difficulty in work with streamwriter in asp.net

private void chng_OnClick(object sender, EventArgs e)
   {

       string pass = txt_pass.Text;
       string passhash = MD5Hash(pass);
       string path = Server.MapPath(".");
       StreamWriter file = new StreamWriter(path+"log.txt");

       file.WriteLine(passhash);
       file.Close();

       txt_pass.Text = "";
   }
   public static string MD5Hash(string text)
   {
       MD5 md5 = new MD5CryptoServiceProvider();

       //compute hash from the bytes of text
       md5.ComputeHash(ASCIIEncoding.ASCII.GetBytes(text));

       //get hash result after compute it
       byte[] result = md5.Hash;

       StringBuilder strBuilder = new StringBuilder();
       for (int i = 0; i < result.Length; i++)
       {
           //change it into 2 hexadecimal digits
           //for each byte
           strBuilder.Append(result[i].ToString("x2"));
       }

       return strBuilder.ToString();
   }