为什么在我的工厂模式实现c＃中得到NullReferenceException？

Question

Main class:

public class ClP_Login
{
    private Form vrcView;
    private I_Repository<I_Identifiable> vrcRepository = null;

    public ClP_Login(Form vrpView)
    {
        vrcView = vrpView;
        SetTheme();
    }

    private void SetTheme()
    {
        if(vrcView !=null)
        vrcView.BackColor = Cl_BaseColor.StandardBackground;
    }

    public void CreateNewUser()
    {
        ClE_User test = new ClE_User();
        test.Name = "test name";
        test.Password = "";
        Cl_RepositoryFactory vrlFactory = new Cl_RepositoryFactory();
        vrcRepository = vrlFactory.CreateRepository(E_Repositories.User);
        vrcRepository.Add(test);
    }
}

Cl_RepositoryFactory class:

public class Cl_RepositoryFactory
{
    public virtual I_Repository<I_Identifiable> CreateRepository(E_Repositories vrpRepository)
    {
        I_Repository<I_Identifiable> vrlRepository = null;
        switch (vrpRepository)
        {
            case E_Repositories.User:
                vrlRepository = new Cl_UserRepository() as I_Repository<I_Identifiable>;
                break;
        }
        return vrlRepository;
    }
}

Enum E_Repositories:

public enum E_Repositories
{
    User
}

I_Identifiable Interface:

public interface I_Identifiable
{
    int Id { get; set; }
}

I_Repository Interface:

public interface I_Repository<T>
{
    T GetById(Guid id);
    T GetByQuery(Queue query);
    void Add(T item);
    void Remove(T item);
    void Update(T item);
}

Cl_UserRepository class:

public class Cl_UserRepository : I_Repository<ClE_User>
{
    public void Add(ClE_User item)
    {
        MessageBox.Show("Created new User");
    }

    public ClE_User GetById(Guid id)
    {
        throw new NotImplementedException();
    }

    public ClE_User GetByQuery(Queue query)
    {
        throw new NotImplementedException();
    }

    public void Remove(ClE_User item)
    {
        throw new NotImplementedException();
    }

    public void Update(ClE_User item)
    {
        throw new NotImplementedException();
    }
}

And ClE_User class:

public class ClE_User : I_Identifiable
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Password { get; set; }
}

The question is, why do I get null reference exception using vrcRepository? vrlFactory.CreateRepository(E_Repositories.User); return null and I don't have any idea why, please help

Answer 1

In CreateRepository method try to remove casting statement as I_Repository<I_Identifiable> . If your code will not compile, that will mean Cl_UserRepository is not compatible with I_Repository<I_Identifiable> .

Otherwise everyting is correct with CreateRepository method

Answer 2

ClE_User inherits from I_Identifiable , but I_Repository<ClE_User> does not inherit from I_Repository<I_Identifiable> . Those are different interfaces as far as C# is concerned.

To elaborate more, you have I_Repository<I_Identifiable> vrcRepository which should in theory take I_Repository of any I_Identifiable kind. So let's say you initialize this member to some other, for instance I_Repository<ClE_SomethingOtherThanUser> . But then you call vrcRepository.Add(test) . That's not going to work, with test being ClE_User .

Now, first remove the as I_Repository<I_Identifiable> part, and then to make it compile make I_Repository just a plain dumb non-generic interface, whose methods take I_Identifiable parameter or return I_Identifiable value. This may not be what you wanted, but it will compile.

Answer 3

EDIT

I realize that the enum will trigger. You are right

new Cl_UserRepository() as I_Repository

CL_UserRepository has to implement the interface you are trying to return, and then you don't need to type cast it at all. Sorry! I owe you a case of beer.