public class FileStore
    {
        public string WorkingDirectory { get; set; }

        public string Save(int id, string message)
        {
            var path = Path.Combine(this.WorkingDirectory + id + ".txt");
            File.WriteAllText(path, message);
            return path;
        }

        public event EventHandler<MessageEventArgs> MessageRead;

        public void Read(int id)
        {
            var path = Path.Combine(this.WorkingDirectory + id + ".txt");
            var msg = File.ReadAllText(path);
            this.MessageRead(this, new MessageEventArgs { Message = msg });    
        }
    }

 public string Read(int id)
 {
     var path = Path.Combine(this.WorkingDirectory + id + ".txt");
     var msg = File.ReadAllText(path);
     this.MessageRead(this, new MessageEventArgs { Message = msg });
     return msg;
  }

public void Save(int id, string message)
{
    var path = Path.Combine(this.WorkingDirectory + id + ".txt");
    File.WriteAllText(path, message);
}

public class FileStore
    {
        public string WorkingDirectory { get; set; }

        public void Save(int id, string message)
        {
            var path = GetFileName(id);  //ok to query from Command
            File.WriteAllText(path, message);            
        }

        public string Read(int id)
        {
            var path = GetFileName(id);
            var msg = File.ReadAllText(path);
            return msg;
        }
     
        public string GetFileName(int id)
        {
            return Path.Combine(this.WorkingDirectory , id + ".txt");     
        }
    }