ControllerActivator.cs

//Example usage
//
//var customControllerActivator = new CustomControllerActivator();
//customControllerActivator.AddType(() => new MyApiController());
//httpConfiguration.Services.Replace(typeof(IHttpControllerActivator),customControllerActivator);

class CustomControllerActivator : IHttpControllerActivator
{
    readonly Dictionary<Type, Func<IHttpController>> _dictionary = new Dictionary<Type, Func<IHttpController>>();

    public IHttpController Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)
    {
        return _dictionary[controllerType]();
    }

    public void AddType<T>(Func<T> creator) where T : class, IHttpController
    {
        _dictionary.Add(typeof(T), creator);
    }
}