How do you write a log file in a thread-safe way?

Question

ICSM Computer · Answer

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");
The lock ensures only one thread writes at a time.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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How do you write a log file in a thread-safe way?

ICSM Computer

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1 Answers

Recommended Approach: Use `lock` for Thread Safety

Example: Basic Thread-Safe Logger

Usage:

Alternative: Use `StreamWriter` with `lock`

Usage:

High-Performance Option: Use `ConcurrentQueue` + Background Writer

Summary

Liked By

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