MockingFileSystem_2

    public interface IFileSystemInteractor
    {
        StreamReader OpenText(string inputFilePath);
        StreamWriter AppendText(string outputFilePath);
    }

    public class FileSystemInteractor : IFileSystemInteractor
    {
        public StreamReader OpenText(string inputFilePath)
        {
            return File.OpenText(inputFilePath);
        }

        public StreamWriter AppendText(string outputFilePath)
        {
            return File.AppendText(outputFilePath);
        }
    }

    public class FileSystemInteractorMock : IFileSystemInteractor
    {
        private readonly MemoryStream _inputMemoryStream;
        public MemoryStream OutputMemoryStream { get; }

        public FileSystemInteractorMock(string fileContent)
        {
            _inputMemoryStream = new MemoryStream(Encoding.UTF8.GetBytes(fileContent));
            OutputMemoryStream = new MemoryStream();
        }

        public StreamReader OpenText(string inputFilePath)
        {
            var streamReader = new StreamReader(_inputMemoryStream);
            return streamReader;
        }

        public StreamWriter AppendText(string outputFilePath)
        {
            var streamWriter = new StreamWriter(OutputMemoryStream);
            return streamWriter;
        }
    }

    [TestFixture]
    public class Tests
    {
        [Test]
        public void Examined_Data_Written_Correctly_To_File()
        {
            var inputFilePath = "inputFile.txt";
            var outputFilePath = "outputFile.txt";
            var inputFileContents = string.Join(Environment.NewLine, new[] {"OK", "WRONG", "OK", "WRONG"});

            var fileInteractorMock = new FileSystemInteractorMock(inputFileContents);
            var sut = new ProductionClass(fileInteractorMock);
            var resultAsync = sut.CheckDataAndStoreResultsAsync(inputFilePath, outputFilePath);
            resultAsync.Wait();

            var outputLines = Encoding.UTF8.GetString(fileInteractorMock.OutputMemoryStream.ToArray())
                .Split(new [] {Environment.NewLine}, StringSplitOptions.RemoveEmptyEntries);
            CollectionAssert.AreEquivalent(new []{"1", "3"}, outputLines, $"Actual:{string.Join(",", outputLines)}");
        }
    }