Question

Sorry for the log4net newb quesiton, this looks like a common setup, but I cannot find an example.

I am trying to get the following setup with my log4net config:

Two appenders:

• File_Appender -> log4net.Appender.RollingFileAppender
• Smtp_Appender -> log4net.Appender.SmtpAppender
  (evaluator threshold set to ERROR)

Many loggers:

• All loggers (root) -> INFO,WARN,ERROR -> File_Appender
• All loggers (root) -> ERROR -> Smtp_Appender
• Specific loggers -> DEBUG -> File_Appender
• Specific loggers -> INFO -> Smtp_Appender

I am not sure how to configure the “root” logger to log different levels to different appenders (I don’t want to explicitly configure each logger to log errors to a Smtp_Appender).

Answer 1

I ended up doing my own appender filter, which allows setting a lower matching level for explicitly specified loggers:

public class MultipleLevelFilter : LevelMatchFilter
{
    public MultipleLevelFilter()
    {
        LevelToMatchForSpecificLoggers = Level.Fatal;
        _specificLoggers = new string[0];
    }

    /// <summary>
    /// Gets or sets the level to match for loggers
    /// specified in the <see cref="SpecificLoggers"/> property.
    /// </summary>
    /// <value>The level to match for specific loggers.</value>
    public Level LevelToMatchForSpecificLoggers { get; set; }

    private string[] _specificLoggers;
    /// <summary>
    /// Gets or sets a comma separated list of "specific" loggers.
    /// For these loggers, <see cref="LevelToMatchForSpecificLoggers"/>
    /// property is checked to see whether they should be logged.
    /// </summary>
    /// <value>The specific loggers.</value>
    public string SpecificLoggers 
    {
        get { return string.Join(",", _specificLoggers); }
        set
        { 
            _specificLoggers = value.Split(',').Select(s=>s.Trim()).ToArray();
        }
    }

    /// <summary>
    /// Tests if filter matches the specified logging event.
    /// </summary>
    /// <param name="loggingEvent">the event to filter</param>
    public override FilterDecision Decide(LoggingEvent loggingEvent)
    {
        // if base.LevelToMatch is matched, always match
        if (loggingEvent.Level >= LevelToMatch)
           return AcceptOnMatch ? FilterDecision.Accept : FilterDecision.Deny; 

        // if specific level is matched, check if logger name matches
        if (loggingEvent.Level >= LevelToMatchForSpecificLoggers)
        {
            string name = loggingEvent.LoggerName;
            for (int i = 0; i < _specificLoggers.Length; i++)
                if (name == _specificLoggers[i])
                    return AcceptOnMatch ?
                        FilterDecision.Accept : FilterDecision.Deny;
        }

        // continue with next filter
        return FilterDecision.Neutral;
    }
}

Usage:

<appender name="Smtp_Appender" 
     type="Ster.Log4Net.DelayedSendSmtpAppender">

  <!-- ... common properties (from, to, etc) -->

  <!-- our custom filter -->      
  <filter type="MyNamespace.MultipleLevelFilter">

    <!-- this level is ALWAYS matched -->
    <levelToMatch value="ERROR" />

    <!-- this level is matched only for specific loggers -->
    <levelToMatchForSpecificLoggers value="INFO" />

    <!-- these are the "specific" loggers (comma separated) -->
    <specificLoggers value="SomeLogger, SomeOtherLogger" />

  </filter>
  <!-- always end the filter chain with DenyAll -->
  <filter type="log4net.Filter.DenyAllFilter" />

</appender>

<root>
  <priority value="INFO" />
  <appender-ref ref="File_Appender" />
  <appender-ref ref="Smtp_Appender" />
</root>