.NET assembly fail at method call

I have the class in C#

using System;
using System.Collections.Generic;
using System.Configuration;
using System.IO;

namespace Logging
{
    /// <summary>
    /// A Logging class implementing the Singleton pattern and an internal Queue to be flushed perdiodically
    /// </summary>
    public class LogWriter
    {
        private static LogWriter instance;
        private static Queue<Log> logQueue;
        private static string logDir = "";
        private static string logFile = "log.txt";
        private static int maxLogAge = int.Parse("10");
        private static int queueSize = int.Parse("5");
        private static DateTime LastFlushed = DateTime.Now;

          /// <summary>
          /// Private constructor to prevent instance creation
          /// </summary>
          private LogWriter() { }

          /// <summary>
          /// An LogWriter instance that exposes a single instance
          /// </summary>
          public static LogWriter Instance
          {
              get
              {
                  // If the instance is null then create one and init the Queue
                  if (instance == null)
                  {
                      instance = new LogWriter();
                      logQueue = new Queue<Log>();
                  }
                  return instance;
              }
          }

          /// <summary>
          /// The single instance method that writes to the log file
          /// </summary>
          /// <param name="message">The message to write to the log</param>
          public void WriteToLog(string message)
          {
              // Lock the queue while writing to prevent contention for the log file
              lock (logQueue)
              {
                  // Create the entry and push to the Queue
                  Log logEntry = new Log(message);
                  logQueue.Enqueue(logEntry);

                  // If we have reached the Queue Size then flush the Queue
                  if (logQueue.Count >= queueSize || DoPeriodicFlush())
                  {
                      FlushLog();
                  }
              }            
          }

          private bool DoPeriodicFlush()
          {
              TimeSpan logAge = DateTime.Now - LastFlushed;
              if (logAge.TotalSeconds >= maxLogAge)
              {
                  LastFlushed = DateTime.Now;
                  return true;
              }
              else
              {
                  return false;
              }
          }

          /// <summary>
          /// Flushes the Queue to the physical log file
          /// </summary>
          private void FlushLog()
          {
              while (logQueue.Count > 0)
              {
                  Log entry = logQueue.Dequeue();
                  string logPath = logDir + entry.LogDate + "_" + logFile;

          // This could be optimised to prevent opening and closing the file for each write
                  using (FileStream fs = File.Open(logPath, FileMode.Append, FileAccess.Write))
                  {
                      using (StreamWriter log = new StreamWriter(fs))
                      {
                          log.WriteLine(string.Format("{0}\t{1}",entry.LogTime,entry.Message));
                      }
                  }
              }            
          }
      }

      /// <summary>
      /// A Log class to store the message and the Date and Time the log entry was created
      /// </summary>
      public class Log
      {
          public string Message { get; set; }
          public string LogTime { get; set; }
          public string LogDate { get; set; }

          public Log(string message)
          {
              Message = message;
              LogDate = DateTime.Now.ToString("yyyy-MM-dd");
              LogTime = DateTime.Now.ToString("hh:mm:ss.fff tt");
          }
      }
  }

I want to call the logger function from Matlab

x=NET.addAssembly('C:\Users\timo\Documents\MATLAB\Datatype\Logging.dll');

works

But then this

writer = Logging.LogWriter.Instance();

gives error: Array formation and indexing are not allowed on .NET objects.