ILogger (ASP.NET Core) Log is called even IsEnabled return false

Question

I am trying to understand the purpose of IsEnabled method in ILogger interface. I would expect that method is automatically called every time something can be log, but method Log is called no mater what returned IsEnabled.

Should I check IsEnabled inside Log method?

    public bool IsEnabled(LogLevel logLevel)
    {
        return logLevel >= this.level;
    }

    public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
    {
        if (this.IsEnabled(logLevel))   //it seams that this is required ????
        {
            string s = formatter(state, exception);
            string formatted = s.Replace("\r\n", "\r\n                          ");
            Console.WriteLine(string.Format("{0,-15} {1,-5} - {2}", logLevel, eventId, formatted));
        }
    }

Then what is the purpose and who(why) calls IsEnabled.

Answer 1

IsEnabled is especially important when you pass parameters.

Without IsEnabled

logger.LogDebug("My debug message with {payload}", Serialize(myPayload));

In this case, myPayload is serialized on EVERY call to LogDebug regardless of your log level settings. This could be an unnecessary hit in a production environment, and potentially lead to unexpected errors. What if Serialize method fails due to unexpected data?

With IsEnabled

if (logger.IsEnabled(LogLevel.Debug))
{
    logger.LogDebug("My debug message with {payload}", Serialize(myPayload));
}

In this case, myPayload is only serialized when LogDebug is enabled in your settings.

Balanced approach

Personally, I skip IsEnabled checks with "constant text" calls to Log() methods (e.g., logger.LogDebug("constant string")), and only test IsEnabled when I need to log with parameters.