I have this logger class i want to unit test but im a bit lost on how i do it: Specifically with how to assert if the streamwriter wrote the correct message? I appreciate any help.

public class Logger
{
    private readonly StreamWriter _writer;
    private readonly IDateTimeProvider _dateTimeProvider;

    public Logger(string path) : this(path, new DateTimeProvider(), new         

StreamWriterProvider()) { }

    public Logger(string path, IDateTimeProvider dateTimeProvider, IStreamWriterProvider streamWriterProvider)
    {
        _dateTimeProvider = dateTimeProvider;
        _writer = streamWriterProvider.CreateStreamWriter(path);

        Log("Logger initialized");
    }

    public void Log(string str)
    {
        _writer.WriteLine($"[{_dateTimeProvider.Now:dd.MM.yy HH:mm:ss}] {str}");
    }
}

--

public interface IStreamWriterProvider
{
    StreamWriter CreateStreamWriter(string path);
}



public class StreamWriterProvider : IStreamWriterProvider
{
    public StreamWriter CreateStreamWriter(string path)
    {
        return new StreamWriter(File.Open(path, FileMode.Append))
        {
            AutoFlush = true
        };
    }
}

--

[Test]
public void Log_PrefixesInputWithDateTime()
{
    // Arrange
    var dateTime = new DateTime(2022, 1, 1, 12, 0, 0);
    var expectedLogMessage = $"[{dateTime:dd.MM.yy HH:mm:ss}] Test message";

    var dateTimeProvider = Substitute.For<IDateTimeProvider>();
    dateTimeProvider.Now.Returns(dateTime);

    var memoryStream = new MemoryStream();
    var streamWriterProvider = Substitute.For<IStreamWriterProvider>();
    streamWriterProvider.CreateStreamWriter(Arg.Any<string> ()).ReturnsForAnyArgs(new StreamWriter(new MemoryStream()));


    Logger logger = new Logger("testPath", dateTimeProvider,               streamWriterProvider);


    // Act
    logger.Log("Test message");

    //Assert
    // ASSERT THAT LOG IS EUQAL TO EXPECTED LOG MESSAGE

    // Ensure that StreamWriter was created with the expected path
    streamWriterProvider.Received().CreateStreamWriter("testPath");



}

​