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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; }

Dependency Injection in ASP.NET MVC

I've been using Dependency Injection in ASP.NET MVC the way I've explained in the below code, but I'm not sure if that is the right and standard way to do it. So I just wanna know if what I'm doing is wrong or if there is a better and more professional way to do it.
public interface IService {
public Boolean AddRecord(Object _Model);
}
public class Employee: IService {
DataBase Context = new DataBase();
public Boolean AddRecord(Object _Model) {
Context.Add((EmployeeModel) _Model);
return Context.SaveChanges() != 0;
}
}
public class DataController: Controller {
IService service;
public DataController(IService service) {
this.service = service;
}
public ActionResult AddRecord(Object _Model, String Caller) {
if (service.Add(_Model)) {
TempData["Result"] = "Data Added";
return RedirectToAction(Caller);
}
TempData["Result"] = "Please try again";
return View (Caller, _Model);
}
}
When I wanna use the controller with the DI, I do: (Is this the right way to consume the DataController)
public class TestController: Controller {
public ActionResult Index () {
return View();
}
public ActionResult TestIt (EmployeeModel _Model) {
DataController DC = new DataController(new Employee());
return DC.AddRecord(_Model, "../Test/TestIt");
}
}

You have the general concept of Dependency Injection / Inversion down. That is, you've understood that instead of this:
public class Example {
public void SomeFunc() {
var service = new Service();
service.DoStuff();
}
}
..you do this:
public class Example {
private readonly IService _service;
public Example(IService service) {
_service = service;
}
public void SomeFunc() {
_service.DoStuff();
}
}
.. and you supply the dependency manually via the calling code.. such as a Controller:
public HomeController : Controller {
[HttpGet]
public ActionResult Index() {
var example = new Example(new Service());
example.SomeFunc();
// .. the rest ..
}
}
So, the first part is Dependency Inversion. You have inverted the dependency chain from being top down to bottom up. The second part (the above code block) is Dependency Injection.
Notice in the above block of code that the Controller has no dependencies injected. This is where Inversion of Control comes in.
Inversion of Control is a pattern where code completely external to the currently running code decides how it functions. In this context, it means some external code - somewhere else - decides how to supply dependencies to your controller.
(Note, I am quite familiar with Ninject - so the below examples are using Ninject. There are plenty of other available DI/IoC containers available)
Ninject is a framework that can help with this (and many others). Ninject has an extension for ASP.NET MVC which automatically builds and supplies controller instances for you - plus your dependencies.
Without providing a full tutorial on using Ninject (which I will leave as an exercise to the OP to Google), the basics of it is this.
You declare a "module" with the configuration for your dependencies. Using the above examples, your module might look like this:
public class YourModule : NinjectModule {
public override void Load() {
Bind<IExample>().To<Example>().InRequestScope();
Bind<IService>().To<Service>().InRequestScope();
}
}
This will wire up all requests for an IExample to an Example, and IService to an instance of a Service. So your Controller would become:
public class HomeController : Controller {
private readonly IExample _example;
public HomeController(IExample example) {
_example = example;
}
[HttpGet]
public ActionResult Index() {
_example.SomeFunc();
// .. the rest ..
}
}
The container (in this case, Ninject) looks at your external code (the YourModule class) and determines what IExample should be. It sees that you've said it should be an instance of an Example class. Example also requires a dependency of type IService. So Ninject will again look at YourModule and determines that it should be an instance of Service. It continues to go down the object hierarchy until it completes the construction of the objects.
Hopefully that makes sense - it is definitely hard to explain these concepts in text.
I haven't viewed this video (I am running off of really really terrible WiFi hotspot while I wait for an internet connection!) so I can't verify it's quality, but a quick Google search turned this up for setting up Ninject and MVC: http://www.youtube.com/watch?v=w_MehI2qBTo
You would definitely benefit from Googling around some Inversion of Control and/or Ninject videos to understand what frameworks like it are for.
It is important to note too that frameworks like Ninject can also control scope. In the example above, I used InRequestScope against the bindings. This means that Ninject will instantiate the dependency at the start of the web request - and dispose of it afterward. This removes the need for you to worry about that.

What is Ninject and when do you use it?

I have been helping a few friends on a project and there is a class that uses Ninject. I am fairly new to C# and I have no idea what that class is doing, which is why I need to understand Ninject. Can anyone explain what Ninject is and when does one use it(with example if possible)? Or if you can point to some links that would be great too.
I tried this question: Ninject tutorials/documentations? but it didn't really help a beginner like me.

Ninject is dependency injector for .NET, practical realisation of pattern Dependency Injection (form of Inversion of Control pattern).
Suppose you have two classes DbRepository and Controller:
class Controller {
private DbRepository _repository;
// ... some methods that uses _repository
}
class DbRepository {
// ... some bussiness logic here ...
}
So, now you have two problems:
You must initialize _repository to use it. You have several options for doing this:
Manually, within the constructor. But what if the constructor of DbRepository changes? You would need to rewrite your Controller class because code it's dependent upon was changed. It's not hard if you have only one Controller, but if you have a couple of classes that have a dependency on your Repository you have a real problem.
You can use a service locator or factory. But now you have a dependency on your service locator. You have a global service locator and all code must use it. How you will you change the behavior of your service locator when you need to use it in one part of your code for activation logic but for something else in another part of your code? There is only one way - passing the service locator through constructors. But with more and more classes you will need to pass it more and more times. Anyway, it's a good thought but in the long run, it's a bad idea.
class Controller {
private DbRepository _repository;
public Controller() {
_repository = GlobalServiceLocator.Get<DbRepository>()
}
// ... some methods that uses _repository
}
You can use dependency injection. Look at the code:
class Controller {
private IRepository _repository;
public Controller(IRepository repository) {
_repository = repository;
}
}
Now when you need your controller you write: ninjectDevKernel.Get<Controller>(); or ninjectTestKernel.Get<Controller>();. You can switch beetween dependency resolvers as fast as you want. See? It's simple, you don't need to write a lot.
You can't create unit tests for it. Your Controller has a dependency on DbRepository and if you want to test some method that uses repository, your code will go to the database and ask it for data. That's slow, very slow. If your code in DbRepository changes, your unit test on Controller will fall. Only integration test must warn you of 'problems' in this case. What you need in unit tests - is to isolate your classes and test only one class in one test (in ideal - only one method). If your DbRepository code fails, you will think that Controller code failed - and that's bad (even if you have tests for DbRepository and Controller - they both will fail and you can start from the wrong place). It takes a lot of time to determine where the error really is. You need to know that class A is ok, and it was class B where something failed.
When you want to replace DbRepository with something else in all your classes, you have to do a lot of work.
You can't easily control the lifetime of DbRepository. An object of this class is created on initialization of Controller and deleted when Controller is deleted. There is no sharing between different instances of the Controller class and there is no sharing between other classes. With Ninject you can simply write:
kernel.Bind<IRepository>().To<DbRepository>().InSingletonScope();
A special feature of dependency injection - agile development! You describe that your controller uses a repository with interface IRepository. You don't need to write DbRepository, you can simply create a MemoryRepository class and develop Controller while another person develops DbRepository. When work on DbRepository is finished, you just rebind in your dependency resolver that default IRepository is now DbRepository. Have a lot of controllers? All of them will now use DbRepository. That's cool.
Read more:
Inversion of control (wiki)
Dependency injection (wiki)
Inversion of Control Containers and the Dependency Injection pattern (Martin Fowler)

Ninject is an Inversion of Control container.
What does it do?
Suppose you have a Car class that depends on a Driver class.
public class Car
{
public Car(IDriver driver)
{
///
}
}
In order to use the Car class you build it like so:
IDriver driver = new Driver();
var car = new Car(driver);
A IoC containter centralizes the knowledge about how to build classes. It is a central repository that knows a few things. For example, it knows that the concrete class that you need to use to build a car is a Driver and not any other IDriver.
For example, if you are developing a MVC application, you can tell Ninject how to build your controllers. You do so by registering which concrete classes satisfy specific interfaces. At run time Ninject will figure out which classes are needed to build the required controller, and all behind the scenes.
// Syntax for binding
Bind<IDriver>().To<Driver>();
This is beneficial because it lets you build systems that are more easily unit testable. Suppose that Driver encapsulates all database access for Car. In a unit test for Car you can do this:
IDriver driver = new TestDriver(); // a fake driver that does not go to the db
var car = new Car(driver);
There are entire frameworks that take care of automatically creating testing classes for you and they are called mocking frameworks.
For more information:
GitHub/Ninject Home
Inversion of Control
Inversion of Control Containers and the Dependency Injection pattern
Mock Object

Other answers are great but I would also like to point out this Implementing Dependency Injection using Ninject article.
This is one of the best articles I ever read which explains Dependency Injection and Ninject with a very elegant example.
Here's the snippet from the article:
Below Interface will be implemented by our (SMSService) and (MockSMSService), basically the new Interface (ISMSService) will expose the same behaviors of both services as the code below:
public interface ISMSService
{
void SendSMS(string phoneNumber, string body);
}
(SMSService) implementation to implement the (ISMSService) interface:
public class SMSService : ISMSService
{
public void SendSMS(string mobileNumber, string body)
{
SendSMSUsingGateway(mobileNumber, body);
}
private void SendSMSUsingGateway(string mobileNumber, string body)
{
/*implementation for sending SMS using gateway*/
Console.WriteLine("Sending SMS using gateway to mobile:
{0}. SMS body: {1}", mobileNumber, body);
}
}
(MockSMSService) with totally different implementation using the same interface:
public class MockSMSService :ISMSService
{
public void SendSMS(string phoneNumber, string body)
{
SaveSMSToFile(phoneNumber,body);
}
private void SaveSMSToFile(string mobileNumber, string body)
{
/*implementation for saving SMS to a file*/
Console.WriteLine("Mocking SMS using file to mobile:
{0}. SMS body: {1}", mobileNumber, body);
}
}
we need to implement a change to our (UIHandler) class constructor to pass the dependency through it, by doing this, the code which uses the (UIHandler) can determine which concrete implementation of (ISMSService) to use:
public class UIHandler
{
private readonly ISMSService _SMSService;
public UIHandler(ISMSService SMSService)
{
_SMSService = SMSService;
}
public void SendConfirmationMsg(string mobileNumber) {
_SMSService.SendSMS(mobileNumber, "Your order has been shipped successfully!");
}
}
Now, we have to create a separate class (NinjectBindings) which inherits from (NinjectModule). This class will be responsible to resolve dependencies at run time, then we’ll override the load event which is used to configure the binding in it. The nice thing about Ninject is that we do not need to change our code in (ISMSService), (SMSService), and (MockSMSService).
public class NinjectBindings : Ninject.Modules.NinjectModule
{
public override void Load()
{
Bind<ISMSService>().To<MockSMSService>();
}
}
Now in UI form code, we’ll use the binding for Ninject which will determine which implementation to use:
class Program
{
static void Main(string[] args)
{
IKernel _Kernal = new StandardKernel();
_Kernal.Load(Assembly.GetExecutingAssembly());
ISMSService _SMSService = _Kernal.Get<ISMSService>();
UIHandler _UIHandler = new UIHandler(_SMSService);
_UIHandler.SendConfirmationMsg("96279544480");
Console.ReadLine();
}
}
Now the code is using the Ninject Kernal to resolve all chain of dependencies, if we want to use the real service (SMSService) in Release mode (on production environment) instead of the mock one, we need to change on the Ninject binding class (NinjectBindings) only to use the right implementation or by using the #if DEBUG directive as below:
public class NinjectBindings : Ninject.Modules.NinjectModule
{
public override void Load()
{
#if DEBUG
Bind<ISMSService>().To<MockSMSService>();
#else
Bind<ISMSService>().To<SMSService>();
#endif
}
}
Now our binding class (NinjectBindings) is living on the top of all our execution code and we can control the configuration easily in once place.
Also, see What is Inversion of Control? some very simple examples are mentioned to understand IoC.

You have to understand the Dependency Injection(DI) first. Notice here,
public interface IService
{
void Serve();
}
public class Service1 : IService
{
public void Serve() {
Console.WriteLine("Service1 Called");
}
}
public class Service2 : IService
{
public void Serve() {
Console.WriteLine("Service2 Called");
}
}
public class Service3 : IService
{
public void Serve() {
Console.WriteLine("Service3 Called");
}
}
public class Client
{
private IService service;
public Client(IService _service) //Constructor injection
{
service = _service;
}
public void ServeMethod() {
service.Serve(); //Notice here, this Serve() method has no idea what to do.
} // runtime will assign the object, that is Ninject
}
class Program
{
static void Main(string[] args)
{
IService s1 = new Service1(); //N.B. Ninject assigns object with interface
Client c1 = new Client(s1);
c1.ServeMethod();
IService s2 = new Service2(); //N.B. Ninject assigns object with interface
c1 = new Client(s2);
c1.ServeMethod();
IService s3 = new Service3(); //N.B. Ninject assigns object with interface
c1 = new Client(s3);
c1.ServeMethod();
Console.ReadKey();
}
}
// Ninject creates object in runtime for interface in runtime in ASP.NET MVC project.
/*
Output:
Service1 Called
Service2 Called
Service3 Called
*/

Mock implementation for use in automatic UI testing

I am working on adding basic automatic UI tests to the block of unit tests we run with each nightly build. We used MSTest coded UI and created a script.
The code-behind is dependent upon IClientManager which both the real manager and mock implement.
My problem is that I don't know how to switch automatically between the real and mock implementations inside the button click handler, when running a test.
My two other constraints are that I can't have a dependency on the mock assembly in the code-behind and that I can't use a DI framework, since the client is "security conscious" and getting a framework approved might take months.
Is there any way of doing this manually, and hopefully, not a bigger problem than the problem I am looking to solve?
Thank you!

You could build your own simple object container if you can't use a third party one (which is silly but I understand, I've been there before)
here is something that I whipped up that could get you started... haven't tested it and it is really rough, but hopefully you get the idea
public static class ObjectFactory
{
static IDictionary<Type, object> _factory = new Dictionary<Type, object>();
public static void Register<T>(Func<T> builder)
{
if (_factory.ContainsKey(typeof(T)))
_factory[typeof(T)] = builder;
else
_factory.Add(typeof(T), builder);
}
public static T GetInstance<T>()
{
if (_factory.ContainsKey(typeof(T)))
throw new ArgumentException(string.Format("Type <{0}> not registered in ObjectFactory", typeof(T).Name));
return ((Func<T>)_factory[typeof(T)])();
}
}
public interface IClientManager { }
public class RealClientManager : IClientManager { }
public class MockClientManager : IClientManager { }
public class MyView
{
public MyView()
{
// probably better to do this registry in some sort of application initialization
ObjectFactory.Register<IClientManager>(() => new RealClientManager());
}
public void SomeMethodThatNeedsClientManager()
{
var clientManager = ObjectFactory.GetInstance<IClientManager>();
}
}
public class MyTester
{
[TestMethod()]
public void SomeTest()
{
var view = new MyView();
// swap the client manager in the test
ObjectFactory.Register<IClientManager>(() => new MockClientManager());
// Asserts
}
}
you can see that if you've used StructureMap or some other DI container before they do a lot of the same thing with a lot of added niceties such as traversing your object graph and registering objects automatically based on conventions, managing object lifecycles, scoping of containers, etc... a lot of this stuff you could implement yourself too... but you should just really used a tried and true solution such as StructureMap

Does CompositionContainer.SatisfyImportsOnce reuse components?

In a WCF service project, I have created a simple wrapper for MEF CompositionContainer to simplify its instantiation :
internal class CompositionProxy
{
private static Lazy<CompositionContainer> m_lazyCC;
static CompositionProxy()
{
m_lazyCC = new Lazy<CompositionContainer>(() =>
{
var batch = new CompositionBatch();
var dc1 = new DirectoryCatalog(
HttpContext.Current.Server.MapPath("~/bin")
);
return new CompositionContainer(dc1);
}
);
}
public static CompositionContainer DefaultContainer
{
get
{
return m_lazyCC.Value;
}
}
}
The idea is to have one CompositionContainer for the application lifetime, which search for export in the bin directory.
Then, I set up some webservices, that requires to have on imported property :
All of them are built like this :
public class MyService: IMyService
{
public MyService()
{
CompositionProxy.DefaultContainer.SatisfyImportsOnce(this);
}
[Import]
private IContext Context { get; set; }
public void DoTheJob()
{
// Logic goes here
}
}
Elsewhere, I have one class that match this export :
[Export(typeof(IContext))]
public class MyContext
{
public MyContext(){
Log("MyContext created");
}
}
In the constructor, I ask the composition container to populate the IContext Context property.
This seems to work, in my service, I can see the Context property is correctly populated.
However, I'm experiencing memory leaks, and my tracing show me the MyContext class is instantiated only once.
Can you clarify if I'm misusing the composition framework ?
I supposed it's a good idea to have one composition container for the application lifetime, was I wrong ?
the multiple calls to SatisfyImportsOnce seems to populate the target with the same unique instance. Is it true ? If true, how can I simply change my code to have a new instance each time the method is called ?
Any suggestion to improve my code ?

I supposed it's a good idea to have one composition container for the application lifetime
Yes, you are supposed to create one container for the application lifetime.
the multiple calls to SatisfyImportsOnce seems to populate the target with the same unique instance. Is it true ? If true, how can I simply change my code to have a new instance each time the method is called ?
You need [Import(RequiredCreationPolicy=CreationPolicy.NonShared)].
Any suggestion to improve my code ?
If possible, do not expose the container as a global and litter your code with calls to it. That's the Service Locator pattern, which has some disadvantages when compared to Dependency Injection. Instead of your service trying to compose itself, just declare what it needs:
[Export(typeof(IMyService))]
public class MyService: IMyService
{
private readonly IContext context;
public MyService(
[Import(typeof(IContext),
RequiredCreationPolicy=CreationPolicy.NonShared)]
IContext context)
{
if (context == null)
throw new ArgumentNullException("context");
this.context = context;
}
public void DoTheJob()
{
// Logic goes here
}
}
In a WCF service, I think you should only need to call the container in your ServiceHostFactory implementation. I'm not really familiar with WCF though.