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Home Interviews How do you ensure thread-safe access when multiple threads are writing to the same file? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 11 May 2025
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To ensure thread-safe access when multiple threads write to the same file in C#, you must synchronize access to prevent race conditions, data corruption, or file lock exceptions.

Here are several approaches:

1. Use a lock for synchronization

Ensure only one thread writes at a time:

private static readonly object fileLock = new object();

public static void WriteToFile(string path, string text)
{
    lock (fileLock)
    {
        File.AppendAllText(path, text + Environment.NewLine);
    }
}

2. Use ConcurrentQueue with a background writer thread

This pattern allows threads to enqueue write requests, and a single thread handles the file writes.

private static readonly ConcurrentQueue<string> logQueue = new ConcurrentQueue<string>();
private static readonly AutoResetEvent logSignal = new AutoResetEvent(false);
private static readonly string logFilePath = "log.txt";

public static void StartLogger()
{
    Task.Run(() =>
    {
        while (true)
        {
            logSignal.WaitOne();  // Wait for signal
            while (logQueue.TryDequeue(out string? line))
            {
                File.AppendAllText(logFilePath, line + Environment.NewLine);
            }
        }
    });
}

public static void Log(string message)
{
    logQueue.Enqueue(message);
    logSignal.Set();  // Signal the logger
}

3. Use SemaphoreSlim for async scenarios

private static readonly SemaphoreSlim fileSemaphore = new SemaphoreSlim(1, 1);

public static async Task WriteToFileAsync(string path, string text)
{
    await fileSemaphore.WaitAsync();
    try
    {
        await File.AppendAllTextAsync(path, text + Environment.NewLine);
    }
    finally
    {
        fileSemaphore.Release();
    }
}

What to avoid:

  1. Writing to the same file from multiple threads without coordination (e.g., no lock, no queue).
  2. Opening the file in append mode from multiple places simultaneously.
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