Testing this command

I'm quite new to unit tests and I've got some troubles testing this command

internal async Task OnDeleteTreasurerCommandExecute(TesorieraItemResult tesoriera)
    {
        try
        {
            if (await MessageService.ShowAsync("Confermare l'operazione?", string.Empty, MessageButton.YesNo, MessageImage.Question) == MessageResult.Yes)
            {
                await repository.DeleteTesorieraItemAsync(tesoriera.ID_ISTITUTO,tesoriera.ID_DIVISA,tesoriera.PROGRESSIVO);

                await MessageService.ShowInformationAsync("Operazione completata");

                if (SelectedInstitute != null)
                    await OnLoadDataCommandExecute();
            }
        }
        catch (Exception ex)
        {
            ErrorService.HandleError(GetType(), ex);
        }
    }

I'm using Catel as MVVM framework

how do I simulate the yes/no answers? Thanks

You need to substitute the MessageService with a class that can return yes or no answer. Here's an example using NSubstitute.

1. Install-Package NSubstitute

2. Install-Package NUnit

3. Let us say you have a class that has a method that needs Yes, then No:

   public class AccountViewModel
   {
       readonly IMessageService _messageService;
       readonly ICustomerRepository _customerRepository;
   
       public AccountViewModel(IMessageService messageService, ICustomerRepository customerRepository)
       {
           _messageService = messageService;
           _customerRepository = customerRepository;
       }
   
       public async Task OnDeleteCustomer(Customer customer)
       {
           if (await MessageService.ShowAsync(
              "Confirm?", 
               string.Empty, 
               MessageButton.YesNo, 
               MessageImage.Question) == MessageResult.Yes)
           {
               _customerRepository.Delete(customer);
               await MessageService.ShowInformationAsync("Completed");
           }
       }
   }
   

Then your test case looks like this:

public class TestAccountViewModel
{
    [TestCase]
    public class TestDeleteCustomer()
    {
        // arrange
        var messageService = Substitute.For<IMessageService>();
        messageService
            .ShowAsync(
                Arg.Any<string>(),
                Arg.Any<string>(),
                Arg.Any<MessageButton>(),
                Arg.Any<MessageImage>())
            .Returns(Task.FromResult(MessageResult.Yes);

        messageService
            .ShowInformationAsync(Arg.Any<string>())
            .Returns(Task.FromResult<object>(null));

        var customerRepository = Substitute.For<ICustomerRepository>();

        // act
        var sut = new AccountViewModel(messageService, customerRepository);
        var customer = new Customer();
        sut.OnDeleteCustomer(customer);

        // assert
        Assert.IsTrue(customerRepository.Received().DeleteCustomer(customer));
    }
}