Model bind to Interface via DI

[benaadams]

Requirement

I'd like to bind to a model interface via DI but it doesn't seem to work? Am I doing something wrong?

Steps to reproduce

Models

public interface IMyModel
{
    string Name { get; set; }
    int Count { get; set; }
}

public class MyModel : IMyModel
{
    public string Name { get; set; }
    public int Count { get; set; }
}

ConfigureServices

public void ConfigureServices(IServiceCollection services)
{
    services.AddTransient<IMyModel>((provider) => new MyModel());
    services.AddMvc();
}

Controller Method

public IActionResult Contact(IMyModel model)
{
    ViewData["Message"] = $"{model.Name} {model.Count}";

    return View();
}

Expected behavior

Transient object obtained from DI; then its properties set as per non DI type

Actual behavior

fail: Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware[0]
      An unhandled exception has occurred: Could not create an instance of type 'mvc.IMyModel'. 
Model bound complex types must not be abstract or value types and must have a parameterless constructor.
System.InvalidOperationException: Could not create an instance of type 'mvc.IMyModel'. Model bound complex types must not be abstract or value types and must have a parameterless constructor.
   at Microsoft.AspNetCore.Mvc.ModelBinding.Binders.ComplexTypeModelBinder.CreateModel(ModelBindingContext bindingContext)
   at Microsoft.AspNetCore.Mvc.ModelBinding.Binders.ComplexTypeModelBinder.<BindModelCoreAsync>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Mvc.Internal.DefaultControllerArgumentBinder.<BindModelAsync>d__8.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Mvc.Internal.DefaultControllerArgumentBinder.<BindArgumentsCoreAsync>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeNextResourceFilter>d__22.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ResourceExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeAsync>d__20.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Builder.RouterMiddleware.<Invoke>d__4.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Diagnostics.ExceptionHandlerMiddleware.<Invoke>d__6.MoveNext()

/cc @DamianEdwards @rynowak

[pranavkm]

public IActionResult Contact([FromServices] IMyModel model)

We don't look in DI by default (similar to how formatting doesn't kick in until you say FromBody.

[benaadams]

@pranavkm Perfect thank you!

[rynowak]

then its properties set as per non DI type

[FromServices] does not do this.

[rynowak]

@benaadams - here's a sample that someone else wrote https://github.com/Serjster/IOCModelBinderExample/tree/ASPNETRTMVersion

[benaadams]

[FromServices] does not do this.

You give with one hand then take away with the other 😢

[rynowak]

I gave you a sample?

And yeah [FromServices] is a missed opportunity

[benaadams]

[FromServicesAndBody]? 😉

[rynowak]

What I really mean is that it would have been dope to be able to create a model using deserialization (like [FromBody]) - and enrich that with model binding some other sources as an opt-in model. It would have been cool if we decided that this was how things should work by default in 1.0.0 but I think that 🚢 has ⛵️ 'ed

[benaadams]

Wouldn't need an object mapper for simple cases though if could just use the language feature of interfaces to do it

[frankabbruzzese]

I wrote a post about how to model bind interfaces in the way you need:

http://www.dotnet-programming.com/post/2017/03/17/Custom-Model-Binding-in-Aspnet-Core-2-Model-Binding-Interfaces.aspx

There you will find a custom binder for interfaces, and how to make validation works properly when using interfaces.

[frankabbruzzese]

@benaadams , anyway, I dont know if interfaces are enough to avoid object mapping at all:

• You need a IQueryable projection operator, in any case, otherwise you are forced to retrive from DB also data you don't need, in case you need just a subset of the data object properties.

• While client side validation is done according to the attributes you place on the interface, server side validation is done according to the actual type that is behind the interface. This means that if you want everything works properly you must add the interface as a MetaDataType of the class that implements it. However, in turn, this means you may use just one interface implemented by that type. Which in turn means you can't use a different interface in case you want to modify just some of the object properties in the user interface.

I dont know if the validation behavior is correct or a kinda bug, but for sure there are classes where you need the actual runtime type be validated instead of the declared type (model binding subclasses for instance).

[benaadams]

Scenario: I have one microservice posting to another microservice; they both have their own data models - however they share an interface/contract for data exchange; but don't share models.

So easy approach is to serialise and post the through interface; then deserialize the posted interface (and populate the rest of the local model as needed)