How to add my own properties to Serilog output template

If you want to add properties that aren’t part of the message template, then you need to enrich the log context. That means adding the FromLogContext enricher, and adding your properties to your logged event a bit differently.

Log.Logger = new LoggerConfiguration()
    .Enrich.FromLogContext()
    .MinimumLevel.Information()
    .WriteTo.Console(outputTemplate:
    "[{Timestamp:HH:mm:ss} {Level:u3}] {UserId} {Event} - {Message}{NewLine}{Exception}")
    .CreateLogger();

using (LogContext.PushProperty("UserId", "123"))
using (LogContext.PushProperty("Event", "Test Event"))
{
    Log.Information("Here is my message about order {OrderNumber}", 567);
    Log.Information("Here is my message about product {ProductId}", "SomeProduct");
}

You can learn more about enrichment in the documentation.

Now I’m not sure about Loggly. I’ve never used it before. But you don’t even need to modify the output template if you’re using Seq (which is from the creators of Serilog and works best with it). Properties added will automatically be available for each event.

As Nicholas Blumhardt pointed out via comment, if you just need to one-off add a property to a single logged event, you can do that as well. I sometimes do this when I have a lot of properties for events that don’t necessarily need to show up in the message, and only apply to this single event.

Log
   .ForContext("OrderNumber", orderNumber)
   .ForContext("UserId", user.Id)
   .Information("Order submitted");