var container = new XyzWebApiStructureMapContainerConfigurator().Configure(GlobalConfiguration.Configuration);
GlobalConfiguration.Configuration.Services
.Replace(typeof(IHttpControllerActivator),
new StructureMapHttpControllerActivator(container));

public class XyzWebApiStructureMapContainerConfigurator
{
    public IContainer Configure(HttpConfiguration config)
    {
        var container = new Container(new BlaWebApiRegistry());
        config.DependencyResolver = new StructureMapDependencyResolver(container);
        return container;
    }
}

public class StructureMapDependencyResolver : StructureMapDependencyScope, IDependencyResolver, IHttpControllerActivator
{
    private readonly IContainer _container;

    public StructureMapDependencyResolver(IContainer container)
        : base(container)
    {
        _container = container;
        container.Inject<IHttpControllerActivator>(this);
    }

    public IDependencyScope BeginScope()
    {
        return new StructureMapDependencyScope(_container.GetNestedContainer());
    }

    public IHttpController Create(
        HttpRequestMessage request,
        HttpControllerDescriptor controllerDescriptor,
        Type controllerType)
    {
        var scope = request.GetDependencyScope();
        return scope.GetService(controllerType) as IHttpController;
    }
}

[RoutePrefix("api/XYZ")]
public class BlaController : ApiController
{
    private readonly ISomething _something;

    public BlaController(ISomething something)
    {
        _something = something;
    }

    [Route("")]
    [HttpGet]
    public IHttpActionResult Resources([FromUri] BlaRequest blaRequest)
    {
        // something exciting
        return Ok(returnObject);
    }
}

public static void Register(HttpConfiguration config)
{
    // Web API configuration and services

    // Web API routes
    config.MapHttpAttributeRoutes();

    //var cors = new EnableCorsAttribute("*", "*", "*");
    config.EnableCors();

    config.Routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}",
    defaults: new { id = RouteParameter.Optional }
    );
}

public class WebApiApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        GlobalConfiguration.Configure(WebApiConfig.Register);

        GlobalConfiguration.Configuration.Formatters.Clear();
        GlobalConfiguration.Configuration.Formatters.Add(new JsonMediaTypeFormatter());

        var container = new XyzWebApiStructureMapContainerConfigurator().Configure(GlobalConfiguration.Configuration);
        GlobalConfiguration.Configuration.Services
        .Replace(typeof(IHttpControllerActivator),
            new StructureMapHttpControllerActivator(container));
    }
}

{
swagger: "2.0",
info: {
version: "v1",
title: "Bla.Di.Bla"
},
host: "localhost:51854",
schemes: [
"http"
],
paths: { },
definitions: { }
}

config.DependencyResolver = new StructureMapDependencyResolver(container);

var assemblies = AppDomain.CurrentDomain.GetAssemblies().Where(asm => asm.FullName.StartsWith("MySolution.MyProject"));

// Web API configuration and services
var container = new UnityContainer();
container.RegisterTypes(
    AllClasses.FromAssemblies(assemblies),
    WithMappings.FromMatchingInterface,
    WithName.Default);

config.DependencyResolver = new UnityDependencyResolver(container);