Return types not matching-CodePudding

I have this method that expects MoreOptionsPopupResult as a return. deleteAttachmentModalViewModel.result is of type MoreOptionsPopupResult, however do I get an error when compiling saying this:

Error CS0738 'PopupProvider' does not implement interface member 'IPopupProvider.ShowDeleteAttachmentPopup()'. 'PopupProvider.ShowDeleteAttachmentPopup()' cannot implement 'IPopupProvider.ShowDeleteAttachmentPopup()' because it does not have the matching return type of 'Task<MoreOptionsPopupResult>'.

In my understanding are both types the same so I am confused why it brings this error.

This is MoreOptionsPopupResult:

namespace SiRiAs.Lib.Helpers {
    public enum MoreOptionsPopupResult {
        Delete,
        Update,
        Export
    }
}

This is where I get the error message:

public async Task<MoreOptionsPopupResult?> ShowDeleteAttachmentPopup() {
    var scope = _serviceProvider.CreateScope();
    var deleteAttachmentModalViewModel = scope.ServiceProvider.GetService<DeleteAttachmentModalViewModel>();
    var deleteAttachmentPopup = new DeleteAttachmentModal(deleteAttachmentModalViewModel);
    var asyncEventAwaiter = new AsyncEventAwaiter<NavigatedToEventArgs>();

    App.Current.MainPage.NavigatedTo  = asyncEventAwaiter.EventListener;

    await App.Current.MainPage.Navigation.PushModalAsync(deleteAttachmentPopup);

    await asyncEventAwaiter.Wait();

    App.Current.MainPage.NavigatedTo -= asyncEventAwaiter.EventListener;

    return (MoreOptionsPopupResult?)deleteAttachmentModalViewModel.result;
}

And this is where result is implemented:

public partial class DeleteAttachmentModalViewModel : ObservableObject {
    private readonly IUnitOfWork _unitOfWork;
    private readonly IPopupProvider _popupProvider;
    
    public MoreOptionsPopupResult result;
    
    public DeleteAttachmentModalViewModel(IUnitOfWork unitOfWork, IPopupProvider popupProvider) {
        _unitOfWork = unitOfWork;
        _popupProvider = popupProvider;
    }
    
    [ICommand]
    public void DeleteButtonPressed() {
        result = MoreOptionsPopupResult.Delete;
    }

This method works just fine:

public async Task<MoreOptionsPopupResult?> ShowMoreOptionsPopup() {
    var popup = new MoreOptionsPopup();
    var result = await Shell.Current.CurrentPage.ShowPopupAsync(popup);
    return (MoreOptionsPopupResult?)result;
}

Does anyone can think of why the one method works just fine and the other one doesn't like the return type? It seems like they should be the same.

CodePudding user response：

A nullable enum (ie Nullable<MoreOptionsPopupResult>) is an entirely different type than its corresponding non-nullable enum.

This means that Task<MoreOptionsPopupResult> and Task<MoreOptionsPopupResult?> are completely different, as the compiler is telling you.

Take this stripped down version of your code:

public interface IPopupProvider
{
    Task<MoreOptionsPopupResult> ShowDeleteAttachmentPopup();
}

public class PopupProvider : IPopupProvider
{
    public async Task<MoreOptionsPopupResult?> ShowDeleteAttachmentPopup()
    {
        throw new NotImplementedException();
    }
}

public enum MoreOptionsPopupResult
{
    Delete,
    Update,
    Export
}

This has the same compile-time error you're getting. However, if you remove the ? from the ShowDeleteAttachmentPopup method in the PopupProvider class, it will compile correctly.

public async Task<MoreOptionsPopupResult> ShowDeleteAttachmentPopup()

Or, if you want to allow the method to return a nullable enum value, you'll need to modify the interface method's return type.