c# - Dependency Injection in Model classes (entities)

Question

I am building an ASP.NET Core MVC application with Entity Framework Code-First. I chose to implement 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 http://docs.asp.net and DI is one of these.

In .NET 5, dependency injection works very well for any class that we do not directly instanciate (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 I have 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 explicitely instanciated, and therefore, I would need to provide myself 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.

Answer 1

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.