AutoMapper, Autofac, Web API, and Per-Request Dependency Lifetime Scopes
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.
- The implementation of
AutoMapper.IMappingEnginewe’ll be using isAutoMapper.MappingEngine(the only implementation available). -
MappingEnginetakes in anIConfigurationProvideras a constructor parameter. -
IConfigurationProviderhas a propertyServiceCtorthat is the factory we need to manipulate to resolve things out of a per-request lifetime scope. - The main
AutoMapper.Mapperhas aConfigurationproperty of typeIConfiguration… but the backing store for it is really anAutoMapper.ConfigurationStore, which is also anIConfigurationProvider. (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.