"Message":"An error has occurred.","ExceptionMessage":"An error occurred
when trying to create a controller of type 'MyController'. Make sure that the
controller has a parameterless public
constructor.","ExceptionType":"System.InvalidOperationException"

public class MyController : ApiController
{
    private IMyManager MyManager { get; set; }

    public MyController(IMyManager myManager)
    {
        this.MyManager = myManager;
    }

    ...
}

container.RegisterType<IMyManager, MyManager>();

{"Message":"An error has occurred.","ExceptionMessage":"An error occurred when trying to create a controller of type 'MyController'. Make sure that the controller has a parameterless public constructor.","ExceptionType":"System.InvalidOperationException","StackTrace":"   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)
at System.Web.Http.Controllers.HttpControllerDescriptor.CreateController(HttpRequestMessage request)
at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__1.MoveNext()","InnerException":{"Message":"An error has occurred.","ExceptionMessage":"Type 'MyProject.Web.Api.Controllers.MyController' does not have a default constructor","ExceptionType":"System.ArgumentException","StackTrace":"   at System.Linq.Expressions.Expression.New(Type type)
at System.Web.Http.Internal.TypeActivator.Create[TBase](Type instanceType)
at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.GetInstanceOrActivator(HttpRequestMessage request, Type controllerType, Func`1& activator)
at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)"}}

using Microsoft.Practices.Unity;
using System.Web.Http;
using System.Web.Mvc;

namespace WebApplication1
{
    public static class UnityConfig
    {
        public static void RegisterComponents()
        {
            var container = new UnityContainer();

            // register all your components with the container here
            // it is NOT necessary to register your controllers

            // e.g. container.RegisterType<ITestService, TestService>();

            // Configures container for ASP.NET MVC
            DependencyResolver.SetResolver(new Unity.Mvc5.UnityDependencyResolver(container));

            // Configures container for WebAPI
            GlobalConfiguration.Configuration.DependencyResolver = new Unity.WebApi.UnityDependencyResolver(container);
        }
    }
}

old.managers.IMyManager

new.managers.IMyManager

container.RegisterType<IMyManager, MyManager>();

public MyController() {}

public static void RegisterComponents()
    {
        var container = new UnityContainer();

        container.RegisterType<IMyManager, MyManager>();

        MvcUnityContainer.Container = container;

        DependencyResolver.SetResolver(new UnityDependencyResolver(container));
    }

public class UnityControllerFactory : DefaultControllerFactory
{
    public override IController CreateController(RequestContext requestContext, string controllerName)
    {
        Type controllerType = null;
        if (TypeHelper.LooksLikeTypeName(controllerName))
        {
            controllerType = TypeHelper.GetType(controllerName);
        }

        if (controllerType == null)
        {
            controllerType = this.GetControllerType(requestContext, controllerName);
        }

        return controllerType != null ? this.GetControllerInstance(requestContext, controllerType) : null;
    }

    protected override IController GetControllerInstance(RequestContext requestContext, Type controllerType)
    {
        try
        {
            if (controllerType == null)
                throw new ArgumentNullException("controllerType");

            if (!typeof(IController).IsAssignableFrom(controllerType))
                throw new ArgumentException(string.Format(
                    "Type requested is not a controller: {0}",
                    controllerType.Name),
                    "controllerType");

            return MvcUnityContainer.Container.Resolve(controllerType) as IController;
        }
        catch
        {
            return null;
        }
    }
}

public static class MvcUnityContainer
{
    public static IUnityContainer Container { get; set; }
}

UnityConfig.RegisterComponents();