Resolve dependency with autofac based on constructor parameter attribute

autofac, inversion-of-control

Solution

It sounds like you want to provide different implementations of `IObjectContainer` to `CustomerRepository` and `FooRepository`. If that is the case, attributes might be a thin metal ruler. Instead, I'll show you how I would implement multiple implementations with Autofac.

(Calls such as `.ContainerScoped()` have been left out for brevity.)

First, register a version of `IObjectContainer` for each connection string by naming the registrations:

builder
    .Register(c => new ObjectContainer(ConnectionStrings.CustomerDB))
    .As<IObjectContainer>()
    .Named("CustomerObjectContainer");

builder
    .Register(c => new ObjectContainer(ConnectionStrings.FooDB))
    .As<IObjectContainer>()
    .Named("FooObjectContainer");

Then, resolve the specific instances in the repository registrations:

builder.Register(c => new CustomerRepository(
    c.Resolve<IObjectContainer>("CustomerObjectContainer"));

builder.Register(c => new FooRepository(
    c.Resolve<IObjectContainer>("FooObjectContainer"));

This leaves the repositories free of configuration information:

class CustomerRepository
{
    public CustomerRepository(IObjectContainer db) { ... }
}

class FooRepository
{
    public FooRepository(IObjectContainer db) { ... }
}

Problem

I'm using Autofac. I want to inject a different implementation of a dependency based on an attribute I apply to the constructor parameter. For example: ``` class CustomerRepository { public CustomerRepository([CustomerDB] IObjectContainer db) { ... } } class FooRepository { public FooRepository([FooDB] IObjectContainer db) { ... } } builder.Register(c => /* return different instance based on attribute on the parameter */) .As<IObjectContainer>(); ``` The attributes will be providing data, such as a connection string, which I can use to instance the correct object. How can I do this?

Original source