] PasswordHash { get; set; }
public string Message(IAccountService accountService)
{
if (accountService == null)
throw new ArgumentNullException(nameof(accountService));
return accountService.Message();
}
}
This does move the responsibility of supplying the dependency from the Composition Root to the entity's direct consumer, though. But as discussed above, this is intentional, as the Composition Root in general, and a DI Container in particular, should not be responsible of creating entities and other data-centric, short-lived objects.
This does mean, however, that UserAccount's direct consumer should inject that dependency, and with that, know about existence of the dependency. But as that consumer would be a behavior-centric class, the typical solution is to use Constructor Injection at that stage:
public class UserService : IUserService
{
private readonly IAccountService accountService;
private readonly IUserAccountRepository repo;
public UserService(IAccountService accountService, IUserAccountRepository repo)
{
this.accountService = accountService;
this.repo = repo
}
public void DoSomething(Guid id)
{
UserAccount entity = this.repo.GetById(id);
var message = entity.Message(this.accountService);
}
}

Using method 3, you need to register AccountService, i.e.
builder.RegisterType<AccountService>().As<IAccountService>();
builder.Register(c => new UserAccount { AccountService = c.Resolve<IAccountService>()});
And when you use UserAccount, make sure it is created using Autofac.

IOption pattern - unit testing and passing through layers

I have some code (C# .Net Core WebAPI) I wish to unit test but need some help as the dependencies looks a bit odd to me.
The code came from some sample code (I found on the web) for accessing MongoDb using .Net Core WebAPI, which initially looked ok, until now..
Both the DbContext and the Repository have the same dependency - and the Repository just passes it through to the DbContext anyway - as the Repository instantiates the DbContext:
public class LogItemRepository : ILogItemRepository
{
private readonly DbContext _context = null;
public LogItemRepository(IOptions<DbSettings> settings)
{
_context = new DbContext(settings);
}
...
public class DbContext
{
private readonly IMongoDatabase _database = null;
public DbContext(IOptions<DbSettings> settings)
{
var client = new MongoClient(settings.Value.ConnectionString);
if (client != null)
_database = client.GetDatabase(settings.Value.Database);
}
public IMongoCollection<LogItem> LogItemsCollection
{
get
{
return _database.GetCollection<LogItem>("LogItem");
}
}
}
}
I'm not familiar with the Options pattern, but from a quick read it looks good. But I'm not convinced it's good practice to make child dependencies (the options), dependencies of the parent (as in the example above).
Instead should I be making an interface, IDbContext, and using that as the dependency for the repository? That's what I would have done in the past - but not sure if this breaks the options pattern.
I suspect this is subjective, but I'd like some others input.
Thanks
Tim

While primarily opinion based, common practice is to not instantiate the db context within the constructor of the repository. That tightly couples the repository to the context. Inject an abstraction as you stated in your OP.
I may be splitting hairs here but there is still too much tight coupling in the example provided.
First abstract the context
public interface IDbContext {
IMongoCollection<LogItem> LogItemsCollection { get; }
}
and also have IMongoDatabase be an explicit dependency
public class DbContext : IDbContext {
private readonly IMongoDatabase database = null;
public DbContext(IMongoDatabase database)
this.database = database;
}
public IMongoCollection<LogItem> LogItemsCollection {
get {
return database.GetCollection<LogItem>("LogItem");
}
}
}
configure service with what ever options are needed at the composition root (Startup). You would even consider encapsulating it in an extension method.
services.AddScoped<IMongoDatabase>(provider => {
var settings = provider.GetService<IOptions<DbSettings>>();
var client = new MongoClient(settings.Value.ConnectionString);
return client.GetDatabase(settings.Value.Database);
});
services.AddScoped<IDbContext, DbContext>();
services.AddScoped<ILogItemRepository, LogItemRepository>();
//...NOTE: Use the desired service lifetime. This is just an example
That now leaves the repository to be explicitly dependent on the context abstraction
public class LogItemRepository : ILogItemRepository {
private readonly IDbContext context = null;
public LogItemRepository(IDbContext context) {
this.context = context;
}
//...other code
}
All layers are now decoupled and explicitly state what their dependencies are, allowing for more isolated unit tests to be done as needed.

Dependency Injection in Model classes (entities)

I am building an ASP.NET Core MVC application with Entity Framework Code-First.
I implemented a simple repository pattern, providing basic CRUD operations for all the model classes I have created.
I chose to follow all the recommendations provided in docs and DI is one of these.
In ~~.NET 5~~ (6 years later update: .net 5 was the alpha name of .net core 1.0) dependency injection works very well for any class that we do not directly instantiate (e.g.: controllers, data repositories, ...).
We simply inject them via the constructor, and register the mappings in the Startup class of the application :
// Some repository class
public class MyRepository : IMyRepository
{
private readonly IMyDependency _myDependency;
public MyRepository(IMyDependency myDependency)
{
_myDependency = myDependency;
}
}
// In startup.cs :
services.AddScoped<IMyDependency, MyDependency>();
services.AddScoped<IMyRepository, MyRepository>();
The problem is that in some of my model classes, I would like to inject some of the dependencies I have declared.
But I think that I cannot use the constructor injection pattern because model classes are often explicitly instantiated. Therefore, I would need to provide myself with the dependencies, which I can't.
So my question is: is there another way than constructor injection to inject dependencies, and how? I was for example thinking of an attribute pattern or something like that.

As I already explained in a comment, when creating an object using new, there is nothing from the dependency injection framework that is involved in the process. As such, it’s impossible for the DI framework to magically inject things into that object, it simply doesn’t know about it.
Since it does not make any sense to let the DI framework create your model instances (models are not a dependency), you will have to pass in your dependencies explicitly if you want the model to have them. How you do that depends a bit on what your models are used for, and what those dependencies are.
The simple and clear case would be to just have your model expect the dependencies on the constructor. That way, it is a compile time error if you do not provide them, and the model has access to them right away. As such, whatever is above, creating the models, is required to have the dependencies the model type needs. But at that level, it’s likely that this is a service or a controller which has access to DI and can request the dependency itself.
Of course, depending on the number of dependencies, this might become a bit complicated as you need to pass them all to the constructor. So one alternative would be to have some “model factory” that takes care of creating the model object. Another alternative would also be to use the service locator pattern, passing the IServiceCollection to the model which can then request whatever dependencies it needs. Note that is generally a bad practice and not really inversion of control anymore.
Both these ideas have the issue that they modify the way the object is created. And some models, especially those handled by Entity Framework, need an empty constructor in order for EF to be able to create the object. So at that point you will probably end up with some cases where the dependencies of your model are not resolved (and you have no easy way of telling).
A generally better way, which is also a lot more explicit, would be to pass in the dependency where you need it, e.g. if you have some method on the model that calculates some stuff but requires some configuration, let the method require that configuration. This also makes the methods easier to test.
Another solution would be to move the logic out of the model. For example the ASP.NET Identity models are really dumb. They don’t do anything. All the logic is done in the UserStore which is a service and as such can have service dependencies.

The pattern often used in domain driven design (rich domain model to be specific) is to pass the required services into the method you are calling.
For example if you want to calculate the vat, you'd pass the vat service into the CalculateVat method.
In your model
public void CalculateVat(IVatCalculator vatCalc)
{
if(vatCalc == null)
throw new ArgumentNullException(nameof(vatCalc));
decimal vatAmount = vatcalc.Calculate(this.TotalNetPrice, this.Country);
this.VatAmount = new Currency(vatAmount, this.CurrencySymbol);
}
Your service class
// where vatCalculator is an implementation IVatCalculator
order.CalculateVat(vatCalculator);
Finally your service can inject another services, like a repository which will fetch the tax rate for a certain country
public class VatCalculator : IVatCalculator
{
private readonly IVatRepository vatRepository;
public VatCalculator(IVatRepository vatRepository)
{
if(vatRepository == null)
throw new ArgumentNullException(nameof(vatRepository));
this.vatRepository = vatRepository;
}
public decimal Calculate(decimal value, Country country)
{
decimal vatRate = vatRepository.GetVatRateForCountry(country);
return vatAmount = value * vatRate;
}
}

I know my answer is late and may not exactly what you're asking for, but I wanted to share how I do it.
First of all: If you want to have a static class that resolves your dependencies this is a ServiceLocator and it's Antipattern so try not to use it as you can.
In my case I needed it to call MediatR inside of my DomainModel to implement the DomainEvents logic.
Anyway, I had to find a way to call a static class in my DomainModel to get an instance of some registered service from DI.
So I've decided to use the HttpContext to access the IServiceProvider but I needed to access it from a static method without mention it in my domain model.
Let's do it:
1- I've created an interface to wrap the IServiceProvider
public interface IServiceProviderProxy
{
T GetService<T>();
IEnumerable<T> GetServices<T>();
object GetService(Type type);
IEnumerable<object> GetServices(Type type);
}
2- Then I've created a static class to be my ServiceLocator access point
public static class ServiceLocator
{
private static IServiceProviderProxy diProxy;
public static IServiceProviderProxy ServiceProvider => diProxy ?? throw new Exception("You should Initialize the ServiceProvider before using it.");
public static void Initialize(IServiceProviderProxy proxy)
{
diProxy = proxy;
}
}
3- I've created an implementation for the IServiceProviderProxy which use internally the IHttpContextAccessor
public class HttpContextServiceProviderProxy : IServiceProviderProxy
{
private readonly IHttpContextAccessor contextAccessor;
public HttpContextServiceProviderProxy(IHttpContextAccessor contextAccessor)
{
this.contextAccessor = contextAccessor;
}
public T GetService<T>()
{
return contextAccessor.HttpContext.RequestServices.GetService<T>();
}
public IEnumerable<T> GetServices<T>()
{
return contextAccessor.HttpContext.RequestServices.GetServices<T>();
}
public object GetService(Type type)
{
return contextAccessor.HttpContext.RequestServices.GetService(type);
}
public IEnumerable<object> GetServices(Type type)
{
return contextAccessor.HttpContext.RequestServices.GetServices(type);
}
}
4- I should register the IServiceProviderProxy in the DI like this
public void ConfigureServices(IServiceCollection services)
{
services.AddHttpContextAccessor();
services.AddSingleton<IServiceProviderProxy, HttpContextServiceProviderProxy>();
.......
}
5- Final step is to initialize the ServiceLocator with an instance of IServiceProviderProxy at the Application startup
public void Configure(IApplicationBuilder app, IHostingEnvironment env,IServiceProvider sp)
{
ServiceLocator.Initialize(sp.GetService<IServiceProviderProxy>());
}
As a result now you can call the ServiceLocator in your DomainModel classes "Or and needed place" and resolve the dependencies that you need.
public class FakeModel
{
public FakeModel(Guid id, string value)
{
Id = id;
Value = value;
}
public Guid Id { get; }
public string Value { get; private set; }
public async Task UpdateAsync(string value)
{
Value = value;
var mediator = ServiceLocator.ServiceProvider.GetService<IMediator>();
await mediator.Send(new FakeModelUpdated(this));
}
}

The built-in model binders complain that they cannot find a default ctor. Therefore you need a custom one.
You may find a solution to a similar problem here, which inspects the registered services in order to create the model.
It is important to note that the snippets below provide slightly different functionality which, hopefully, satisfies your particular needs. The code below expects models with ctor injections. Of course, these models have the usual properties you might have defined. These properties are filled in exactly as expected, so the bonus is the correct behavior when binding models with ctor injections.
public class DiModelBinder : ComplexTypeModelBinder
{
public DiModelBinder(IDictionary<ModelMetadata, IModelBinder> propertyBinders) : base(propertyBinders)
{
}
/// <summary>
/// Creates the model with one (or more) injected service(s).
/// </summary>
/// <param name="bindingContext"></param>
/// <returns></returns>
protected override object CreateModel(ModelBindingContext bindingContext)
{
var services = bindingContext.HttpContext.RequestServices;
var modelType = bindingContext.ModelType;
var ctors = modelType.GetConstructors();
foreach (var ctor in ctors)
{
var paramTypes = ctor.GetParameters().Select(p => p.ParameterType).ToList();
var parameters = paramTypes.Select(p => services.GetService(p)).ToArray();
if (parameters.All(p => p != null))
{
var model = ctor.Invoke(parameters);
return model;
}
}
return null;
}
}
This binder will be provided by:
public class DiModelBinderProvider : IModelBinderProvider
{
public IModelBinder GetBinder(ModelBinderProviderContext context)
{
if (context == null) { throw new ArgumentNullException(nameof(context)); }
if (context.Metadata.IsComplexType && !context.Metadata.IsCollectionType)
{
var propertyBinders = context.Metadata.Properties.ToDictionary(property => property, context.CreateBinder);
return new DiModelBinder(propertyBinders);
}
return null;
}
}
Here's how the binder would be registered:
services.AddMvc().AddMvcOptions(options =>
{
// replace ComplexTypeModelBinderProvider with its descendent - IoCModelBinderProvider
var provider = options.ModelBinderProviders.FirstOrDefault(x => x.GetType() == typeof(ComplexTypeModelBinderProvider));
var binderIndex = options.ModelBinderProviders.IndexOf(provider);
options.ModelBinderProviders.Remove(provider);
options.ModelBinderProviders.Insert(binderIndex, new DiModelBinderProvider());
});
I'm not quite sure if the new binder must be registered exactly at the same index, you can experiment with this.
And, at the end, this is how you can use it:
public class MyModel
{
private readonly IMyRepository repo;
public MyModel(IMyRepository repo)
{
this.repo = repo;
}
... do whatever you want with your repo
public string AProperty { get; set; }
... other properties here
}
Model class is created by the binder which supplies the (already registered) service, and the rest of the model binders provide the property values from their usual sources.
HTH

Is there another way than constructor injection to inject dependencies, and how?
The answer is "no", this cannot be done with "dependency injection". But, "yes" you can use the "service locator pattern" to achieve your end-goal.
You can use the code below to resolve a dependency without the use of constructor injection or the FromServices attribute. Additionally you can new up an instance of the class as you see fit and it will still work -- assuming that you have added the dependency in the Startup.cs.
public class MyRepository : IMyRepository
{
public IMyDependency { get; } =
CallContextServiceLocator.Locator
.ServiceProvider
.GetRequiredService<IMyDependency>();
}
The CallContextServiceLocator.Locator.ServiceProvider is the global service provider, where everything lives. It is not really advised to use this. But if you have no other choice you can. It would be recommended to instead use DI all the way and never manually instantiate an object, i.e.; avoid new.

I'm simply adding some supplemental information here to the answers provided that can help.
IServiceProvider was provided in the accepted answer, but not the important IServiceProvider.CreateScope() method. You can use it to create scopes as necessary that you added through ConfigureServices.
I'm not sure if IServiceProvider is actually a Service Locator pattern behind the scenes or not, but it's how you create scopes as far as I know. At least in the case if it is a Service Locator pattern, it's the official one for today in .NET, and so it's not compounded by the problems of writing your own Service Locator, which I also agree is anti-pattern.
Example, Startup.cs/ConfigureServices and Configure:
public void ConfigureServices(IServiceCollection services)
{
services.AddDbContext<SomeDbContext>(options =>
{
options.UseSqlServer(Configuration.GetSection("Databases").GetSection("SomeDb")["ConnectionString"]);
options.UseQueryTrackingBehavior(QueryTrackingBehavior.NoTracking);
}, ServiceLifetime.Scoped);
services.AddMvcCore().AddNewtonsoftJson();
services.AddControllersWithViews();
}
public async void Configure(IApplicationBuilder app, IWebHostEnvironment env, IServiceProvider provider)
{
...
IServiceScope scope = provider.CreateScope();
SomeDbContext context = scope.ServiceProvider.GetRequiredService<SomeDbContext>();
SomeModelProxyClass example = new SomeModelProxyClass(context);
await example.BuildDefaults(
Configuration.GetSection("ProfileDefaults").GetSection("Something"),
Configuration.GetSection("ProfileDefaults").GetSection("SomethingSomething"));
scope.Dispose();
}
The above is for doing some default interactions on Startup, maybe if you need to build some default records in your database on a first usage, just as an example.
Ok so let's get to your repository and dependency though, will they work?
Yep!
Here's a test in my own CRUD project, I made a simple minimalist implementation of your IMyDependency and IMyRepository like so, then added them scoped as you did to Startup/ConfigureServices:
public interface IMyRepository
{
string WriteMessage(string input);
}
public interface IMyDependency
{
string GetTimeStamp();
}
public class MyDependency : IMyDependency
{
public MyDependency()
{
}
public string GetTimeStamp()
{
return DateTime.Now.ToLongDateString() + " " + DateTime.Now.ToLongTimeString();
}
}
public class MyRepository : IMyRepository
{
private readonly IMyDependency _myDependency;
public MyRepository(IMyDependency myDependency)
{
_myDependency = myDependency;
}
public string WriteMessage(string input)
{
return input + " - " + _myDependency.GetTimeStamp();
}
}
Here ContextCRUD is a Model class from my own project not derived from Scaffold-DbContext tooling like my other database classes, it's a container of logic from those scaffold Model classes, and so I put it in the namespace Models.ProxyModels to hold its own business logic for doing CRUD operations so that the Controllers are not gummed up with logic that should be in the Model:
public ContextCRUD(DbContext context, IServiceProvider provider)
{
Context = context;
Provider = provider;
var scope = provider.CreateScope();
var dep1 = scope.ServiceProvider.GetService<IMyRepository>();
string msg = dep1.WriteMessage("Current Time:");
scope.Dispose();
}
Debugging I get back the expected results in msg, so it all checks out.
The calling code from the Controller for reference, just so you can see how IServiceProvider is passed from upstream by constructor injection in the Controller:
[Route("api/[controller]")]
public class GenericController<T> : Controller where T: DbContext
{
T Context { get; set; }
ContextCRUD CRUD { get; set; }
IConfiguration Configuration { get; set; }
public GenericController(T context, IConfiguration configuration, IServiceProvider provider)
{
Context = context;
CRUD = new ContextCRUD(context, provider);
Configuration = configuration;
}
...

You can do it, check out [InjectionMethod] and container.BuildUp(instance);
Example:
Typical DI constructor (NOT NEEDED IF YOU USE InjectionMethod) public
ClassConstructor(DeviceHead pDeviceHead) {
this.DeviceHead = pDeviceHead; }
This attribute causes this method to be called to setup DI.
[InjectionMethod] public void Initialize(DeviceHead pDeviceHead) {
this.DeviceHead = pDeviceHead; }

Inversion of Control / Dependency Injection and conversion operators

I have a UserModel class used by many MVC apps that I'm trying to convert to use IoC:
public class UserModel
{
public string Login { get; set; }
[DisplayName("Display Name")]
public string DisplayName { get; set; }
public static explicit operator UserModel(string userLogin)
{
IKernel IoC; // Where do I get this from?
var ActiveDirRepo = IoC.Get<IActiveDirectoryRepository>();
var User = ActiveDirRepo.FindById<ActiveDirectoryUser>(userLogin.ToUpper());
return new UserModel()
{
Login = userLogin,
DisplayName = User == null ? userLogin.ToUpper() : User.Name
};
}
}
It has an explicit conversion operator from string to UserModel. Previously I just newed an ActiveDirectoryRepository and used it, but I want to start using IoC/DI.
The problem I've got now is, how do I reference my IoC container (IKernel)?
I can't inject it into UserModel from the outside, as the operator is what is creating it.

This kind of conversion are suppose to be simple, this is not suppose to need external dependencies. If you need to inject some sort of dependencies, then forgot the conversion, there is no solutions.
Simple enough, pass the IKernel to a build method :
public static UserModel Build(string userLogin, IKernel IoC)
{
var ActiveDirRepo = IoC.Get<IActiveDirectoryRepository>();
var User = ActiveDirRepo.FindById<ActiveDirectoryUser>(userLogin.ToUpper());
return new UserModel()
{
Login = userLogin,
DisplayName = User == null ? userLogin.ToUpper() : User.Name
};
}
Then you can ask yourself if it make sense that this method belong to the IKernel interface :
interface IKernel
{
UserModel Build(string userLogin);
}

There's no magic in IoC. Sometimes you still need a starting point, from which you can access application's IoC container, something like this:
var ioc = Application.Instance.Container.Get<IActiveDirectoryRepository>();
You don't need to access IoC container from everywhere, because objects, that were created via container already have their dependencies injected, but at least some root objects need to gain access to container.

Ideologically, your IoC container should only exist in the composite root of your application. With that in mind, it's a matter of figuring out your classes dependencies and injecting them via the constructor (if you cant do constructor DI then many IoC containers offer alternative methods of injection).
Here, you need access to IActiveDirectoryRepository in order to create a UserModel. However, your creation method, explicit operator UserModel is a static method with no access to instance variables. Unfortunately, that restricts your ability to use dependency injection to leverage IoC so here's some ideas:
Straightforward: Use the service locator pattern. You use a service locator to get an instance of IActiveDirectoryRepository. This would directly adapt into your current implementation nicely (swap the kernel lines to a service locator). However, there are drawbacks to this pattern; most notably, the fact you're internalizing your IActiveDirectoryRepository source which would make unit testing less effective.
Traditional: Make your UserModel creator into a factory that accepts IActiveDirectoryRepository via it's constructor. Then, any UserModel's created by the factory use the supplied IActiveDirectoryRepository.
public sealed class UserFactory
{
private readonly IActiveDirecotryRepository repository;
public UserFactory(IActiveDirectoryRepository repository)
{
this.repository = repository;
}
public UserModel CreateNewUser(string userLogin)
{
var User = repository.FindById<ActiveDirectoryUser>(userLogin.ToUpper());
return new UserModel()
{
Login = userLogin,
DisplayName = User == null ? userLogin.ToUpper() : User.Name
}
}
}
Fancy: Create a closure over IActiveDirectoryRepository and pass the creator around as first-class function.
static Func<string, UserModel> CreateFactory(IActiveDirectoryRepository repository)
{
return (userLogin) =>
{
var User = repository.FindById<ActiveDirectoryUser>(userLogin.ToUpper());
return new UserModel()
{
Login = userLogin,
DisplayName = User == null ? userLogin.ToUpper() : User.Name
}
}
}

You can use service locator pattern form Microsoft.Practices.ServiceLocation I would be supprised if ninject would not provide implementation. If it does not it's straight forward to implement.
Regardless that you can do what you want with this code I would strongly recommend you to not implement explicit conversion operator as you did. It is a bad practice.

Where to keep dictionaries in app using Dependency Injection

I have a legacy code, and I have a problem with reconstructor it.
At start of my application I load from WCF to property on App (this is SL application) list of users.
Then every control (for sending emails, view calendar and assigning tasks) use this property as
(App.Current as App).Users
Now, I'm trying to create Unit Test for one of controls that use this lists, and I'm stuck.
Should I make a Constructor Injection(I'm using Unity) with App as parameter? Or maybe introduce some class to hold this list?

Updated with OP's implementation as the pseudocode was incomplete.
I propose create an interface for all your application services
Inject IApplicationService to your modules.
You can use this interface for all the services the application provides(probably you will need more). Mock the interface for the unit tests
OP's implemantation
public interface IApplicationService
{
List<User> Users{get;set;}
}
public class ApplicationService : IApplicationService
{
public List<User> Users
{
get { return (App.Current as App).Users; }
set { (App.Current as App).Users = value; }
}
}
public partial class MainWindow : UserControl
{
readonly IApplicationService _applicationService
public MainWindow(IApplicationService applicationService)
{
_applicationService=applicationService;
}
}

I would create a wrapper class that will expose the list of users. In production code this class will just be a wrapper around your App.Current property and it can be injected in the constructor trough Unity.
In your Unit Tests you can easily mock the App parameter and pass it when constructing a new SUT.
Something like:
public interface IUserList
{
List<User> Users { get; }
}
public class SUT
{
private IUserList UserList { get; set; }
public SUT(IUserList userList)
{
this.UserList = userList;
}
}
public class AppUserList : IUserList
{
public List<User> Users
{
get
{
return ((App)App.Current).Users;
}
}
}

For Silverlight there is an extension model called Application Extension Services.
For infrastructure purposes that might be a better alternative than adding properties to your app class and casting App.Currentback and forth.
Downside of that model is the creation of a singleton you would have to initialize for your unit tests. It would also hide the dependency on Users in your consuming classes.
Your users seem to be just data. Making that data an ambient context which can be accessed and edited everywhere in your application will bite you. You don't know who does what with that data and when he does it. This is like a session state.
So making the dependency on your data explicit would be a first step to be able to track abuse of that data.
If it makes sense to you to create a "data holder object" that has a property for Users or directly inject that data into your consumers is up to you. If there is more data than just Usersit is tempting to put all of them into the same central data store object, even if your specific consumers don't need them.

Jimmy's answer is great, but can be provide quite a bit, and some errors fixed. Differences are explained at the bottom below the code/instructions:
Create a public interface: IUserService
public interface IUserService
{
// Implemented functionality as methods where possible for better
// extendability (like IoC)
IEnumerable<User> Users();
// Add any other user service stuff as you see fit.
void AddUser(User user);
}
Write a UserService that implements IUserService
public class UserService : IUserService
{
// If you need DI for this service, follow the same pattern of using
// fields and controller injection. I left examples in comment below.
// private readonly IRepository _repository;
// Constructor is unnecessary if you do not need DI example.
public UserService(/* IRepository repository */)
{
// _repository = repository;
}
// Methods
public IEnumerable<User> Users()
{
return ((App)App.Current).Users;
}
public void AddUser(User user)
{
((App)App.Current).Users.Add(user);
}
}
Inject IUserService into classes via their Constructor
In this case your MainWindow as an example:
public partial class MainWindow : UserControl
{
private readonly IUserService _userService;
public MainWindow(IUserService userService)
{
_userService = userService;
}
// Example method consuming the service
public IEnumerable<User> GetUsers()
{
return _userService.Users();
}
}
Differences:
Separate your User Services from a central Application Service
Better modularity. In addition I use an IApplicationService for more central/global data like Api Keys, Timeouts, cleanup, DB prepping, etc.
Return IEnumerable<T> instead of List<T>
This is just a golden rule of thumb for keeping things dry and not imposing hard instantiations on your consuming classes. Refactoring is easier/safer, and your code more extensible.
Use methods instead of properties
This is preference, but I think it smart in a service layer to use methods where possible so that you can introduce filters and overloads or continue to use dependency injection - for example, you could add GetUsers(string lastName), GetUsers(string lastName, string firstName) and maintain a clean interface for your consuming classes.
Cast App.Current without the as keyword
This is a good practice because using the as keyword means when the cast fails it will return null, rather than throw an exception. I prefer the exception because 99% of the time, if your cast fails, your next operations will too. :)
Enjoy!