Generic interface implementation confusion

Question

I'm getting myself in a bit of a mess regarding interface implementations, all my attempted 'fixes' seem to make the whole solution more complex and broken. I'm sure there is a simple answer, but I can't quite see it at the moment!

I have these two interfaces (the second is used in a list of the first):

public interface IUserDefinedListEditViewModel<T> where T : IUserDefinedListEntryEditViewModel<IBaseUserDefinedListModel>
{
    string TypeName { get; set; }
    IList<T> UserDefinedListEntries { get; set; }
}

public interface IUserDefinedListEntryEditViewModel<T> where T : IBaseUserDefinedListModel
{
    string Display { get; set; }
    T UserDefinedListEntry { get; set; }
}

I have a third interface which is implemented by several different classes:

public interface IBaseUserDefinedListModel
{
    Guid Id { get; set; }
    string Name { get; set; }
    bool IsSystem { get; set; }
}

Below is my (incorrect) implementation attempt:

public class APEditViewModel : IUserDefinedListEditViewModel<APEntryEditViewModel>
{
    public string TypeName { get; set; }
    public IList<APEntryEditViewModel> UserDefinedListEntries { get; set; } = new List<APEntryEditViewModel>();
}

public class APEntryEditViewModel : IUserDefinedListEntryEditViewModel<APModel>
{
    public string Display { get; set; }
    public APModel UserDefinedListEntry { get; set; }
}

public class BaseUserDefinedListModel : IBaseUserDefinedListModel
{
    public Guid Id { get; set; }
    [Required(ErrorMessage = "The Name field is required.")]
    public string Name { get; set; }
    public bool IsSystem { get; set; }
}

public class APModel : BaseUserDefinedListModel
{
    public string NewValue { get; set; }
}

The main error I'm getting at the moment is in the APEditViewModel, here is the (cut down) error:

The type 'APEntryEditViewModel' cannot be used as type parameter 'T' in the
generic type or method 'IUserDefinedListEditViewModel<T>'. There is no 
implicit reference conversion from 'APEntryEditViewModel' to 
'IUserDefinedListEntryEditViewModel<IBaseUserDefinedListModel>'.

I'm not sure whether I need this level of generic interfaces, but from my research and experiments, I believe I do. I'm just not quite getting there and I'm thinking that the IUserDefinedListEditViewModel interface needing a type in the type interface ( IUserDefinedListEntryEditViewModel ) seems wrong.

Sorry that I'm not making myself that clear, it's quite tricky to explain because I'm not sure where I'm going wrong, so any questions I'll try and answer/update my question.

Answer 1

Change your IUserDefinedListEditViewModel interface to:

public interface IUserDefinedListEditViewModel<T1,T2> 
           where T1 : IUserDefinedListEntryEditViewModel<T2> 
           where T2 : IBaseUserDefinedListModel

After that, and update the implementation in APEditViewModel :

public class APEditViewModel : IUserDefinedListEditViewModel<APEntryEditViewModel, APModel>

---

The error indicates that with the generic constraint

where T : IUserDefinedListEntryEditViewModel<IBaseUserDefinedListModel>

only IUserDefinedListEntryEditViewModel<IBaseUserDefinedListModel> and types which implement this interface will be accepted as T . If you want any IBaseUserDefinedListModel to be accepted in the type parameter of T , you need to make it generic as well.

Answer 2

You need to give us the whole business scenario for expecting the right design suggestions.

As of above examples the issue is: Your defined interface::

interface IUserDefinedListEditViewModel<T> where T : IUserDefinedListEntryEditViewModel<IBaseUserDefinedListModel>

But while calling you are passing APEntryEditViewModel as Interface specific type. Which is wrong with OOPS.

To successfully compile this your

APEntryEditViewModel must be an implimentation of IUserDefinedListEntryEditViewModel