Support for template names

[martinRocks]

When will log4net allow the use of template names? For example, a log message like this:
_log.InfoFormat("Started {time}", DateTime.Now);
so that the message comes out in Json format like this:

{
  "date": "2025-06-27T12:09:20.6955070-04:00",
  "level": "INFO",
  "appname": "App.exe",
  "logger": "Log",
  "thread": "1",
  "ndc": "(null)",
  "message": {
    "Format": "Started {time}",
    "time": [
      "2025-06-27T12:09:20.6935074-04:00"
    ]
  }
}

I'm currently using the library log4net.Ext.Json to export the json. If this does work, please tell me how, because when I try it, I get the error:

log4net:WARN Exception while rendering format [Started {time}]
System.FormatException: Input string was not in a correct format.
   at System.Text.StringBuilder.AppendFormatHelper(IFormatProvider provider, String format, ParamsArray args)
   at System.String.FormatHelper(IFormatProvider provider, String format, ParamsArray args)
   at System.String.Format(IFormatProvider provider, String format, Object[] args)
   at log4net.Util.SystemStringFormat.StringFormat(IFormatProvider provider, String format, Object[] args) in /home/alle/git/apache/logging-log4net/src/log4net/Util/SystemStringFormat.cs:line 86

Also, I'm using these libraries.
log4net, version=3.1.0
log4net.Ext.Json, version=3.0.3
log4net.Ext.Json.Net, version=3.0.3
Newtonsoft.Json, version=13.0.3

[FreeAndNil]

I am not aware of a possible solution to your problem.
@gdziadkiewicz do you have an idea?

[martinRocks]

Thank you for the quick reply. I have created a small demo app for how I have things configured. If I have anything set wrong, please let me know. I hope this helps explain my problem. I am surprised that more people haven't asked for this. Maybe, now that I think about it, there are 2 problems. One, the json output seems incorrect, IE missing the parameter names. And two, the console output throws an error.

Log4netJson.zip

[FreeAndNil]

Thanks for the detailed explanation and the sample app.

I believe there might be a misunderstanding regarding how log4net handles message templates. The kind of structured logging you're describing (e.g., using {time} placeholders with named parameters in JSON output) isn't something log4net supports natively.

As far as I know, log4net doesn't parse message templates like "Started {time}" in the same way that libraries like Serilog do. Instead, it expects traditional .NET format strings with positional arguments (e.g., {0}, {1}, etc.), which is likely why you're seeing the System.FormatException.

If you've seen this working elsewhere or have a reference to how it might be done in log4net, I'd be really interested to see it—maybe there's an extension or approach I'm not aware of.

[martinRocks]

Thank you so very much for the detailed and quick response. I have an idea on how to get it to work. If it works, I will share what I have with you. However, there is still the problem of the StringFormat method blowing up. Do you think there is anything that can be done with that?

[FreeAndNil]

There are some possibilities:

You can

• derive your own LoggingEvent
• override RenderedMessage and WriteRenderedMessage (with your special format logic)
• derive your own logger
• override ForcedLog
• create your own LoggingEvent class (instead of LoggingEvent)

This would work without changing anything in log4net itself.

Alternatively, we could

• unseal SystemStringFormat
• add a factory for SystemStringFormat

You could then

• derive from SystemStringFormat and override ToString()
• configure the factory to create your derived class

This needs work inside log4net and on your side.

[martinRocks]

What do you think about doing something like this? I'm not sure if any of the names are something that my cloud log reader would ingest, but that should fix part of my problem. To make this work, just add this class to the project and replace the Layout in the file appender.

public class MyLayout : LayoutSkeleton
    {
        public override void ActivateOptions()
        {
            
        }

        public override void Format(TextWriter writer, LoggingEvent loggingEvent)
        {
            var messageObject = loggingEvent.MessageObject;
            if (messageObject is string)
            {
                writer.WriteLine(messageObject);
                return;
            }
            var messageObjectFormat = ((log4net.Util.SystemStringFormat)messageObject).Format;
            var templateNames = GetTemplateNames(messageObjectFormat);
            var args = ((log4net.Util.SystemStringFormat)messageObject).Args;

            var i = 0;
            var dic = new Dictionary<string, string>();
            foreach (var name in templateNames)
            {
                dic.Add(name, args[i].ToString());
                i++;
            }

            var serializer = new JsonSerializer();
            serializer.Serialize(writer, new TemplateWithValue(messageObjectFormat, dic));
            writer.WriteLine();
        }

        private static List<string> GetTemplateNames(string template)
        {
            var regex = new Regex(@"\{(\w+)\}");
            var matches = regex.Matches(template);

            return (from Match match in matches select match.Groups[1].Value).ToList();
        }

        private class TemplateWithValue
        {
            public TemplateWithValue(string template, Dictionary<string, string> values)
            {
                Template = template;
                Values = values;
            }
            public string Template { get; private set; }
            public Dictionary<string,string> Values { get; private set; }
        }
    }

[FreeAndNil]

Have you tried your solution?
You can easily exchange the PatternLayout with your own layout in the log4net configuration: https://logging.apache.org/log4net/manual/configuration/layouts.html

In case you log many events in a short time, be careful not to produce too much garbage for the garbage collector.
Now you have at least six more objects in your hot path for every log event:

• 1 Regex (could be cached as static readonly)
• 1 MatchCollection
• 1 List
• 1 TemplateWithValue
• 1 Dictionary
• 1 JsonSerializer (could also be cached static)

[martinRocks]

I have tried it. It seems to get me what I need in the json, however, the console still blows up with the error. My next step is to test it with splunk. However, I have to wait for my IT guys to get that set up.

I will keep try and make my code more performative. Thank you for all the insight.