How to control Nlog logLevel using environment variables

Question

Trying to override LogLevel in Nlog Configuration using environment variable and it does not work: e.g

Answer 1

I know this is a little more wordy, but maybe it works, until someone creates a proper fix:

 <logger name="*"  writeTo="console">
          <filters>
              <when condition="(level >= LogLevel.Fatal and equals('${environment:LOG_LEVEL}','LogLevel.Fatal)" action="Log"/>
              <when condition="(level >= LogLevel.Error and equals('${environment:LOG_LEVEL}','LogLevel.Error')" action="Log"/>
              <when condition="(level >= LogLevel.Info and equals('${environment:LOG_LEVEL}','LogLevel.Info')" action="Log"/>
              <when condition="(level >= LogLevel.Debug and equals('${environment:LOG_LEVEL}','LogLevel.Debug')" action="Log"/>
              <when condition="(level >= LogLevel.Trace and equals('${environment:LOG_LEVEL}','LogLevel.Trace')" action="Log"/>
          </filters>      
 </logger>

Btw. curious why your original question has it as less-than instead of greater-than. Would expect when having configured LOG_LEVEL to Warn, then it should log all warnings or worse.

The above example will have a performance hit because lookup of environment-variables are not fast. NLog 4.6.8 introduces the cachedSeconds-features, that reduces the performance hit:

 <logger name="*"  writeTo="console">
          <filters>
              <when condition="(level >= LogLevel.Fatal and equals('${environment:LOG_LEVEL:cachedSeconds=5}','LogLevel.Fatal)" action="Log"/>
              <when condition="(level >= LogLevel.Error and equals('${environment:LOG_LEVEL:cachedSeconds=5}','LogLevel.Error')" action="Log"/>
              <when condition="(level >= LogLevel.Info and equals('${environment:LOG_LEVEL:cachedSeconds=5}','LogLevel.Info')" action="Log"/>
              <when condition="(level >= LogLevel.Debug and equals('${environment:LOG_LEVEL:cachedSeconds=5}','LogLevel.Debug')" action="Log"/>
              <when condition="(level >= LogLevel.Trace and equals('${environment:LOG_LEVEL:cachedSeconds=5}','LogLevel.Trace')" action="Log"/>
          </filters>
 </logger>

NLog 4.6.8 also makes it easier to use Layout in LoggingRules, but one is still required to make an explicit call to LogManager.ReconfigExistingLoggers() to activate.

See also: https://github.com/nlog/NLog/wiki/Filtering-log-messages#semi-dynamic-routing-rules