generics - C# - No implicit reference conversion

Question

I know the problem but I couldn't find solution. How can I create such a structure. What is the correct way?

public class BuildingListPageViewModel : ListPageViewModel<BuildingItemViewModel>
{
}

public interface ItemViewModel<T> where T:IEntity
{
    T Model { get; set; }
}

public abstract class ListPageViewModel<TVm> : PageViewModel where TVm : ItemViewModel<IEntity>
{
}


public class BuildingItemViewModel : ItemViewModel<Building>
{
}

public partial class Building : IEntity
{
    public int Id;
}

It gives BuildingItemViewModel cannot be used as type parameter TVm in the generic type or method ListPageViewModel<TVm> . There is no implicit reference conversion from BuildingItemViewModel to ItemViewModel<IEntity> error.

Answer 1

You need second generic parameter on ListPageViewModel :

public abstract class ListPageViewModel<TVm, TModel>
    where TVm : ItemViewModel<TModel>
    where TModel : IEntity
{
}

Then you declare classes that derive from ListPageViewModel with both the TVm and TModel specified:

public class BuildingListPageViewModel
    : ListPageViewModel<BuildingItemViewModel, Building>
{
}

Answer 2

ItemViewModel needs to be covariant . Otherwise, ItemViewModel<Building> won't be a subtype of ItemViewModel<IEntity> .

To make it covariant, you need to declare the type parameter as an out parameter and remove the setter from ItemViewModel:

public interface ItemViewModel<out T> where T:IEntity
{
    T Model { get; }
}

public class BuildingItemViewModel : ItemViewModel<Building>
{
    Building b;

    private BuildingItemViewModel(Building b) { this.b = b; }

    public Building Model { get { return b; } }
}

Answer 3

The problem you are facing is caused because you have created your BuildingItemViewModel as ItemViewModel<Building> . The problem is, that you want to accept ItemViewModel<IEntity> , where IEntity is an interface, but your BuildingViewModel is defined with a concrete type Building , instead of the interface, thus violating the structure and giving you an error.