In Umbraco (or any other ASP.NET application using Log4Net, we can output to the log file using the following example. Example

Class1.cs

public class Class1
{
	private static readonly ILog Log = LogManager.GetLogger(MethodBase.GetCurrentMethod().DeclaringType);
	
	public void MyMethod1()
	{
		// do something
		Log.Debug("Example log message");
		Log.Warn("This is a warning.");
	}
}