How can I Mock async method holding CrossFilePicker?

Question

I am working on Xamarin.Forms project, that is using Autofac , Moq , and Plugin.FilePicker .

One of button commands is calling method:

private async void OnLoadFileExecute(object obj)
{
    await PickUpFile();
    LoadedPhrases = LoadFromFile(FileLocation);
    PopulateDb(LoadedPhrases);
    LoadGroups();
}

And PickUpFile() method is async :

public async Task<string> PickUpFile()
{
    try
    {
        FileLocation = "";
        var file = await CrossFilePicker.Current.PickFile();
        if (file != null)
        {
            FileLocation = file.FilePath;
            return FileLocation;
        }
        else
        {
            FileLocation = "";
            return "";
        }
    }
    catch (Exception ex)
    {
        Debug.WriteLine("Exception choosing file: " + ex.ToString());
        return "";
    }
}

I wanted to test whole command, so all methods in OnLoadFileExecute will be tested. In that case I am not sure how can I Setup PickUpFile() method to return some string . As far as I know, I can not use in the interface async methods. Correct me if I am wrong. If I could, I would be able to mock it.

Answer 1

You can use Task in the interface. When you mock it, you will need to return Task.FromResult

Answer 2

From my point of view, it should look like this

public interface IFileService {
    Task<string> PickUpFile();
}

public class FileService : IFileService {
    public async Task<string> PickUpFile() {
        // here you have your implementation
    }
}

// Here is the test method
public async Task TestTheService() {
    const string fileName = "filename.txt";

    var fileMocker = new Mock<IFileService>();
    fileMocker.Setup( x => x.PickUpFile() ).Returns( Task.FromResult( fileName ) );
    var mainClass = new MainClass( fileMocker.Object );
    await mainClass.OnLoadFileExecute( null );
    Assert.Equal( fileName, mainClass.FileLocation );
}

// here is the real class

public class MainClass {
    private IFileService FileService { get; }
    public string FileLocation { get; set; }

    public MainClass( IFileService fileService ) {
        FileService = fileService;
    }

    private async Task OnLoadFileExecute( object obj )
    {
        FileLocation = await FileService.PickUpFile();
        LoadedPhrases = LoadFromFile( FileLocation );
        PopulateDb( LoadedPhrases );
        LoadGroups();
    }
}