Model Binder Injection in Autofac ASP.NET MVC 3 Integration

The Autofac MVC integration supported model binder injection in the MVC 2 version, but improvements in the dependency injection support offered by MVC 3 has allowed the implementation to be made cleaner. ASP.NET MVC 3 introduces the IModelBinderProvider interface that allows the implementer to determine what model binder should be used for a particular type.

Developers who implement this interface can optionally return an implementation of IModelBinder for a given type (they should return null if they cannot create a binder for the given type).

Let’s start by looking at how the model binder injection is configured in the MVC integration. You first create a class that implements IModelBinder like you would when creating any other model binder in MVC. Next you apply the ModelBinderType attribute provided as part of the integration to indicate what types the model binder supports binding. The simple example below declares that the model binder supports binding for string types.

[ModelBinderType(typeof(string))]
public class StringBinder : IModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        // Do implementation here.
    }
}

You then use the RegisterModelBinders extension method on the ContainerBuilder to register all the IModelBinder types that are present in one or more assemblies.

ContainerBuilder builder = new ContainerBuilder();
builder.RegisterModelBinders(Assembly.GetExecutingAssembly());

The interesting part about the implementation of the assembly scanning is that it finds the types the model binder supports through the ModelBinderType attributes and then adds this information as metadata to the registration.

public static IRegistrationBuilder<object, ScanningActivatorData, DynamicRegistrationStyle>
    RegisterModelBinders(this ContainerBuilder builder, params Assembly[] modelBinderAssemblies)
{
    return builder.RegisterAssemblyTypes(modelBinderAssemblies)
        .Where(type => typeof(IModelBinder).IsAssignableFrom(type))
        .As<IModelBinder>()
        .InstancePerHttpRequest()
        .WithMetadata(AutofacModelBinderProvider.MetadataKey, type => 
            (from ModelBinderTypeAttribute attribute in type.GetCustomAttributes(typeof(ModelBinderTypeAttribute), true)
             from targetType in attribute.TargetTypes
            select targetType).ToList());
}

You must also remember to register the AutofacModelBinderProvider using the RegisterModelBinderProvider extension method. This is Autofac's implementation of the new IModelBinderProvider interface.

builder.RegisterModelBinderProvider();

The constructor of the AutofacModelBinderProvider requests that an IEnumerable<Meta<Lazy<IModelBinder>>> be provided. When the GetBinder method is called through the IModelBinderProvider interface, the list of Meta<T> about the components is queried to locate any potential matches based on the types stored in the metadata. The Lazy<T> part of dependency makes sure that we do not actually create an instance of the IModelBinder until it is actually needed.

public IModelBinder GetBinder(Type modelType)
{
    Meta<Lazy<IModelBinder>> modelBinder = _modelBinders
        .FirstOrDefault(binder => ((List<Type>)binder.Metadata[MetadataKey]).Contains(modelType));
    return (modelBinder != null) ? modelBinder.Value.Value : null;
}

This dynamic approach to handling model binder injection removes the need for a special wrapper around each IModelBinder component, and avoids having to register this wrapper directly into the static ModelBinders.Binders dictionary.