How does Resolve works in case of AsImplementedInterfaces?

Question

I have a simple application as shown below:

public interface PaymentProcessor
{
    void ProcessPayment();
}

public class PaymentProcessorOne : PaymentProcessor
{
    public void ProcessPayment()
    {
        Console.WriteLine("Payment processed by PaymentProcessorOne.");
    }
}

public class PaymentProcessorTwo : PaymentProcessor
{
    public void ProcessPayment()
    {
        Console.WriteLine("Payment processed by PaymentProcessorTwo.");
    }
}

public class PaymentProcessorThree : PaymentProcessor
{
    public void ProcessPayment()
    {
        Console.WriteLine("Payment processed by PaymentProcessorThree.");
    }
}

And I'm registering my types in Program.cs as:

    var builder = new ContainerBuilder();
    builder.RegisterAssemblyTypes(Assembly.GetExecutingAssembly()).AsImplementedInterfaces();

    IContainer container = builder.Build();
    var processor = container.Resolve<PaymentProcessor>();
    processor.ProcessPayment();

First time when I executed this code, the output was "Payment processed by PaymentProcessorTwo". However, when I executed the same code on a different machine the output was "Payment processed by PaymentProcessorThree".

I would like to know how is autofac resolving the PaymentProcessor in the main logic.

And what is the behavior you need? Do you always want to have one particular implementation? — Steven, Oct 07 '16 at 08:17
Related: https://stackoverflow.com/questions/5218803/is-order-of-dependencies-guaranteed-when-injecting-ienumerablet — Steven, Oct 07 '16 at 08:18
Yeah, I would like `processor` to always have an instance of `PaymentProcessorOne`. — Gaurav Gahlot, Oct 07 '16 at 08:56

score 1 · Accepted Answer · answered Oct 13 '16 at 13:42

The problem is not in AsImplementedInterfaces(), but in .RegisterAssemblyTypes().

The magic of resolving types, when there are more than one type registered for interface is pretty simple - last registration wins. It means, the order of types is unknown when you use RegisterAssemblyTypes(), and can vary on different machines you run your code.

There are several ways to o make it work as expected, e.g. you can explicitly register your types or sort them somehow before you pass them to RegisterAssemblyTypes() method

How does Resolve works in case of AsImplementedInterfaces?

1 Answers1