Write to a file from multiple threads asynchronously c#

While thread based locks can solve this, there is a manner which works across threads, but is probably best used when you have multiple processes writing to the end of a single file.

To get this behavior across processes (or threads too), specify that you want atomic append writes to the operating system when the OS file handles are created. This is done by specifying O_APPEND under Posix(Linux,Unix), and FILE_APPEND_DATA under Windows.

In C# you don't call the OS 'open', or 'CreateFile' system calls directly, but there are ways to get this result.

I asked how to do this under Windows a while ago, and got two good answers here: How can I do an atomic write/append in C#, or how do I get files opened with the FILE_APPEND_DATA flag?

Basically, you can use FileStream() or PInvoke, I would suggest FileStream() over PInvoke for obvious reasons.

You can use constructor arguments to FileStream() to specify asynchronous file I/O in addition to the FileSystemRights.AppendData flag, which should give you both async I/O and atomic append writes to a file.

Warning: Some OSes have limits on the maximum number of bytes that can be atomically written this way, and exceeding that threshold will remove the OS promise of atomicity.

Because of this last gotcha, I would recommend staying with lock() style contention management when trying to address your problem within a single process.

Save to log with Queue and multiple threads (.Net Core 2.2 linux sample - tested)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Text.RegularExpressions;
// add
using System.Security.Cryptography.X509Certificates;
using System.Threading;
using System.Net;
using System.Net.Sockets;
using System.Net.Security;
using System.Security.Authentication;
using System.IO;
using System.Timers;

namespace LogToFile
{
    class Program
    {
        public static Logger logger = new Logger("debug.log");

        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");

            logger.add("[001][LOGGER STARTING]");

            Thread t0 = new Thread(() => DoWork("t0"));
            t0.Start();

            Thread t1 = new Thread(() => DoWork("t1"));
            t1.Start();

            Thread t2 = new Thread(() => DoWork("t2"));
            t2.Start();

            Thread ts = new Thread(() => SaveWork());
            ts.Start();
        }

        public static void DoWork(string nr){
            while(true){
                logger.add("Hello from worker .... number " + nr);
                Thread.Sleep(300);
            }
        }

        public static void SaveWork(){
            while(true){
                logger.saveNow();
                Thread.Sleep(50);
            }
        }
    }

    class Logger
    {
        // Queue import: 
        // using System.Collections
        public Queue logs = new Queue();
        public string path = "debug.log";

        public Logger(string path){
            this.path = path;
        }

        public void add(string t){
            this.logs.Enqueue("[" + currTime() +"] " + t);
        }

        public void saveNow(){
            if(this.logs.Count > 0){
                // Get from queue
                string err = (string) this.logs.Dequeue();
                // Save to logs
                saveToFile(err, this.path);
            }
        }

        public bool saveToFile(string text, string path)
        {
            try{
                // string docPath = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
                // text = text + Environment.NewLine;
                using (StreamWriter sw = File.AppendText(path))
                {
                    sw.WriteLine(text);
                    sw.Close();
                }
            }catch(Exception e){
                // return to queue
                this.logs.Enqueue(text + "[SAVE_ERR]");
                return false;
            }
            return true;
        }

        public String currTime(){
            DateTime d = DateTime.UtcNow.ToLocalTime();
            return d.ToString("yyyy-MM-dd hh:mm:ss");
        }
    }
}

Compile (Save to: LogToFile/Program.cs):

dotnet new console -o LogToFile
cd LogToFile
dotnet build
dotnet run

Stop app CTRL+C and see log file

cat debug.log

You can use events for logger:

using System;
using System.IO;
using System.Threading;
using System.Threading.Tasks;

namespace EventLogger
{
    class Program
    {
        static void Main(string[] args)
        {                   
            // Event handler
            LogData ld = new LogData();
            // Logger
            Logger lo = new Logger();                    
            // Subscribe to event
            ld.MyEvent += lo.OnMyEvent;     
            // Thread loop
            int cnt = 1;
            while(cnt < 5){         
                Thread t = new Thread(() => RunMe(cnt, ld));
                t.Start();
                cnt++;
            }
            Console.WriteLine("While end");
        }

        // Thread worker
        public static void RunMe(int cnt, LogData ld){
            int nr = 0;
            while(true){
                nr++;
                // Add user and fire event
                ld.AddToLog(new User(){Name = "Log to file Thread" + cnt + " Count " + nr, Email = "em@em.xx"});
                Thread.Sleep(1);
            }
        }
    }

    class LogData
    {
        public delegate void MyEventHandler(object o, User u);
        public event MyEventHandler MyEvent;

        protected virtual void OnEvent(User u)
        {
            if(MyEvent != null){
                MyEvent(this, u);
            }

        }

        // Wywołaj
        public void AddToLog(User u){
            Console.WriteLine("Add to log.");

            // Odpal event
            OnEvent(u);

            Console.WriteLine("Added.");
        }
    }

    class User
    {
        public string Name = "";
        public string Email =  "";
    }

    class Logger
    {
        // Catch event
        public void OnMyEvent(object o, User u){
            try{
                Console.WriteLine("Added to file log! " + u.Name + " " + u.Email);
                File.AppendAllText(@"event.log", "Added to file log! " + u.Name + " " + u.Email+"\r\n");
            }catch(Exception e){
                Console.WriteLine("Error file log " + e);
            }
        }
    }
}