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better way of invoking thread and logging to text file in C#
namespace logging
{
    using System;
    using System.IO;
    using System.Reflection;
    using System.Runtime.InteropServices;
    using System.Text;

    public class log
    {
        private static string lpath;

        [DllImport("kernel32")]
        private static extern int GetPrivateProfileString(string section, string key, string def, StringBuilder retVal, int size, string filePath);
        public static void Info(string user, string info, string txt)
        {
            StreamWriter writer;
            string str = Assembly.GetExecutingAssembly().GetName().CodeBase.ToString();

            .....

            if (!File.Exists(path))
            {
                writer = new StreamWriter(path, true);
                writer.WriteLine(str3 + " " + info + " => " + txt);
                writer.Flush();
                writer.Close();
            }
            else
            {
                writer = File.AppendText(path);
                writer.Write(str3 + " " + info + " => " + txt + "n");
                writer.Flush();
                writer.Close();
            }
        }
    }
}

private void bgw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    Thread thread_BP_Print = new Thread(BoardingPass_Print);
    thread_BP_Print.Start();

    // Simultaneously, do something on the main thread.
    BaggageTag_Print();
    bgw.RunWorkerAsync();
}

logging.log.Info(username, "Query Trace > ", sql);

private static object locker = new object();
public static void Info(string user, string info, string txt)
{
    string str = Assembly.GetExecutingAssembly().GetName().CodeBase.ToString();
    string str3 = ...
    lock (locker)
    {
       File.AppendAllText(path, str3 + " " + info + " => " + txt + "n");
    }
}