MessageChecksumConverter.cs

public class MessageChecksumConverter : PatternLayoutConverter
{
    private static readonly TraceSource Source = new TraceSource(nameof(MessageChecksumConverter));
    private static readonly MD5 ChecksumGenerator = MD5.Create();

    protected override void Convert(TextWriter writer, LoggingEvent loggingEvent)
    {
        var output = TryGetHash(loggingEvent);
        writer.Write(output);
    }

    private string TryGetHash(LoggingEvent loggingEvent)
    {
        string output = string.Empty;
        try
        {
            byte[] hash;
            var fmt = loggingEvent.MessageObject as SystemStringFormat;
            if (fmt != null)
            {
                // read fmt.m_format and use that as input for hash
                var systemStringFormatType = typeof(SystemStringFormat);
                var fieldInfo =
                    systemStringFormatType.GetField("m_format", BindingFlags.NonPublic | BindingFlags.Instance);
                var formatString = fieldInfo?.GetValue(fmt) as string;
                hash = ChecksumGenerator.ComputeHash(Encoding.ASCII.GetBytes(formatString ?? string.Empty));
            }
            else
            {
                var str = (loggingEvent.ExceptionObject ?? loggingEvent.MessageObject).ToString();
                hash = ChecksumGenerator.ComputeHash(Encoding.ASCII.GetBytes(str));
            }

            var builder = new StringBuilder();
            foreach (var b in hash.Take(2))
            {
                builder.Append(b.ToString("x2"));
            }

            output = builder.ToString();
        }
        catch (Exception ex)
        {
            // can be found if log4net debugging is enabled
            Source.TraceError(ex.ToString());
        }

        return output;
    }
}