Question

I have a Singleton class that uses the thread-safe Singleton pattern from Jon Skeet as seen in the TekPub video. The class represents a cached list of reference data for dropdowns in an MVC 3 UI.

To get the list data the class calls a static method on a static class in my DAL.

Now I’m moving into testing an I want to implement an interface on my DAL class but obviously cannot because it is static and has only one static method so there’s no interface to create. So I want to remove the static implementation so I can do the interface.

By doing so I can’t call the method statically from the reference class and because the reference class is a singleton with a private ctor I can’t inject the interface. How do I get around this? How do I get my interface into the reference class so that I can have DI and I can successfully test it with a mock?

Here is my DAL class in current form

public static class ListItemRepository {

    public static List<ReferenceDTO> All() {
        List<ReferenceDTO> fullList;
        ... /// populate list
        return fullList;
    }
}

This is what I want it to look like

public interface IListItemRepository {
    List<ReferenceDTO> All();
}

public class ListItemRepository : IListItemRepository {

    public List<ReferenceDTO> All() {
        List<ReferenceDTO> fullList;
        ... /// populate list
        return fullList;
    }
}

And here is my singleton reference class, the call to the static method is in the CheckRefresh call

public sealed class ListItemReference {

    private static readonly Lazy<ListItemReference> instance = 
        new Lazy<ListItemReference>(() => new ListItemReference(), true);

    private const int RefreshInterval = 60;
    private List<ReferenceDTO> cache;
    private DateTime nextRefreshDate = DateTime.MinValue;

    public static ListItemReference Instance {
        get { return instance.Value; }
    }

    public List<SelectListDTO> SelectList {
        get {
            var lst = GetSelectList();
            lst = ReferenceHelper.AddDefaultItemToList(lst);
            return lst;
        }
    }

    private ListItemReference() { }

    public ReferenceDTO GetByID(int id) {
        CheckRefresh();
        return cache.Find(item => item.ID == id);
    }

    public void InvalidateCache() {
        nextRefreshDate = DateTime.MinValue;
    }

    private List<SelectListDTO> GetSelectList() {
        CheckRefresh();
        var lst = new List<SelectListDTO>(cache.Count + 1);
        cache.ForEach(item => lst.Add(new SelectListDTO { ID = item.ID, Name = item.Name }));
        return lst;
    }

    private void CheckRefresh() {
        if (DateTime.Now <= nextRefreshDate) return;
        cache = ListItemRepository.All(); // Here is the call to the static class method
        nextRefreshDate = DateTime.Now.AddSeconds(RefreshInterval);
    }
    }
}

Answer 1

You can use the singleton based on instance(not based on static), for which you can declare interface like this.

public interface IListItemRepository
{
    List<ReferenceDTO> All();
}


public class ListItemRepository : IListItemRepository
{
    static IListItemRepository _current = new ListItemRepository();

    public static IListItemRepository Current
    {
        get { return _current; }
    }

    public static void SetCurrent(IListItemRepository listItemRepository)
    {
        _current = listItemRepository;
    }

    public List<ReferenceDTO> All()
    {
        .....
    }
}

Now, you can mock IListItemRepository to test.

    public void Test()
    {
        //arrange
        //If Moq framework is used,
        var expected = new List<ReferneceDTO>{new ReferneceDTO()};

        var mock = new Mock<IListItemRepository>();           
        mock.Setup(x=>x.All()).Returns(expected);

        ListItemRepository.SetCurrent(mock.Object);

        //act
        var result = ListItemRepository.Current.All();

        //Assert
        Assert.IsSame(expected, result);
    }