AutoMapper, Autofac, Web API, and Per-Request Dependency Lifetime Scopes

net, Web Development comments edit

I’m working on a new Web API project where I want to use AutoMapper for some type conversion. As part of that, I have a custom AutoMapper type converter that takes in some constructor parameters so the converter can read configuration values. I’m using Autofac for dependency injection (naturally).

Historically, I’ve been able to hook AutoMapper into dependency injection using the ConstructServicesUsing method and some sort of global dependency resolver, like:

Mapper.Initialize(cfg =>
{
  cfg.ConstructServicesUsing(t => DependencyResolver.Current.GetService(t));
  cfg.CreateMap();
});

That works great in MVC or in other applications where there’s a global static like that. In those cases, the “request lifetime scope” either doesn’t exist or it’s managed by the implementation of IDependencyResolver the way it is in the Autofac integration for MVC.

Retrieving the per-request lifetime scope is much more challenging in Web API because the request lifetime scope is managed by the inbound HttpRequestMessage. Each inbound message gets a lifetime scope associated, so there’s no “global static” from which you can get the request lifetime. You can get the global dependency resolver, but resolving from that won’t be per-request; it’ll be at the application level.

It’s also a challenging situation because AutoMapper really leans you toward using the static Mapper object to do your mapping and you can’t really change the value of ConstructServicesUsing on the static because, well, you know, threading.

So… what to do?

The big step is to change your mindset around the static Mapper object. Instead of using Mapper to map things, take an IMappingEngine as a dependency in your class doing mapping. Yes, that’s one more dependency you’d normally not have to take, but there’s not really a better way given the way the IMappingEngine has to resolve dependencies is actually different per-request.

This frees us up to now think about how to register and resolve a per-request version of IMappingEngine.

Before I show you how to do this, standard disclaimers apply: Works on my machine; I’ve not performance tested it; It might not work for you; etc.

Oooookay.

First, we need to understand how the IMappingEngine we build will come together.

  1. The implementation of AutoMapper.IMappingEngine we’ll be using is AutoMapper.MappingEngine (the only implementation available).
  2. MappingEngine takes in an IConfigurationProvider as a constructor parameter.
  3. IConfigurationProvider has a property ServiceCtor that is the factory we need to manipulate to resolve things out of a per-request lifetime scope.
  4. The main AutoMapper.Mapper has a Configuration property of type IConfiguration… but the backing store for it is really an AutoMapper.ConfigurationStore, which is also an IConfigurationProvider. (This is where the somewhat delicate internal part of things comes in. If something breaks in the future, chances are this will be it.)

Since we need an IConfigurationProvider, let’s make one.

We want to leverage the main configuration/initialization that the static Mapper class provides because there’s a little internal work there that we don’t want to copy/paste. The only thing we really want to change is that ServiceCtor property, but that’s not a settable property, so let’s write a quick wrapper around an IConfigurationProvider that lets us override it with our own method.

public class ConfigurationProviderProxy : IConfigurationProvider
{
  private IComponentContext _context;
  private IConfigurationProvider _provider;
  
  // Take in a configuration provider we're going to wrap
  // and an Autofac context from which we can resolve things.
  public ConfigurationProviderProxy(IConfigurationProvider provider, IComponentContext context)
  {
    this._provider = provider;
    this._context = context;
  }
  
  // This is the important bit - we use the passed-in
  // Autofac context to resolve dependencies.
  public Func<Type, object> ServiceCtor
  {
    get
    {
      return this._context.Resolve;
    }
  }
  
  //
  // EVERYTHING ELSE IN THE CLASS IS JUST WRAPPER/PROXY
  // CODE TO PASS THROUGH TO THE BASE PROVIDER.
  //
  public bool MapNullSourceCollectionsAsNull { get { return this._provider.MapNullSourceCollectionsAsNull; } }
  
  public bool MapNullSourceValuesAsNull { get { return this._provider.MapNullSourceValuesAsNull; } }
  
  public event EventHandler<TypeMapCreatedEventArgs> TypeMapCreated
  {
    add { this._provider.TypeMapCreated += value; }
    remove { this._provider.TypeMapCreated -= value; }
  }
  
  public void AssertConfigurationIsValid()
  {
    this._provider.AssertConfigurationIsValid();
  }
  
  public void AssertConfigurationIsValid(TypeMap typeMap)
  {
    this._provider.AssertConfigurationIsValid(typeMap);
  }
  
  public void AssertConfigurationIsValid(string profileName)
  {
    this._provider.AssertConfigurationIsValid(profileName);
  }

  public TypeMap CreateTypeMap(Type sourceType, Type destinationType)
  {
    return this._provider.CreateTypeMap(sourceType, destinationType);
  }
  
  public TypeMap FindTypeMapFor(ResolutionResult resolutionResult, Type destinationType)
  {
    return this._provider.FindTypeMapFor(resolutionResult, destinationType);
  }
  
  public TypeMap FindTypeMapFor(Type sourceType, Type destinationType)
  {
    return this._provider.FindTypeMapFor(sourceType, destinationType);
  }
  
  public TypeMap FindTypeMapFor(object source, object destination, Type sourceType, Type destinationType)
  {
    return this._provider.FindTypeMapFor(source, destination, sourceType, destinationType);
  }
  
  public TypeMap[] GetAllTypeMaps()
  {
    return this._provider.GetAllTypeMaps();
  }
  
  public IObjectMapper[] GetMappers()
  {
    return this._provider.GetMappers();
  }
  
  public IFormatterConfiguration GetProfileConfiguration(string profileName)
  {
    return this._provider.GetProfileConfiguration(profileName);
  }
}

That was long, but there’s not much logic to it. You could probably do some magic to make this smaller with Castle.DynamicProxy but I’m keeping it simple here.

Now we need to register IMappingEngine with Autofac so that it:

  • Creates a per-request engine that
  • Uses a per-request lifetime scope to resolve dependencies and
  • Leverages the root AutoMapper configuration for everything else.

That’s actually pretty easy:

// Register your mappings here, but don't set any
// ConstructServicesUsing settings.
Mapper.Initialize(cfg =>
{
  cfg.AddProfile<SomeProfile>();
  cfg.AddProfile<OtherProfile>();
});

// Start your Autofac container.
var builder = new ContainerBuilder();

// Register your custom type converters and other dependencies.
builder.RegisterType<DemoConverter>().InstancePerApiRequest();
builder.RegisterApiControllers(Assembly.GetExecutingAssembly());

// Register the mapping engine to use the base configuration but
// a per-request lifetime scope for dependencies.
builder.Register(c =>
{
  var context = c.Resolve<IComponentContext>();
  var config = new ConfigurationProviderProxy(Mapper.Configuration as IConfigurationProvider, context);
  return new MappingEngine(config);
}).As<IMappingEngine>()
.InstancePerApiRequest();

// Build the container.
var container = builder.Build();

Now all you have to do is take an IMappingEngine as a dependencyand use that rather than AutoMapper.Mapper for mapping.

public class MyController : ApiController
{
  private IMappingEngine _mapper;

  public MyController(IMappingEngine mapper)
  {
    this._mapper = mapper;
  }

  [Route("api/myaction")]
  public SomeValue GetSomeValue()
  {
    // Do some work and use the IMappingEngine for maps.
    return this._mapper.Map<SomeValue>(otherValue);
  }
}

Following that pattern, any mapping dependencies will be resolved out of the per-request lifetime scope rather than the application root container and you won’t have to use any static references or fight with request contexts. When the API controller is resolved (out of the request scope) the dependent IMappingEngine will be as well, as will all of the chained-in dependencies for mapping.

While I’ve not tested it, this technique should also work in an MVC app to allow you to get away from the static DependencyResolver.Current reference. InstancePerApiRequest and InstancePerHttpRequest do effectively the same thing internally in Autofac, so the registrations are cross-compatible.

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