change ApplicationError in Global.asax

This is for checking the text file size

 string filename = Server.MapPath(".") + "\\TextFile.txt";
 FileInfo fi = new FileInfo(filename);
 var size = fi.Length;
 Response.Write(string.Format("File Size in Bytes: {0}", size));

This is for writing empty text if the file size is greayer than 30000 bytes

if (size > 30000)
            {
                File.WriteAllText(Server.MapPath(".") + "\\TextFile.txt", String.Empty);
            }

Thank You.

I have Edited that code

string filename = Server.MapPath(".") + "\\log_" + DateTime.Now.ToString("MM-dd-yyyy hh mm ss") + ".txt";
            if (!System.IO.File.Exists(filename))
                File.Create(filename).Dispose();
            System.IO.StreamWriter sw = new System.IO.StreamWriter(filename, true);
            sw.Write(DateTime.Now + "\n\r" + Server.GetLastError() + "\n\r\n\r");
            sw.Close();
            Response.Redirect("error.aspx");

Thank You.