Register MEF for both ASP.NET MVC and Web API using conventions.

ContainerConfig.cs

namespace Portal.Web
{
    using System.Composition.Convention;
    using System.Composition.Hosting;
    using System.Reflection;
    using System.Web.Http;
    using System.Web.Http.Controllers;

    public static class ContainerConfig
    {
        public static void RegisterMef(HttpConfiguration configuration)
        {
            var appAssemblies =
                new[]
                {
                    Assembly.GetAssembly(typeof(IValidationService)), // .Service
                    Assembly.GetAssembly(typeof(ILookups)), // .Data
                    Assembly.GetAssembly(typeof(ISpreadsheet)), // .Excel
                    Assembly.GetExecutingAssembly()
                };

            var resolver = CreateWithConventions(appAssemblies);

            System.Web.Mvc.DependencyResolver.SetResolver(resolver); // MVC.

            configuration.DependencyResolver = (System.Web.Http.Dependencies.IDependencyResolver)resolver; // WebAPI.
        }

        private static System.Web.Mvc.IDependencyResolver CreateWithConventions(Assembly[] appAssemblies)
        {
            var conventions = new ConventionBuilder();

            conventions.ForTypesDerivedFrom<IHttpController>()
                .Export();

            conventions.ForTypesMatching(
                t => t.Namespace != null && t.Namespace.StartsWith("Unsettling."))
                .Export()
                .ExportInterfaces();

            var container = new ContainerConfiguration()
                .WithAssemblies(appAssemblies, conventions)
                .WithExport(AutoMapperConfig.MappingEngineInstance)
                .CreateContainer();

            return new MefDependencyResolver(container);
        }
    }
}