exelent let me test it

I see your code has dependencies on System.IO classes such as File and StreamWriter and System.DateTime. Would it improve the code if you inject these dependencies as interfaces via the constructor?

The append parameter of private void WriteLine(string text, bool append = true) is never used in the code. It will always default to true.

The way the constructor is using the append parameter is not clear. You might consider something like this where the local instance of append is set to false to force the creation of the log file if it does not exist:

        public SimpleLogger(bool append = false)
        {
            DatetimeFormat = "yyyy-MM-dd HH:mm:ss.fff";
            Filename = Assembly.GetExecutingAssembly().GetName().Name + ".log";

            // Log file header line
            string logHeader = Filename + " is created.";
            if (!File.Exists(Filename))
            {
                append =false; // force the file to be created
            }

            if (append == false)
            {
                WriteLine(DateTime.Now.ToString(DatetimeFormat) + " " + logHeader, false);
            }
        }

Neat, just fine for simple usage to 0-external-deps apps. Just a small suggestion regarding multithreaded environment:

    private readonly object fileLock = new object();
...
    private void WriteLine(string text, bool append = true)
    {
        if (string.IsNullOrEmpty(text)) 
        {
            return;
        }
        lock (fileLock) 
        {
            using (System.IO.StreamWriter writer = new System.IO.StreamWriter(logFilename, append, System.Text.Encoding.UTF8))
            {
                writer.WriteLine(text);
            }
        }
    }

Just to avoid exception during multiple access.

Thanks, I needed a simple logger and it is exactly what I wanted.