C#实现多线程写入同一个文件的方法

namespace WfpApp
{
 public partial class Form2 : Form
 {
  object obj = new object();
  public Form2()
  {
   InitializeComponent();
   System.Threading.Thread thread;
   string[] users = new string[] { "zkk", "admin", "administrator", "soft", "iany", "nec", "necsl" };
   for (int i = 0; i < users.Length; i++)
   {
    thread = new System.Threading.Thread(new System.Threading.ParameterizedThreadStart(WriteLock));
    thread.Start(users[i]);
   }
  }
  public void WriteThread(object user)
  {
   string path = System.Windows.Forms.Application.StartupPath + "\\app\\";
   if (!System.IO.Directory.Exists(path))
    System.IO.Directory.CreateDirectory(path);
   path = path + "\\" + DateTime.Now.ToString("yyyyMMdd") + ".txt";
   StringBuilder sb = new StringBuilder();
   sb.AppendLine("----------------------------" + DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + "--------------------------");
   sb.AppendLine(user.ToString());
   sb.AppendLine("---------------------------------------------------------------------------------");
   sb.AppendLine();
   //if (!System.IO.File.Exists(path))
   // System.IO.File.Create(path).Close();
   System.IO.FileStream fileStream = new System.IO.FileStream(path, System.IO.FileMode.Append, System.IO.FileAccess.Write, System.IO.FileShare.ReadWrite);
   System.IO.StreamWriter sw = new System.IO.StreamWriter(fileStream, Encoding.Default);
   sw.Write(sb.ToString());
   sw.Close();
   sw.Dispose();
   fileStream.Close();
   fileStream.Dispose();
  }
  public void WriteLock(object user)
  {
   lock (obj)
   {
    string path = System.Windows.Forms.Application.StartupPath + "\\app\\";
    if (!System.IO.Directory.Exists(path))
     System.IO.Directory.CreateDirectory(path);
    path = path + "\\" + DateTime.Now.ToString("yyyyMMdd") + ".txt";
    StringBuilder sb = new StringBuilder();
    sb.AppendLine("----------------------------" + DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss.fff") + "--------------------------");
    sb.AppendLine(user.ToString());
    sb.AppendLine("---------------------------------------------------------------------------------");
    sb.AppendLine();
    if (!System.IO.File.Exists(path))
     System.IO.File.Create(path).Close();
    System.IO.FileStream fileStream = new System.IO.FileStream(path, System.IO.FileMode.Append, System.IO.FileAccess.Write);
    System.IO.StreamWriter sw = new System.IO.StreamWriter(fileStream, Encoding.Default);
    sw.Write(sb.ToString());
    sw.Close();
    sw.Dispose();
    fileStream.Close();
    fileStream.Dispose();
   }
  }
 }
}