Property Injection in Asp.Net Core

Question

I am trying to port an asp.net application to asp.net core. I have property injection (using ninject) on my UnitOfWork implementation like this.

[Inject]
public IOrderRepository OrderRepository { get; set; }
[Inject]
public ICustomerRepository CustomerRepository { get; set; }

Is there a way to achieve the same functionality using build in DI on .net core? Also is it possible to use convention based binding?

Answer 1

No, the built-in DI/IoC container is intentionally kept simple in both usage and features to offer a base for other DI containers to plug-in.

So there is no built-in support for: Auto-Discovery, Auto-Registrations, Decorators or Injectors, or convention based registrations. There are also no plans to add this to the built-in container yet as far as I know.

You'll have to use a third party container with property injection support.

Please note that property injection is considered bad in 98% of all scenarios, because it hides dependencies and there is no guarantee that the object will be injected when the class is created.

With constructor injection you can enforce this via constructor and check for null and the not create the instance of the class. With property injection this is impossible and during unit tests its not obvious which services/dependencies the class requires when they are not defined in the constructor, so easy to miss and get NullReferenceExceptions.

The only valid reason for Property Injection I ever found was to inject services into proxy classes generated by a third party library, i.e. WCF proxies created from an interface where you have no control about the object creation.

Avoid it everywhere else.

Answer 2

Is there a way to achieve the same functionality using build in DI on .net core?

No, but here is how you can create your own [inject] attributes with the help of autofac's property injection mecanism.

First Create your own InjectAttribute:

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class InjectAttribute : Attribute
{
  public InjectAttribute() : base() { }
}

Then create your own InjectPropertySelector that uses reflection to check for properties marked with [inject]:

public class InjectPropertySelector : DefaultPropertySelector
{
  public InjectPropertySelector(bool preserveSetValues) : base(preserveSetValues)
  { }

  public override bool InjectProperty(PropertyInfo propertyInfo, object instance)
  {
    var attr = propertyInfo.GetCustomAttribute<InjectAttribute>(inherit: true);
    return attr != null && propertyInfo.CanWrite
            && (!PreserveSetValues
            || (propertyInfo.CanRead && propertyInfo.GetValue(instance, null) == null));
  }
}

Then use your selector in your ConfigureServices where you wire up your AutofacServiceProvider:

public class Startup
{
  public IServiceProvider ConfigureServices(IServiceCollection services)
  {
    var builder = new ContainerBuilder();
    builder.Populate(services);

    // use your property selector to discover the properties marked with [inject]
    builder.RegisterType<MyServiceX>().PropertiesAutowired((new InjectablePropertySelector(true)););

    this.ApplicationContainer = builder.Build();
    return new AutofacServiceProvider(this.ApplicationContainer);
  }
}

Finally in your service you can now use [inject]:

public class MyServiceX 
{
    [Inject]
    public IOrderRepository OrderRepository { get; set; }
    [Inject]
    public ICustomerRepository CustomerRepository { get; set; }
}

You surely can take this solution even further, e.g. by using an attribute for specifying your service's lifecycle above your service's class definition...

[Injectable(LifetimeScope.SingleInstance)]
public class IOrderRepository

...and then checking for this attribute when configuring your services via autofac. But this would go beyond the scope of this answer.