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Home Interviews How can you restrict file size when writing to a log file? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 12 May 2025
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To restrict the size of a log file in C#, you typically check the file size before writing, and take action if it exceeds a limit — like rotating, truncating, or archiving the file.

Example: Limit log file to 5 MB

using System;
using System.IO;

class Program
{
    static readonly string logPath = @"C:\Logs\app.log";
    static readonly long maxSizeBytes = 5 * 1024 * 1024; // 5 MB

    static void Main()
    {
        Log("This is a log entry.");
    }

    static void Log(string message)
    {
        try
        {
            // Check file size
            if (File.Exists(logPath))
            {
                var fileInfo = new FileInfo(logPath);
                if (fileInfo.Length > maxSizeBytes)
                {
                    // Rotate the log
                    string backupPath = logPath + "." + DateTime.Now.ToString("yyyyMMdd_HHmmss");
                    File.Move(logPath, backupPath);
                }
            }

            // Append log
            File.AppendAllText(logPath, $"{DateTime.Now:yyyy-MM-dd HH:mm:ss} - {message}{Environment.NewLine}");
        }
        catch (Exception ex)
        {
            Console.WriteLine("Logging failed: " + ex.Message);
        }
    }
}

Behavior:

  1. When app.log exceeds 5 MB:
  2. It’s renamed (e.g., app.log.20250513_161230).
  3. A new empty app.log is started.

Variants:

  1. Truncate file instead of rotating:
  2. Delete old logs after N days or keep only the last N files.
  3. Would you like a full logging utility class with rotation, size control, and thread-safety?
File.WriteAllText(logPath, ""); // clear the file
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