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Home Forums How do you write a log file in a thread-safe way? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 30 May 2025
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To write a log file in a thread-safe way in C#, you need to ensure that multiple threads don't write to the file simultaneously, which can cause corruption or data loss.

Recommended Approach: Use lock for Thread Safety

Example: Basic Thread-Safe Logger

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

public static class ThreadSafeLogger
{
    private static readonly object _lock = new object();
    private static readonly string _logFilePath = "log.txt";

    public static void Log(string message)
    {
        string logEntry = $"{DateTime.Now:yyyy-MM-dd HH:mm:ss.fff} - {message}";
        lock (_lock)
        {
            File.AppendAllText(_logFilePath, logEntry + Environment.NewLine);
        }
    }
}

Usage:

ThreadSafeLogger.Log("User logged in");
  1. The lock ensures only one thread writes at a time.
  2. AppendAllText is used to append without overwriting the file.

Alternative: Use StreamWriter with lock

If you're doing frequent logging, reusing a StreamWriter is more efficient:

public class ThreadSafeStreamLogger : IDisposable
{
    private readonly StreamWriter _writer;
    private readonly object _lock = new object();

    public ThreadSafeStreamLogger(string path)
    {
        _writer = new StreamWriter(path, append: true)
        {
            AutoFlush = true
        };
    }

    public void Log(string message)
    {
        string logEntry = $"{DateTime.Now:yyyy-MM-dd HH:mm:ss.fff} - {message}";
        lock (_lock)
        {
            _writer.WriteLine(logEntry);
        }
    }

    public void Dispose()
    {
        _writer.Dispose();
    }
}

Usage:

var logger = new ThreadSafeStreamLogger("app.log");
logger.Log("App started");

Dispose when done (or use with a using block).

High-Performance Option: Use ConcurrentQueue + Background Writer

For high throughput or async apps, log to a queue and let a background thread write to the file.

Let me know if you'd like a full example of that pattern.

Summary

Technique Thread-Safe Suitable For
lock + File.AppendAllText Yes Low-frequency logging
lock + shared StreamWriter Yes Frequent logging
ConcurrentQueue + worker thread Yes High-performance logging
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