Every once in a while I get the following exception:
System.ArgumentOutOfRangeException: Index and length must refer to a location within the string.
Parameter name: length
at System.String.InternalSubStringWithChecks(Int32 startIndex, Int32 length, Boolean fAlwaysCopy)
at System.Text.StringBuilder.ToString(Int32 startIndex, Int32 length)
at log4net.Util.PatternConverter.Format(TextWriter writer, Object state) in xxx\Log4Net\src\Util\PatternConverter.cs:line 187
at log4net.Layout.PatternLayout.Format(TextWriter writer, LoggingEvent loggingEvent) in xxx\Log4Net\src\Layout\PatternLayout.cs:line 1009
at Nemmco.Common.Initialization.Internal.NemLoggingAppender.Execute(DateTime lastTrigger, DateTime currentTrigger) in xxxInitialization\Internal\InitializationLogging.cs:line 765
From my own investigation it looks like the problem occurs because the shared string buffer (from the m_formatWriter.GetStringBuilder() call) may end up in a state where its size is adjusted differently on separate threads, causing one thread to over-estimate the available length.
I wonder if the re-use of a StringWriter / StringBuilder in this scenario actually outweighs the threading implications? The simplest fix would be to replace use of m_formatWriter with use of a local StringWriter / StringBuilder.