Writing a custom formatter for log4net

2008, Apr 29 3 mins read

Log4Net is certainly one of the most important instrumentation library for logging. Let’s see how to extend, writing a custom formatter for exceptions. Exceptions are important things to log, and it is important to log more information as possible, here is how can you write a simple class to log exception

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public class ExceptionLayoutExtended : log4net.Layout.LayoutSkeleton {
   private PatternLayout patternLayout;
   PatternLayout PatternLayout {
      get { return patternLayout ?? (patternLayout = new PatternLayout(Pattern)); }
   }
public ExceptionLayoutExtended() {
   this.IgnoresException = false;
}

public String Pattern { get; set; }

As you can see the class must inherit from LayoutSkeleton base class, then I insert a property of type PatternLayout, we will see shortly how it will be used, for now notice that it use lazy construction to actually create the object only at first access, and it pass it a property called Pattern

This is written in C# 3.0, so I can use the short form to define properties. The core of the logging is in the Format() Method.

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 1 override public void Format(TextWriter writer, LoggingEvent loggingEvent) {
 2    if (loggingEvent == null) {
 3       throw new ArgumentNullException("loggingEvent");
 4    }
 5    ;
 6    String header = String.Empty;
 7    writer.Write("\n-EXCEPTION EXTENDED LOG START");
 8    //writer.Write("\nBASE MESSAGE:" + loggingEvent.MessageObject);
 9    PatternLayout.Format(writer, loggingEvent);
10    Exception curEx = loggingEvent.ExceptionObject;
11  
12    while (curEx != null) {
13       writer.Write(header + "\nExceptionMessage:  ");
14       writer.Write(header + curEx.Message);
15       writer.Write(header + "\nExceptionType:    ");
16       writer.Write(header + curEx.GetType().FullName);
17 
18       writer.Write(header + "\nStackTrace:    ");
19       if (curEx.StackTrace != null) {
20          writer.Write(header + curEx.StackTrace.ToString());
21       }
22       curEx = curEx.InnerException;
23       if (curEx != null) {
24          writer.Write("\n" + header + "INNER EXCEPTION");
25          header += new string('-', 4);
26       }
27    }
28    writer.Write("\nEXCEPTION EXTENDED LOG END");
29 
30 }

This is a very simple way to format the exception, first of all I write an header (line7) then I use the log4Net class PatternLayout to write a formatted header, then I begin to log some information from exception object. At each iteration I goes deeper into exception hierarchy dumping information for the innerException object, until I found a null innerExcception.

In the configuration file now I can use my new layout in this way

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<appender name="ExceptionAppender" type="log4net.Appender.FileAppender">
   <param name="File" value="Exceptions.txt" />
   <param name="AppendToFile" value="true" />
   <layout type="mynamespace.ExceptionLayoutExtended, MyAssembly" >
      <param name="Pattern" value="%d - %exception% Detail:%m%newline" />
   </layout>
</appender>

As you can see I created a new appender of type FileAppender, the interesting part is in the layout property: I put my new layout class in type, and then I set a param of name Pattern with value %d – %exception% Detail:%m%newline. With such a definition the log4net configurator look into my ExceptionLAyoutExtended class for a property named Pattern and set it with the value specified in the configuration. Into my layout class I simply use the base PatternLayout class to use the familiar and standard notation usable with log4Net.

Code Sample

Alk.

Tags: log4Net Instrumentation