how to unit test controller that in turn calls repository with two parameters

At present, I am writing unit tests for my controller. Below is the structure of my code in the project.

MyController Class

public class MyController : Controller
{              
    private readonly MyRepository _myRepository;        

    public MyController()
        : this(new MyRepository())
    {

    }

    [HttpGet]        
    public ActionResult Index()
    {
        var items = _myRepository.GetAllItems();
        if (items.Count() == 0)
            return View("EmptyItems");
        else
        {
            return View("List", items);
        }
    }
}

MyRepository Class

public class MyRepository : IDisposable, IMyRepository
{
    private readonly MyDbContext _dbcontext;
    private readonly ISecurityService _securityService;


    public TodoListItemsRepository() : this(new MyDbContext(), new SecurityService())
    {            

    }
    public TodoListItemsRepository(MyDbContext context, ISecurityService securityService)
    {
        _dbcontext = context;
        _securityService = securityService;
    }
    public IEnumerable<MyModel> GetAllItems()
    {
        var userid = _securityService.GetUser();
        var todoList = _dbcontext.MyList.Where(e => e.UserId == userid);

        return todoList;
    }
    //Other Methods etc...
    ......
}

SecurityService class

public class SecurityService : ISecurityService
{    
    public int GetUser()
    {
        return (int)Membership.GetUser().ProviderUserKey;
    } 
}

Here all methods inside my repository depends on GetUser method. Hence, I have initialized it inside the constructor. The repository class is initialized from the controller constructor.

My issue is - I couldn't unit the Index action unless I need to initialize dbcontext and the securityservice. Could someone please advise me if I am doing the right thing or any changes required in the structure of my code so that I can unit test my application ? I am new to MVC. So, any suggestions would be much appreciated.