Log4Net – Write logs to a second file only when a failure occurs (C# 10)

@Danny Nguyen (WICLOUD CORPORATION)

Thank you very much for your answer.

I used the Log one time , not multi-threaded, but I want to give this support for future.

These are my answers:

1.Be careful configuring log4net in code

BasicConfigurator.Configure(...) changes the global log4net setup. If you call your SetAdminFile() / SetAdminAndUserFailure() more than once (restart logging, tests, etc.), you can end up with multiple appenders and duplicated log lines. You should reset/reconfigure the repository (or let the host configure log4net and only swap your adapter).

This is my fix code is ok ?

Fix
Reset repository before configuring.
private static void Configure(params IAppender[] appenders)
{
    var repo = LogManager.GetRepository();
    repo.ResetConfiguration();
    BasicConfigurator.Configure(appenders);
}
Replace
BasicConfigurator.Configure(appender);
with
Configure(appender); (In SetAdminFile class)
and
BasicConfigurator.Configure(fileAppender, memoryAppender);
with
Configure(fileAppender, memoryAppender); (In SetAdminAndUserFailure class)

2.Make the “flush once” thread-safe

If two threads log an error at the same time, you might flush twice. Use Interlocked or a lock around the flush.

This is my fix code is ok ?

Current can be changed from multiple threads.
Fix
Use volatile + Interlocked.
private static volatile IAppLogger _current = new NullLogger();
public static IAppLogger Current
{
    get => _current;
    set => Interlocked.Exchange(ref _current, value ?? new NullLogger());
}
Add:
using System.Threading;

3.Memory can grow forever

If the app runs a long time and no error happens, MemoryAppender can keep growing.

Usually you want to keep only the last N messages (a small ring buffer), not everything.

This is my fix code is ok ? I need I have 100 lines of logs at maximum, need it ?

MemoryAppender stores unlimited logs.
If the app runs long before failure → huge memory usage.
This parameter will come from outside the function
Simpler fix:
if (_memoryAppender.GetEvents().Length > 500)
    _memoryAppender.Clear();
Add before flushing.

4.Your example doesn’t match the code after the first error

Right now you flush the buffered messages once, but after that you do NOT keep writing new Info/Warn/Debug to the user file. That’s fine if you only want “pre-error context”, but then say that clearly. If you want “after error, keep logging to the user file too”, you need to add that behavior.

I want to Info/Warn/Debug + Error log and when there is error to log last error and to exit.

I will exit in my code after Log.Error. This my bussiness.

Do you see problem ?

5.Your ‘UserFailureOnly’ mode doesn’t buffer

UserLogOnlyAdapter ignores Info/Warn/Debug, so the user won’t get the “what happened before the error” context. If you want that, it also needs buffering + flush.

This is my fix code is ok ?

Fixed UserLogOnlyAdapter (Buffered + Flush)
using System;
using System.Threading;
using log4net.Appender;
using log4net.Core;
using log4net.Layout;
public class UserLogOnlyAdapter : IAppLogger
{
    private readonly MemoryAppender _memoryAppender = new MemoryAppender();
    private readonly RollingFileAppender _userAppender;
    private int _flushed;
    public UserLogOnlyAdapter(string userLogPath)
    {
        var fullPath = Log.ResolvePath(userLogPath);
        Log.EnsureDirectoryExists(fullPath);
        Log.ProbeWriteAccess(fullPath);
        var layout = Log.CreateDefaultLayout();
        _userAppender = new RollingFileAppender
        {
            File = fullPath,
            AppendToFile = true,
            RollingStyle = RollingFileAppender.RollingMode.Size,
            MaximumFileSize = "5MB",
            MaxSizeRollBackups = 1,
            StaticLogFileName = true,
            Layout = layout
        };
        _userAppender.ActivateOptions();
        _memoryAppender.ActivateOptions();
    }
    public void Debug(string message, Exception ex = null)
        => Buffer(Level.Debug, message, ex);
    public void Info(string message, Exception ex = null)
        => Buffer(Level.Info, message, ex);
    public void Warn(string message, Exception ex = null)
        => Buffer(Level.Warn, message, ex);
    public void Error(string message, Exception ex = null)
    {
        Buffer(Level.Error, message, ex);
        if (Interlocked.Exchange(ref _flushed, 1) == 1)
            return;
        foreach (var e in _memoryAppender.GetEvents())
            _userAppender.DoAppend(e);
        _memoryAppender.Clear();
    }
    private void Buffer(Level level, string message, Exception ex)
    {
        var loggingEvent = new LoggingEvent(
            new LoggingEventData
            {
                LoggerName = "User",
                Level = level,
                Message = message,
                ExceptionString = ex?.ToString(),
                TimeStampUtc = DateTime.UtcNow
            });
        _memoryAppender.DoAppend(loggingEvent);
        // optional safety cap
        if (_memoryAppender.GetEvents().Length > 500)
            _memoryAppender.Clear();
    }
}
In additions:

6.Extract Repeated Layout Creation

I repeat this everywhere:
var layout = new PatternLayout("%date %-5level %logger - %message%newline%exception");
layout.ActivateOptions();
Fix
Add helper:
internal static PatternLayout CreateDefaultLayout()
{
    var layout = new PatternLayout("%date %-5level %logger - %message%newline%exception");
    layout.ActivateOptions();
    return layout;
}
Then use:
Layout = Log.CreateDefaultLayout()

7.Fix EnsureDirectoryExists Null Edge Case

Problem
Path.GetDirectoryName() can return null.
Fix
internal static void EnsureDirectoryExists(string fullPath)
{
    var dir = Path.GetDirectoryName(fullPath);
    if (!string.IsNullOrEmpty(dir) && !Directory.Exists(dir))
        Directory.CreateDirectory(dir);
}

8. Fix ProbeWriteAccess File Lock Risk

Current:
using (File.Open(fullPath, FileMode.Append, FileAccess.Write)) { }
This can fail if log4net already opened the file.
Safer
internal static void ProbeWriteAccess(string fullPath)
{
    using (var fs = new FileStream(fullPath, FileMode.OpenOrCreate, FileAccess.Write, FileShare.ReadWrite))
    {
    }
}

9.Improve UserLogOnlyAdapter LoggingEvent Creation

Currently:
var loggingEvent = new LoggingEvent(
    typeof(UserLogOnlyAdapter),
    LogManager.GetRepository(),
    "User",
    Level.Error,
    message,
    ex);
Better to include timestamp + thread.
Fix
var loggingEvent = new LoggingEvent(
    new LoggingEventData
    {
        Level = Level.Error,
        LoggerName = "User",
        Message = message,
        ExceptionString = ex?.ToString(),
        TimeStampUtc = DateTime.UtcNow
    });

10.Avoid Logger Type Fragmentation

Currently:
Log4NetAdapter
Log4NetAdapterWithFailureFlushWithPath
This creates different logger names in log4net.
Better:
private readonly ILog _logger = LogManager.GetLogger("App");
instead of
LogManager.GetLogger(typeof(...))
This keeps single logger identity.

11. Prevent MemoryAppender Growth (Important)

MemoryAppender stores unlimited logs.
If the app runs long before failure → huge memory usage.
Fix (recommended)
Create custom capped memory appender.
Simpler fix:
if (_memoryAppender.GetEvents().Length > 500)
    _memoryAppender.Clear();
Add before flushing.

12.Small Performance Improvement

Change facade calls from:
public static void Debug(string message, Exception ex = null) => Current.Debug(message, ex);
to avoid property access:
public static void Debug(string message, Exception ex = null)
{
    _current.Debug(message, ex);
}
Minor but cleaner.

This my final code with all your suggestion above with some refactoring is it ok now?

Fixed version.txt

In addition:

MemoryAppender is the only solution to this problem?