NLOG：如何使用变量来指定目标

Question

Is it possible to use a variable to specify a specific target?

I want to switch between database and filelogging depending on the environment i'm running my application on.

This however does not seem to work:

<logger name="MyLogger" minlevel="Warn" writeTo="${var:myTargetName}" />

In my application startup I have (error occurs on first line):

var logger = NLog.Web.NLogBuilder.ConfigureNLog("nlog.config").GetCurrentClassLogger(); 
var config = LoadConfiguration();
NLog.LogManager.Configuration.Variables["myTargetName"] = config.GetSection("LogTargets:TargetName").Value;   
NLog.LogManager.KeepVariablesOnReload = true;
NLog.LogManager.Configuration.Reload();

When startin the application the following exception is thrown:

"Target ${var:myTargetName} not found."

I guess the variables are not available when parsing the config file. How can I set the variables so NLOG uses them while parsing the config-file? Is it possible to use a variable to set an attribute value? Or is this not supported by NLOG?

Note: I do have another variable in the NLOG config which does work

<target xsi:type="Database" name="databaseTarget">
  <connectionString>${var:connectionString}</connectionString>
  ...........
</target>

I guess the loggers are checked once on startup and the actual target is evaluated when a logevent occurs.

Update: got it working without variable in config file. I had to remove the logger from the config and create it through code

var myCustomTarget = NLog.LogManager.Configuration.FindTargetByName(config.GetSection("LogTargets:TargetName").Value);
NLog.LogManager.Configuration.AddRuleForAllLevels(myCustomTarget , "MyLogger", true);

Answer 1

One way to do it is to have a different Web.config file based on the environment and there you will change the connection string. That is the method I am using.

Also NLOG is initialized once the application starts thus you cannot change what's written in the NLOG.config

Answer 2

Setting variables via NLog.LogManager.Configuration.Variables does not work for all attributes in nlog.config file. I don't know why, but that's a known problem and that is unfortunate how it works. But there is a simple workaround, here is an example how I solved this problem for the attribute connectionString of a target.

<target xsi:type="Database" name="tDatabase"
            dbProvider="Microsoft.Data.Sqlite.SqliteConnection, Microsoft.Data.Sqlite"
            connectionString="Data Source =.\${environment:DATABASE_FILE_NAME};"
            commandText="INSERT INTO ...">
      <parameter name="@MachineName" layout="${machinename}" />
      ...
</target>

You can than simple set the environment variable in your code like this

Environment.SetEnvironmentVariable("DATABASE_FILE_NAME", "foo.db");

So you just use ${ environment :DATABASE_FILE_NAME} and

Environment.SetEnvironmentVariable("DATABASE_FILE_NAME", "foo.db");

instead of ${ var :DATABASE_FILE_NAME} and

NLog.LogManager.Configuration.Variables["DATABASE_FILE_NAME"] = "foo.db";

Answer 3

NLog 4.6.7 makes it easier to update the logging-rules at runtime like this:

<nlog>
    <variable name="fileMinLevel" value="Off" />
    <variable name="databaseMinLevel" value="Off" />
    <rules>
      <logger minLevel="${var:fileMinLevel}" writeTo="fileTarget" />
      <logger minLevel="${var:databaseMinLevel}" writeTo="databaseTarget" />
    </rules>
</nlog>

Then you can do this:

if (IsDevelopment())
       LogManager.Configuration.Variables["fileMinLevel"] = "Debug";
else
       LogManager.Configuration.Variables["databaseMinLevel"] = "Debug";
LogManager.ReconfigExistingLoggers();

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