how to save exception in txt file?

Since you want to save the exception to C:\Error.txt, you don't need Directory.Exists, Directory.CreateDirectory, or Server.MapPath("~/Error.txt"). You can simply use StreamWriter like this:

string filePath = @"C:\Error.txt";Exception ex = ...using( StreamWriter writer = new StreamWriter( filePath, true ) ){    writer.WriteLine( "-----------------------------------------------------------------------------" );    writer.WriteLine( "Date : " + DateTime.Now.ToString() );    writer.WriteLine();    while( ex != null )    {        writer.WriteLine( ex.GetType().FullName );        writer.WriteLine( "Message : " + ex.Message );        writer.WriteLine( "StackTrace : " + ex.StackTrace );        ex = ex.InnerException;    }}

The above code will create C:\Error.txt if it doesn't exist, or append C:\Error.txt if it already exists.

    using System;    using System.Collections.Generic;    using System.Linq;    using System.Text;    using System.Threading.Tasks;    using System.IO;    namespace ErrorLoggingSample    {     class Program      {      static void Main(string[] args)      {        try        {            string str = string.Empty;            if (string.IsNullOrEmpty(str))            {                throw new Exception("Wrong Data");            }        }        catch (Exception ex)        {            ErrorLogging(ex);            ReadError();        }    }    public static void ErrorLogging(Exception ex)    {        string strPath = @"D:\Rekha\Log.txt";        if (!File.Exists(strPath))        {            File.Create(strPath).Dispose();        }        using (StreamWriter sw = File.AppendText(strPath))        {            sw.WriteLine("=============Error Logging ===========");            sw.WriteLine("===========Start============= " +       DateTime.Now);            sw.WriteLine("Error Message: " + ex.Message);            sw.WriteLine("Stack Trace: " + ex.StackTrace);            sw.WriteLine("===========End============= " + DateTime.Now);        }    }    public static void ReadError()    {        string strPath = @"D:\Rekha\Log.txt";        using (StreamReader sr = new StreamReader(strPath))        {            string line;            while ((line = sr.ReadLine()) != null)            {                Console.WriteLine(line);            }        }      }   }  }

I use that one

catch (Exception e){    new MessageWriteToFile(e).WriteToFile();}public class MessageWriteToFile{    private const string Directory = "C:\\AppLogs";    public string Message { get; set; }    public Exception Exception { get; set; }    public string DefaultPath    {        get        {            var appName = System.Reflection.Assembly.GetExecutingAssembly().GetName().Name;            var folder = $"{Directory}\\{appName}";            if (!System.IO.Directory.Exists(folder))            {                System.IO.Directory.CreateDirectory(folder);            }            var fileName = $"{DateTime.Today:yyyy-MM-dd}.txt";            return $"{Directory}\\{appName}\\{fileName}";        }    }    public MessageWriteToFile(string message)    {        Message = message;    }    public MessageWriteToFile(Exception ex)    {        Exception = ex;    }    public bool WriteToFile(string path = "")    {        if (string.IsNullOrEmpty(path))        {            path = DefaultPath;        }         try        {            using (var writer = new StreamWriter(path, true))            {                writer.WriteLine("-----------------------------------------------------------------------------");                writer.WriteLine("Date : " + DateTime.Now.ToString(CultureInfo.InvariantCulture));                writer.WriteLine();                if (Exception != null)                {                    writer.WriteLine(Exception.GetType().FullName);                    writer.WriteLine("Source : " + Exception.Source);                    writer.WriteLine("Message : " + Exception.Message);                    writer.WriteLine("StackTrace : " + Exception.StackTrace);                    writer.WriteLine("InnerException : " + Exception.InnerException?.Message);                }                if (!string.IsNullOrEmpty(Message))                {                    writer.WriteLine(Message);                }                writer.Close();            }        }        catch (Exception)        {            return false;        }        return true;    }}