Serilog - convert level to syslog standard - serilog

Question

I am working on an Azure IoT Edge container and we are using Serilog. Microsoft recommends outputting the log text to the console window in the format shown in this article: https://learn.microsoft.com/en-us/azure/iot-edge/how-to-retrieve-iot-edge-logs?view=iotedge-2020-11. It would output a line like this to the console:
<6> 2022-06-15 13:05:44.790 +00:00 [INF] - Received direct method call - ping
My question is - how can I display the level twice? The second is easy ({level:u3}), but how can I convert the level to a syslog number at the beginning of the line? The number reference is at the beginning of the article above. I thought about a text formatter, but I need the level formatted two different ways and was not sure that would work.
Thoughts?

Answer 1

I got it to work with a TextFormatter
internal class ConsoleTextFormatter : ITextFormatter
{
public void Format(LogEvent logEvent, TextWriter output)
{
var sb = new StringBuilder();
sb.AppendLine($"<{GetSysLogLevel(logEvent.Level)}> {logEvent.Timestamp:O} [{GetThreeLetterLevel(logEvent.Level)}] - {logEvent.MessageTemplate}");
if (logEvent.Exception != null)
sb.AppendLine(logEvent.Exception.ToString());
foreach (var property in logEvent.Properties.Where(p => p.Key != "SourceContext"))
{
sb.AppendLine($"{property.Key}={property.Value}");
}
output.Write(sb.ToString());
}
// these are in a static class that I cannot access (https://github.com/serilog/serilog/blob/dev/src/Serilog/Formatting/Display/LevelOutputFormat.cs),
// so I have hard coded them here
private string GetThreeLetterLevel(LogEventLevel level)
{
return level switch
{
LogEventLevel.Debug => "DBG",
LogEventLevel.Error => "ERR",
LogEventLevel.Fatal => "FTL",
LogEventLevel.Information => "INF",
LogEventLevel.Verbose => "VRB",
LogEventLevel.Warning => "WRN",
_ => throw new NotSupportedException("Unknown log level")
};
}
private string GetSysLogLevel(LogEventLevel level)
{
return level switch
{
LogEventLevel.Debug => "7",
LogEventLevel.Error => "3",
LogEventLevel.Fatal => "2", // critical
LogEventLevel.Information => "6",
LogEventLevel.Verbose => "7", // debug also
LogEventLevel.Warning => "4",
_ => throw new NotSupportedException("Unknown log level")
};
}