Log4Net в отдельном файле конфигурации

В качестве шаблона используйте следующее:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>

  <configSections>    
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,log4net"/>
  </configSections>

  <log4net configSource="log4net.config"/>

  <startup>
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5"/>
  </startup>

</configuration>

И поместите конфигурацию log4net в конфигурацию log4net, например.

<log4net>
    <!-- Configuration -->
</log4net>

Если вы хотите следить за изменениями в файле, вы можете сделать это со следующим классом (вызовите LoggingConfigurator.ConfigureLogging()):

public static class LoggingConfigurator
{
    private const string DebugLoggingConfiguration = @"log4net.debug.config";

    /// <summary>
    /// Configures the logging.
    /// </summary>
    /// <exception cref="System.Configuration.ConfigurationErrorsException">Thrown if the logging configuration does not exist.</exception>
    [SuppressMessage("Microsoft.Globalization", "CA1303:Do not pass literals as localized parameters", MessageId = "System.Console.WriteLine(System.String)")]
    [SuppressMessage("Microsoft.Globalization", "CA1303:Do not pass literals as localized parameters", MessageId = "System.Console.WriteLine(System.String,System.Object)")]
    public static void ConfigureLogging()
    {
        try
        {
            string path = GetLogConfigurationPath();

            var fileInfo = new FileInfo(path);

            if (fileInfo.Exists)
            {
                log4net.Config.XmlConfigurator.ConfigureAndWatch(fileInfo);
                Console.WriteLine("Loaded logging configuration from: {0}", path);
            }
            else
            {
                var message = "Logging configuration does not exist: " + path;
                Console.WriteLine(message);
                throw new ConfigurationErrorsException(message);
            }
        }
        catch (ConfigurationErrorsException ex)
        {
            Console.WriteLine("log4net is not configured:\n{0}", ex);
        }
    }

    /// <summary>
    /// Gets the path to the logging configuration file.
    /// </summary>
    /// <returns>The path to the log configuration file.</returns>
    private static string GetLogConfigurationPath()
    {
        var path = GetPathFromAppConfig();
        var directory = Path.GetDirectoryName(path);

        if (directory != null)
        {
            var debugPath = Path.Combine(directory, DebugLoggingConfiguration);
            if (File.Exists(debugPath))
            {
                return debugPath;
            }
        }

        return path;
    }

    /// <summary>
    /// Gets the log4net configuration path file from the app.config.
    /// </summary>
    /// <returns>The path to the log4net configuration file if found, otherwise <c>null</c>.</returns>
    private static string GetPathFromAppConfig()
    {
        string appConfigPath;

        var xml = LoadAppConfig(out appConfigPath);
        var logSectionNode = GetSection(xml, "Log4NetConfigurationSectionHandler");

        if (logSectionNode == null || logSectionNode.Attributes == null)
        {
            return appConfigPath;
        }

        var attribute = logSectionNode.Attributes["configSource"];

        if (attribute == null || string.IsNullOrEmpty(attribute.Value))
        {
            return appConfigPath;
        }

        // Otherwise return the path to the actual log4net config file.
        return ToAbsolutePath(attribute.Value, appConfigPath);
    }

    /// <summary>
    /// Gets the node for a configurations section from an application configuration.
    /// </summary>
    /// <param name="configuration">The <see cref="XmlDocument"/> representing the application configuration.</param>
    /// <param name="type">The section type.</param>
    /// <returns>The node for the section.</returns>
    /// <exception cref="ArgumentNullException"><paramref name="configuration"/> is <c>null</c>.</exception>
    /// <exception cref="ArgumentNullException"><paramref name="type"/> is <c>null</c>.</exception>
    /// <exception cref="ArgumentException"><paramref name="type"/> is an empty string.</exception>
    /// <exception cref="ConfigurationErrorsException">The section could not be found in the application configuration.</exception>
    private static XmlNode GetSection(XmlDocument configuration, string type)
    {
        if (configuration == null)
        {
            throw new ArgumentNullException("configuration");
        }

        if (type == null)
        {
            throw new ArgumentNullException("type");
        }

        if (type.Length == 0)
        {
            throw new ArgumentException("'type' cannot be an empty string.");
        }

        // Get the name of the section from the type
        const string configSectionFormat = @"/configuration/configSections/section[contains(@type,'{0}')]/@name";

        var configSectionPath = string.Format(CultureInfo.InvariantCulture, configSectionFormat, type);
        var configSectionNode = configuration.SelectSingleNode(configSectionPath);

        if (configSectionNode == null)
        {
            throw new ConfigurationErrorsException("App.config does not have a section with a type attribute containing: " + type);
        }

        // Get the section from the name discovered above
        var sectionName = configSectionNode.Value;
        var sectionNode = configuration.SelectSingleNode(@"/configuration/" + sectionName);

        if (sectionNode == null)
        {
            throw new ConfigurationErrorsException("Section not found in app.config: " + sectionName);
        }

        return sectionNode;
    }

    /// <summary>
    /// Loads the application configuration.
    /// </summary>
    /// <param name="appConfigPath">The path to the application configuration.</param>
    /// <returns>The loaded application configuration as an <see cref="XmlDocument"/>.</returns>
    /// <exception cref="ConfigurationErrorsException">The application configuration could not be loaded.</exception>
    private static XmlDocument LoadAppConfig(out string appConfigPath)
    {
        try
        {
            var xml = new XmlDocument();
            appConfigPath = AppDomain.CurrentDomain.SetupInformation.ConfigurationFile;
            xml.Load(appConfigPath);
            return xml;
        }
        catch (Exception ex)
        {
            throw new ConfigurationErrorsException("Could not load app.config.", ex);
        }
    }

    /// <summary>
    /// Converts a path to an absolute path.
    /// </summary>
    /// <param name="path">The path (can be absolute or relative).</param>
    /// <param name="basePath">The base path (used for resolving absolute path).</param>
    /// <returns>The absolute path</returns>
    /// <exception cref="ArgumentException"><paramref name="basePath"/> does not contain a directory.</exception>
    private static string ToAbsolutePath(string path, string basePath)
    {
        if (Path.IsPathRooted(path))
        {
            return path;
        }

        var directory = Path.GetDirectoryName(basePath);

        if (directory == null)
        {
            throw new ArgumentException("'basePath' does not contain a directory.", "basePath");
        }

        return Path.GetFullPath(Path.Combine(directory, path));
    }
}