Develop Reference

Check NLog minlevel before logging?

I want to enable developers to log objects as JSON with NLog. To do this I need to implement some logic before sending to nLog OR before sending to target.
I can build my own Target(TargetWithLayout) but I can´t find a way to check the log level from the config for this specific target/logger? Another drawback is that I need to make a new TargetWithLayout class for each target that we will use (EventLog, File, WebService and so on).
Another solution would be to do it in my LogHandler that uses NLog. The only way to know if I should translate the object is probably to read all the loggers from the config file, if any of them is set to log objects then I serialize. I am however not sure if I can check this information from the LogHandler (without doing it manually)?

You can use the NLog-Logger object to query active logging-rules:
if (myLogger.IsTraceEnabled)
myLogger.Trace("Hello World");

You can use the NLog json layout to write json in you log files, no need to check and do the serialization yourself:
<target name="jsonFile" xsi:type="File" fileName="${logFileNamePrefix}.json">
<layout xsi:type="JsonLayout">
<attribute name="time" layout="${longdate}" />
<attribute name="level" layout="${level:upperCase=true}"/>
<attribute name="message" layout="${message}" />
</layout>
</target>
The log messages formatting is handled by NLog instead of doing it yourself.
release notes nlog

To add some theory ;)
Another drawback is that I need to make a new TargetWithLayout class for each target that we will use (EventLog, File, WebService and so on).
That's the reasons there are Layouts in NLog. Those are the layouts that could be used in the target, but those are independent of the target.
(don't get confused with Layout Renderers, those ${..} things.)
There are multiple layouts (plain text, CSV, JSON) (see list) , and you could easily add your own layout, analogous to adding a custom Target / Layout renderer, see the wiki

Using 'Environment' values in log4net configuration

My application can be started both as a Windows Service and in console environment. For each case, I need some log4net appenders to be active (or not, respectively) as well as some which are active in both cases. (Service: RollingFileAppender, DebugAppender (if enabled), EventLogAppender | Console: ColoredConsoleAppender, DebugAppender (if enabled))
The only way to achieve something similar is using the PropertyFilter together with ThreadContext.Properties like so:
<filter type="log4net.Filter.PropertyFilter">
<key value="ApplicationMode" />
<stringToMatch value="Service" />
</filter>
if(!Environment.UserInteractive)
ThreadContext.Properties["ApplicationMode"] = "Service";
However, since the property is declared on the thread context, it only works on the current thread. If the thread changes, the configuration is being reset and I have to declare it again.
Does log4net support a way to declare a PropertyFilter in configuration to setup the desired environment automatically? Like this:
<filter type="log4net.Filter.PropertyFilter">
<key value="{Environment.UserInteractive}" />
<stringToMatch value="false" />
</filter>
Or... is there a better approach? Since I didn't found a solution yet.. is this an uncommon practice?

See the last part of my answer to this question:
Capture username with log4net
To summarize, you can implement an object that contains the logic that you have above and put that object in GlobalContext.Properties. When log4net retrieves the value (your object) from the Properties, it will call ToString to get the actual value. Put your logic inside ToString.
Maybe something like this:
public class ApplicationModeProvider
{
public override string ToString()
{
return Environment.UserInteractive ? "Console" : "Service";
}
}
Put it in your dictionary at startup:
GlobalContext.Properties["ApplicationMode"] = new ApplicationModeProvider();
In effect, this is sort of like adding
if(!Environment.UserInteractive)
ThreadContext.Properties["ApplicationMode"] = "Service";
before ever logging statement.
I'm not sure, but I think that then you can configure your filter as you describe in your post.

log4net not working

Hey I have this configuration in my web.config
<log4net>
<appender name="LogFileAppender" type="log4net.Appender.FileAppender">
<param name="File" value="mylog.log" />
<param name="AppendToFile" value="true" />
<layout type="log4net.Layout.PatternLayout">
<param name="Header" value="" />
<param name="Footer" value="" />
<param name="ConversionPattern" value="%d [%t] %-5p %m%n" />
</layout>
</appender>
<appender name="ConsoleAppender" type="log4net.Appender.ConsoleAppender" >
<layout type="log4net.Layout.PatternLayout">
<param name="Header" value="[Header]\r\n" />
<param name="Footer" value="[Footer]\r\n" />
<param name="ConversionPattern" value="%d [%t] %-5p %m%n" />
</layout>
</appender>
<root>
<level value="DEBUG" />
<appender-ref ref="LogFileAppender" />
<appender-ref ref="ConsoleAppender" />
</root>
</log4net>
but log4net is not working. My project compiles fine, and I get no errors debugging either. The lines where I tell to log.debug("somemessage") gets run fine, but I can't find the mylog.log file, so where is it?

One gotcha for this type of thing is to make sure to add the XmlConfigurator attribute to the assembly by placing the following line in your AssemblyInfo.cs:
[assembly: log4net.Config.XmlConfigurator]
Otherwise log4net never activates.

I guess that either log4net is not logging at all, or the file is not ending up where you expect it.
Firstly, have you actually called
XmlConfigurator.Configure()
anywhere in your code? If the xml snippet above is in the application configuration file, this call will do the trick. If the xml snippet is in it's own file, you'll need to use the .Configure(string) overload that takes the path to the file. Without this call (or apparently the assembly level attribute mentioned by Kirk Woll), then log4net won't be logging at all.
If you believe this is all done, and log4net should be logging, then maybe you should put a fully qualified path in for the log file while you debug further. That will let you be sure where the file should be.

There is another small gotcha, see here: http://logging.apache.org/log4net/release/manual/configuration.html#dot-config
the [assembly: log4net.Config.XmlConfigurator] method doesn't work with app.config. If you configure log4net from app.config, you must use the log4net.Config.XmlConfigurator.Configure() method.

Here is my checklist for when log4net is proving to be recalcitrant:
ensure that log4net.config file is copied to the bin\ folder when building (set to 'Copy if newer' in compiler)
when dealing with installed code, ensure that the log4net.config came along for the ride (set to 'Content' in compiler)
ensure that the user that the process runs as has write rights to the folder where the logs are to be written
if in doubt, give promiscuous permissions to c:\temp\ and get everything to log to there ()
fire up Sysinternal/Dbgview.exe to see if that tells you anything

For an ASP.NET MVC project adding
log4net.Config.XmlConfigurator.Configure();
to the Global.asax.cs also helps:
public class MvcApplication : System.Web.HttpApplication
{
protected void Application_Start()
{
AreaRegistration.RegisterAllAreas();
WebApiConfig.Register(GlobalConfiguration.Configuration);
FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
RouteConfig.RegisterRoutes(RouteTable.Routes);
BundleConfig.RegisterBundles(BundleTable.Bundles);
AuthConfig.RegisterAuth();
log4net.Config.XmlConfigurator.Configure();
}
}

These are the steps which eventually got my file logging working:
-Check AssemblyInfo.cs contains the following attribute.
[assembly: log4net.Config.XmlConfigurator]. This loads log4net.
Check the log directory has write permissions.
Check the logger has a format specified. This is done by checking each element in your configuration has a layout element specified. Eg:
<appender name="MainLogger"...
<layout type="log4net.Layout.SimpleLayout"/>
Finally, try turning on log4net internal logging to enable console logging and checking the console. To do this, add <add key="log4net.Internal.Debug" value="true"/> to your appSettings.

I've had experiences where logging systems fail silently without raising exceptions. I suppose this makes sense because if the logger is logging errors then how can it log an error that it's unable to perform logging?
So if the file isn't created on disk, start by looking into file system permissions to ensure the user your application is running under can write a new file to that disk location.
For testing purposes you might want to manually create the file on disk that should be written to and open up permissions for everybody to write to it. If the logger starts writing to it then you know it's permission based rather than configuration based.

On my side, I forgot to mark the config file to be copied while the app is being compiled.
Just right click the log4net.config file, select property and then choose Copy to Output Directory to be Copy XXXX

Unfortunately none of the above helped. Explicit configuration in the class to be logged additionaly to the previous settings suggestions did the trick for me.
string assemblyFolder = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
log4net.Config.XmlConfigurator.Configure(new FileInfo(assemblyFolder + "/log4net.config"));

So, in the Accepted Answer, the solution was to put
[assembly: log4net.Config.XmlConfigurator]
into the AssemblyInfo.cs File.
Also, there was a comment sugesting
[assembly: log4net.Config.XmlConfigurator(Watch = true)]
however, if it is still not working, consider configuring the name of your log4net config file (log4net.config in my case):
[assembly: log4net.Config.XmlConfigurator(ConfigFile = "log4net.config")]
after that, logging worked like a charm.

I tried all of the above but nothing worked. Adding this line in app.config's configSections section worked for me.
<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler,log4net, Version=1.2.15.0, Culture=neutral, PublicKeyToken=669e0ddf0bb1aa2a" />
Ensure that Version and PublicKeyToken is correct

<param name="File" value="mylog.log" />
is saying "write mylog.log to actual folder". This means that if your webapp is under IIS, then log will be written to C:\inetpub\wwwroot\appname\mylog.log.
If log file is not there, then maybe account under which is app running doesn't have write permission on folder. You can run Process Monitor from SysInternals to see if and where a file is written.
Also run VS in debug mode, to see if any exceptions are thrown (Debug->Exceptions->CLR Exceptions, check Thrown).

In my case I forget to set the log4Net.config file properties as "Content" so the file was not included in the deploy.
So pay attention on it:
Compile action : Content

Check your Reporting level is set to "Debug" or "ALL".
This caught me and I spent ages trying to debug my log4net setup.
I had it set to Error and as it wasn't erroring it didn't log anything.

Aside from all the other answers, my problem that was I needed a call to GetLogger() in my start-up class (WinForms example):
static class Program
{
public static readonly ILog MainLog = LogManager.GetLogger("Main");
/// <summary>
/// The main entry point for the application.
/// </summary>
[STAThread]
static void Main()
{
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
Application.Run(new Form1());
}
}
All my 'meaningful' logging statements were in a DLL that is apparently loaded too late for log4net.

We also had problems with log4net not working and in our case the problem was that the config file had Windows line breaks instead of Unix line breaks.

Also after some hours, I figured out why it wasn't working for me...
i had:
public static class Program
{
private static CommunicationManager _bcScanner = new CommunicationManager();
private static ILog _log = LogManager.GetLogger(typeof(Program));
private static SocketServer socketListener;
but it should be:
public static class Program
{
private static ILog _log = LogManager.GetLogger(typeof(Program));
private static CommunicationManager _bcScanner = new CommunicationManager();
private static SocketServer socketListener;
So make sure the ILog is on the first line....

Sharing application configurations across a solution

I have a solution which has multiple output projects (a website, and admin tool, and a SOAP API layer).
They each share common projects in the solution (the service layer, data layer etc). In one of these common projects, I am looking to store a config layer.
Right now, we have three seperate appsettings config files for each output project -
development.AppSettings.config
testing.AppSettings.config
production.AppSettings.config
So altogether, there are nine config files. Only one is used in each project, as they are referenced by utilising the configSource attribute in the web.config appsettings node.
Anyhoo, it's getting to be a pain any time we want to add/remove values from our config files, because it means that we have to change all nine files to do this. And here's what I'd like to do:
In the common project, we have three config files as above. These would be set to copy to the output directory, so that each project has a copy of them. These would be the 'base' config.
Then in each project, I would like to have three files again, but they wouldn't necessarily have to contain the same values as the base configs. If they did however, then the base config value would be overridden by the value in the output project config. A form of configuration inheritance, I suppose.
On application start, I'd like to be able to get these two config files - the base config, and the project config file. And then set the app settings accordingly.
What I'm wondering though, is what's a nice way of determining which file to use? Also, I'm wondering if this is a good way of sharing application values across a large solution, and if there's another, perhaps more efficient way of doing it?
If I'm in development mode, then I don't want production.appsettings.config, and vice versa if I'm in production mode.
Is there a simple way to get the mode (development/testing/production) that I'm in before I go off and get the configurations?

You can have one set of files (3 configs) and link/share them in whatever projects you need.
http://www.devx.com/vb2themax/Tip/18855
Hope this helps.

You could use the ConfigurationManager.OpenExeConfiguration static method. This will allow you to work with as many config files as you want.
You may also try creating a custom class to store all of your settings. You could then serialize your object to save it as a file. You could extend your base custom config class to fit all your other projects.

After some careful thought, and a trip to the toilet at 03:30, I came across a solution which works.
Let's say that we have some appSettings in our base config file:
<add key="MyKey1" value="MyValue1" />
<add key="MyKey2" value="MyValue2" />
<!-- And so on... -->
<add key="MyKey5" value="MyValue5" />
And in my output project, I have three appSettings:
<!-- This is used to identify which config to use. -->
<add key="Config" value="Development" />
<!-- Different value to the one in the base -->
<add key="MyKey2" value="NewValue2" />
<!-- This key does not exist in the base config -->
<add key="MyKey6" value="MyValue6" />
In my Application_Start, I have a call to GetConfigs():
ConfigHelper.GetConfig(HostingEnvironment.MapPath("~/bin/BaseConfig"));
And the actual GetConfigs function:
public static void GetConfigs()
{
if (configMode == null)
{
configMode = ConfigurationManager.AppSettings.Get("Config").ToLowerInvariant();
}
//Now load the app settings file and retrieve all the config values.
var config = XElement.Load(#"{0}\AppSettings.{1}.config".FormatWith(directory, configMode))
.Elements("add")
.Select(x => new { Key = x.Attribute("key").Value, Value = x.Attribute("value").Value })
//If the current application instance does not contain this key in the config, then add it.
//This way, we create a form of configuration inheritance.
.Where(x => ConfigurationManager.AppSettings.Get(x.Key) == null);
foreach (var configSetting in config)
{
ConfigurationManager.AppSettings.Set(configSetting.Key, configSetting.Value);
}
}
Now, my output project effectively has the following configuration settings:
<add key="Config" value="Development" />
<add key="MyKey1" value="MyValue1" />
<add key="MyKey2" value="NewValue2" />
<!-- And so on... -->
<add key="MyKey5" value="MyValue5" />
<add key="MyKey6" value="MyValue6" />
Simples!

How can I change the file location programmatically?

I am totally new to Log4net. I have managed to get something going by adding a config file and simple logging. I have hardcoded the value to be "C:\temp\log.txt" but this is not good enough.
The logs must go to the special folders
path = Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData);
and this path changes depending whether you are using Windows Server 2008 or Windows XP or Vista etc...
How can I just change the location of the file in log4net programmatically?
This is what I have done:
<configSections>
<section name="log4net"
type="log4net.Config.Log4NetConfigurationSectionHandler,Log4net"/>
</configSections>
<log4net>
<root>
<level value="DEBUG" />
<appender-ref ref="LogFileAppender" />
</root>
<appender name="LogFileAppender" type="log4net.Appender.RollingFileAppender">
<param name="File" value="C:\temp\log.txt" />
<param name="AppendToFile" value="true" />
<rollingStyle value="Size" />
<maxSizeRollBackups value="10" />
<maximumFileSize value="10MB" />
<staticLogFileName value="true" />
<layout type="log4net.Layout.PatternLayout">
<param name="ConversionPattern" value="%-5p%d{yyyy-MM-dd hh:mm:ss} – %m%n" />
</layout>
</appender>
</log4net>
class Program
{
protected static readonly ILog log = LogManager.GetLogger(typeof(Program));
static void Main(string[] args)
{
log4net.Config.XmlConfigurator.Configure();
log.Warn("Log something");
path = Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData);
// How can I change where I log stuff?
}
}
Just need to figure out how I can change to log stuff to where I want to.
Any suggestions?
Thanks a lot

log4net can handle this for you. Any appender property of type string can be formatted, in this case, using the log4net.Util.PatternString option handler. PatternString even supports the SpecialFolder enum which enables the following elegant config:
<appender name="LogFileAppender" type="log4net.Appender.RollingFileAppender" >
<file type="log4net.Util.PatternString"
value="%envFolderPath{CommonApplicationData}\\test.txt" />
...
</appender>
Here's a unit test that proofs the pudding:
[Test]
public void Load()
{
XmlConfigurator.Configure();
var fileAppender = LogManager.GetRepository()
.GetAppenders().First(appender => appender is RollingFileAppender);
var expectedFile =
Path.Combine(
Environment.GetFolderPath(
Environment.SpecialFolder.CommonApplicationData),
"test.txt");
Assert.That(fileAppender,
Is.Not.Null & Has.Property("File").EqualTo(expectedFile));
}
The following test verifies that log4net actually writes to disk (which basically makes this an "integration" test, not a unit test, but we'll leave it at that for now):
[Test]
public void Log4net_WritesToDisk()
{
var expectedFile =
Path.Combine(
Environment.GetFolderPath(
Environment.SpecialFolder.CommonApplicationData),
"test.txt");
if (File.Exists(expectedFile))
File.Delete(expectedFile);
XmlConfigurator.Configure();
var log = LogManager.GetLogger(typeof (ConfigTest));
log.Info("Message from test");
LogManager.Shutdown();
Assert.That(File.ReadAllText(expectedFile),
Text.Contains("Message from test"));
}
NB: I strongly suggest using the compact property syntax demonstrated in the above sample. Removing all those "<property name=" makes your config that much more readable.

I found a mutation of this code in the interwebs:
XmlConfigurator.Configure();
log4net.Repository.Hierarchy.Hierarchy h =
(log4net.Repository.Hierarchy.Hierarchy) LogManager.GetRepository();
foreach (IAppender a in h.Root.Appenders)
{
if (a is FileAppender)
{
FileAppender fa = (FileAppender)a;
// Programmatically set this to the desired location here
string logFileLocation = #"C:\MySpecialFolder\MyFile.log";
// Uncomment the lines below if you want to retain the base file name
// and change the folder name...
//FileInfo fileInfo = new FileInfo(fa.File);
//logFileLocation = string.Format(#"C:\MySpecialFolder\{0}", fileInfo.Name);
fa.File = logFileLocation;
fa.ActivateOptions();
break;
}
}
This works for me. Our application needs to put the log file in a folder that contains the version number of the app based on the AssemblyInfo.cs file.
You should be able to set the logFileLocation programmatically (e.g. you can use Server.MapPath() if this is a web application) to suit your needs.

Looks like Peter's answer doesn't work for Log4net v1.2.10.0.
An alternative method is described here.
Basically the method is to implement a custom pattern converter for the log4net config file.
First add this class to your project:
public class SpecialFolderPatternConverter : log4net.Util.PatternConverter
{
override protected void Convert(System.IO.TextWriter writer, object state)
{
Environment.SpecialFolder specialFolder = (Environment.SpecialFolder)Enum.Parse(typeof(Environment.SpecialFolder), base.Option, true);
writer.Write(Environment.GetFolderPath(specialFolder));
}
}
Then set up the File parameter of your FileAppender as follows:
<file type="log4net.Util.PatternString">
<converter>
<name value="folder" />
<type value="MyAppName.SpecialFolderPatternConverter,MyAppName" />
</converter>
<conversionPattern value="%folder{CommonApplicationData}\\SomeOtherFolder\\log.txt" />
</file>
Basically the %folder tells it to look at the converter called folder which points it to the SpecialFolderPatternConverter class. It then calls Convert on that class, passing in the CommonApplicationData (or whatever) enum value.

How about a simple:
XmlConfigurator.LogFullFilename = #"c:\ProgramData\MyApp\Myapp.log";
Why is it so complex to do a really simple thing?

To also change the error log's path (based on JackAce's answer):
private static void SetLogPath(string path, string errorPath)
{
XmlConfigurator.Configure();
log4net.Repository.Hierarchy.Hierarchy h =
(log4net.Repository.Hierarchy.Hierarchy)LogManager.GetRepository();
foreach (var a in h.Root.Appenders)
{
if (a is log4net.Appender.FileAppender)
{
if (a.Name.Equals("LogFileAppender"))
{
log4net.Appender.FileAppender fa = (log4net.Appender.FileAppender)a;
string logFileLocation = path;
fa.File = logFileLocation;
fa.ActivateOptions();
}
else if (a.Name.Equals("ErrorFileAppender"))
{
log4net.Appender.FileAppender fa = (log4net.Appender.FileAppender)a;
string logFileLocation = errorPath;
fa.File = logFileLocation;
fa.ActivateOptions();
}
}
}
}

This worked for me:
<log4net>
<appender name="RollingFileAppender" type="log4net.Appender.RollingFileAppender">
..
<file value="${APPDATA}\MyApp\MyApp Client\logs\Log.txt"/>
..
</log4net>
If needing to write to special folders I found help here (2nd and 3rd example).
Edit:
To answer OP.. This works for 'all users' area:
...
<file value="${ALLUSERSPROFILE}\MyApp\MyApp Client\logs\Log.txt"/>
...
Which is normally "C:\ProgramData" in newer versions of Windows.
See these too:
How to specify common application data folder for log4net? == https://stackoverflow.com/a/1889591/503621 and comments
&
https://superuser.com/q/405097/47628
https://stackoverflow.com/a/5550502/503621

JackAce's answer, just more concise using Linq:
C#
XmlConfigurator.Configure();
var appender = (LogManager.GetRepository() as Hierarchy).Root.Appenders
.OfType<FileAppender>()
.First();
appender.File = logPath;
appender.ActivateOptions();
VB.NET
XmlConfigurator.Configure()
Dim appender = CType(LogManager.GetRepository(), Hierarchy).Root.Appenders _
.OfType(FileAppender)() _
.First()
appender.File = logPath
appender.ActivateOptions()

Great use case for LINQs OfType<T> filter:
/// <summary>
/// Applies a transformation to the filenames of all FileAppenders.
/// </summary>
public static void ChangeLogFile(Func<string,string> transformPath)
{
// iterate over all FileAppenders
foreach (var fileAppender in LogManager.GetRepository().GetAppenders().OfType<FileAppender>())
{
// apply transformation to the filename
fileAppender.File = transformPath(fileAppender.File);
// notify the logging subsystem of the configuration change
fileAppender.ActivateOptions();
}
}
If the filename in the app.config is log.txt this will change the log output to logs/some_name_log.txt:
ChangeLogFile(path => Path.Combine("logs", $"some_name_{Path.GetFileName(path)}"));
To Answer the OPs original problem that would be:
ChangeLogFile(path => Path.Combine(
Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData), path));

As an alternative to doing this programatically, you can use environment variables and customizable patterns in the configuration file. See this response to a similar question.
Look at "PatternString for pattern based configuration" in the Log4Net V1.2.10 release notes.
Also if you are thinking of writing to a directory such as Enviroment.SpecialFolder.CommonApplicationData you need to consider:
Will all instances of your application for all users have write access to the log file? E.g. I don't believe non-administrators will be able to write to Enviroment.SpecialFolder.CommonApplicationData.
Contention if multiple instances of your application (for the same or different users) are attempting to the same file. You can use the "minimal locking model" (see http://logging.apache.org/log4net/release/config-examples.html to allow multiple processes to write to the same log file, but there probably will be a performance impact. Or you could give each process a different log file, e.g. by including the process id in the filename using a customizable pattern.

In the current version of Log4Net (2.0.8.0) you could simply use <file value="${ProgramData}\myFolder\LogFiles\" /> for C:\ProgramData\.. and ${LocalAppData} for C:\Users\user\AppData\Local\

If you have to deploy to unknown systems and you want to use the simple solution by Philipp M even with different special folders you can retrieve the special folder path you want and set a custom env variable before loading the log4net configuration.
string localData = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
Environment.SetEnvironmentVariable("MY_FOLDER_DATA", localData);
XmlConfigurator.Configure( ...
... just to be sure the env variable exists and has the value you want.

Those who are looking to set the file path to Project root dicrectory Path can refer below code. I have implemented this in .NET 5 API Project
Changes for Log4net.config file. Remember type="log4net.Util.PatternString" this line is very important
<appender name="RollingFile" type="log4net.Appender.RollingFileAppender">
<file type="log4net.Util.PatternString" value="%property{LoggerFilePath}\Logs\server.log" />
<appendToFile value="true" />
<maximumFileSize value="100KB" />
<maxSizeRollBackups value="2" />
<layout type="log4net.Layout.PatternLayout">
<conversionPattern value="%date %5level %logger.%method [%line] - MESSAGE: %message%newline %exception" />
</layout>
Then, in Program.cs file
public static IHostBuilder CreateHostBuilder(string[] args) =>
Host.CreateDefaultBuilder(args)
.ConfigureLogging((context,loggerBuilder) =>
{
log4net.GlobalContext.Properties["LoggerFilePath"] = context.HostingEnvironment.ContentRootPath;
loggerBuilder.ClearProviders();
loggerBuilder.AddLog4Net();
})
.ConfigureWebHostDefaults(webBuilder =>
{
webBuilder.UseStartup<Startup>();
});
Thanks!

//Make sure you include System.Linq
//Though is an answer to old question,
//but more concise, just two lines of code.
((FileAppender)LogManager.GetRepository().GetAppenders().Select(a => a).First()).File = #"C:\log.txt";
((FileAppender)LogManager.GetRepository().GetAppenders().Select(a => a).First()).ActivateOptions();