Logging Exception.Data using Log4Net

Question

We're just getting started with Log4Net (and wishing we'd done it earlier). Whilst we can see inner exceptions, etc. the one thing that seems to be missing from the output when logging an exception is any key/value information held inside the "Exception.Data". Is there anyway we can do this "out of the box"? If not, as we really are only just starting out where should be looking to find a way to implement this functionality?

As an example please see the very basic pseudo code below. We don't want to pollute the exception message with context information just what the problem was (We'd probably have lost more information in the data which would help in investigating the actual problem). But right now all we see in our logs is the type of exception, the message, any stack trace - but no exception "data". This means in our logs we lose the customer id, etc. How can we easily get this information into our logs (without having to code it by hand in each exception catch).

try
{
   var ex = new ApplicationException("Unable to update customer");
   ex.Data.Add("id", customer.Id);
   throw ex;
}
catch(ApplicationException ex)
{
   logger.Error("An error occurred whilst doing something", ex);
   throw;
}

Answer 1

Following Stefan's lead:

namespace YourNamespace {
    public sealed class ExceptionDataPatternConverter : PatternLayoutConverter {

        protected override void Convert(TextWriter writer, LoggingEvent loggingEvent) {
            var data = loggingEvent.ExceptionObject.Data;
            if (data != null) {
                foreach(var key in data.Keys) {
                    writer.Write("Data[{0}]={1}" + Environment.NewLine, key, data[key]);
                }
            }

        }
    }   
}

And in your configuration add %ex_data and the converter:

<appender ...>
  ...
  <layout type="log4net.Layout.PatternLayout,log4net">
    <conversionPattern value="%date %d{HH:mm:ss.fff} [%t] %-5p %c %l - %m%n %ex_data"/>
    <converter>
      <name value="ex_data" />
      <type value="YourNamespace.ExceptionDataPatternConverter" />
    </converter>
  </layout>

Answer 2

If you have multiple appenders defined you can use a custom renderer rather than defining the converter for every layout.

web/app.config

<log4net>
    ...
    <renderer renderingClass="YourNamespace.ExceptionObjectLogger, YourAssembly" renderedClass="System.Exception" />
    ...
</log4net>

ExceptionObjectLogger

public class ExceptionObjectLogger : IObjectRenderer
{
    public void RenderObject(RendererMap rendererMap, object obj, TextWriter writer)
    {
        var ex = obj as Exception;

        if (ex == null)
        {
            // Shouldn't happen if only configured for the System.Exception type.
            rendererMap.DefaultRenderer.RenderObject(rendererMap, obj, writer);
        }
        else
        {
            rendererMap.DefaultRenderer.RenderObject(rendererMap, obj, writer);

            const int MAX_DEPTH = 10;
            int currentDepth = 0;

            while (ex != null && currentDepth <= MAX_DEPTH)
            {
                this.RenderExceptionData(rendererMap, ex, writer, currentDepth);
                ex = ex.InnerException;

                currentDepth++;
            }
        }
    }

    private void RenderExceptionData(RendererMap rendererMap, Exception ex, TextWriter writer, int depthLevel)
    {
        var dataCount = ex.Data.Count;
        if (dataCount == 0)
        {
            return;
        }

        writer.WriteLine();

        writer.WriteLine($"Exception data on level {depthLevel} ({dataCount} items):");

        var currentElement = 0;
        foreach (DictionaryEntry entry in ex.Data)
        {
            currentElement++;

            writer.Write("[");
            ExceptionObjectLogger.RenderValue(rendererMap, writer, entry.Key);
            writer.Write("]: ");

            ExceptionObjectLogger.RenderValue(rendererMap, writer, entry.Value);

            if (currentElement < dataCount)
            {
                writer.WriteLine();
            }
        }
    }

    private static void RenderValue(RendererMap rendererMap, TextWriter writer, object value)
    {
        if (value is string)
        {
            writer.Write(value);
        }
        else
        {
            IObjectRenderer keyRenderer = rendererMap.Get(value.GetType());
            keyRenderer.RenderObject(rendererMap, value, writer);
        }
    }
}