Build status NuGet Samples

Logging to from log4net

In this tutorial we'll add logging to from a .NET application with log4net. Install the appender:

Install-Package Elmah.Io.Log4Net
dotnet add package Elmah.Io.Log4Net
<PackageReference Include="Elmah.Io.Log4Net" Version="3.*" />
paket add Elmah.Io.Log4Net

Add the following to your AssemblyInfo.cs file:

[assembly: log4net.Config.XmlConfigurator(Watch = true)]

Add the following config section to your web/app.config file:

<section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />

Finally, add the log4net configuration element to web/app.config:

  <appender name="ElmahIoAppender" type=",">
    <logId value="LOG_ID" />
    <apiKey value="API_KEY" />
    <level value="Info" />
    <appender-ref ref="ElmahIoAppender" />

That’s it! log4net is now configured and log messages to Remember to replace API_KEY(Where is my API key?) and LOG_ID (Where is my log ID?) with your actual log Id. To start logging, write your usual log4net log statements:

var log = log4net.LogManager.GetLogger(typeof(HomeController));
    log.Info("Trying something");
    throw new ApplicationException();
catch (ApplicationException ex)
    log.Error("Error happening", ex);

Logging custom properties

log4net offers a feature called context properties. With context properties, you can log additional key/value pairs with every log message. The appender for log4net, supports context properties as well. Context properties are handled like custom properties in the UI.

Let's utilize two different hooks in log4net, to add context properties to

log4net.GlobalContext.Properties["ApplicationIdentifier"] = "MyCoolApp";
log4net.ThreadContext.Properties["ThreadId"] = Thread.CurrentThread.ManagedThreadId;

log.Info("This is a message with custom properties");

Basically, we set two custom properties on contextual classes provided by log4net. To read more about the choices in log4net, check out the log4net manual.

When looking up the log message in, we see the context properties in the Data tab. Besides the two custom variables that we set through GlobalContext and ThreadContext, we see a couple of build-in properties in log4net, both prefixed with log4net:.

In addition, Elmah.Io.Log4Net provides a range of reserved property names, that can be used to fill in data in the correct fields on the UI. Let's say you want to fill the User field:

var properties = new PropertiesDictionary();
properties["User"] = "Arnold Schwarzenegger";
log.Logger.Log(new LoggingEvent(new LoggingEventData
    Level = Level.Error,
    TimeStampUtc = DateTime.UtcNow,
    Properties = properties,
    Message = "Hasta la vista, baby",

This will fill in the value Arnold Schwarzenegger in the User field, as well as add a key/value pair to the Data tab on For a reference of all possible property names, check out the property names on CreateMessage.

Setting category provide a field named Category to better group log messages by class name, namespace, or similar. Category maps to log4net's LoggerName field automatically when using Elmah.Io.Log4Net. The category field can be overwritten using one of the context features available in log4net:

log4net.ThreadContext.Properties["Category"] = "The category";
log.Info("This is an information message with custom category");

Message hooks

Decorating log messages

In case you want to set one or more core properties on each message logged, using message hooks may be a better solution. In that case you will need to add a bit of log4net magic. An example could be setting the Version property on all log messages. In the following code, we set a hard-coded version number on all log messages, but the value could come from assembly info, a text file, or similar:

Hierarchy hier = log4net.LogManager.GetRepository(Assembly.GetEntryAssembly()) as Hierarchy;
var elmahIoAppender = (ElmahIoAppender)(hier?.GetAppenders())
    .FirstOrDefault(appender => appender.Name
        .Equals("ElmahIoAppender", StringComparison.InvariantCultureIgnoreCase));

elmahIoAppender.Client.Messages.OnMessage += (sender, a) =>
    a.Message.Version = "1.0.0";

This rather ugly piece of code would go into an initalization block, depending on the project type. The code starts by getting the configured appender (typically set up in web/app.config or log4net.config). With the appender, you can access the underlying client and subscribe to the OnMessage event. This let you trigger a small piece of code, just before sending log messages to In this case, we set the Version property to 1.0.0. Remember to call the ActiveOptions method, to make sure that the Client property is initialized.

Include source code

You can use the OnMessage event to include source code to log messages. This will require a stack trace in the Detail property with filenames and line numbers in it.

There are multiple ways of including source code to log messages. In short, you will need to install the Elmah.Io.Client.Extensions.SourceCode NuGet package and call the WithSourceCodeFromPdb method in the OnMessage event handler:

elmahIoAppender.Client.Messages.OnMessage += (sender, a) =>

Check out How to include source code in log messages for additional requirements to make source code show up on

Including source code on log messages is available in the Elmah.Io.Client v4 package and forward.

Specify API key and log ID in appSettings

You may prefer storing the API key and log ID in the appSettings element over having the values embedded into the appender element. This can be the case for easy config transformation, overwriting values on Azure, or similar. log4net provides a feature named pattern strings to address just that:

<?xml version="1.0" encoding="utf-8"?>
    <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net" />
    <add key="logId" value="LOG_ID"/>
    <add key="apiKey" value="API_KEY"/>
      <level value="ALL" />
      <appender-ref ref="ElmahIoAppender" />
    <appender name="ElmahIoAppender" type=",">
      <logId type="log4net.Util.PatternString" value="%appSetting{logId}" />
      <apiKey type="log4net.Util.PatternString" value="%appSetting{apiKey}" />

The logId and apiKey elements underneath the appender have been extended to include type="log4net.Util.PatternString". This allows for complex patterns in the value attribute. In this example, I reference an app setting from its name, by adding a value of %appSetting{logId} where logId is a reference to the app setting key specified above.


Like other logging frameworks, logging through log4net from ASP.NET Core is also supported. We have a sample to show you how to set it up. The required NuGet packages and configuration are documented in this section.

To start logging to from Microsoft.Extensions.Logging (through log4net), install the Microsoft.Extensions.Logging.Log4Net.AspNetCore NuGet package:

Install-Package Microsoft.Extensions.Logging.Log4Net.AspNetCore
dotnet add package Microsoft.Extensions.Logging.Log4Net.AspNetCore
<PackageReference Include="Microsoft.Extensions.Logging.Log4Net.AspNetCore" Version="3.*" />
paket add Microsoft.Extensions.Logging.Log4Net.AspNetCore

Include a log4net config file to the root of the project:

<?xml version="1.0" encoding="utf-8" ?>
    <level value="WARN" />
    <appender-ref ref="ElmahIoAppender" />
    <appender-ref ref="ConsoleAppender" />
  <appender name="ElmahIoAppender" type=",">
    <logId value="LOG_ID" />
    <apiKey value="API_KEY" />
    <!--<application value="My app" />-->
  <appender name="ConsoleAppender" type="log4net.Appender.ConsoleAppender">
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level %logger [%property{NDC}] - %message%newline" />

In the Program.cs file, make sure to set up log4net:


All internal logging from ASP.NET Core, as well as manual logging you create through the ILogger interface, now goes directly into

A common request is to include all of the HTTP contextual information you usually get logged when using a package like Elmah.Io.AspNetCore. We have developed a specialized NuGet package to include cookies, server variables, etc. when logging through log4net from ASP.NET Core. To set it up, install the Elmah.Io.AspNetCore.Log4Net NuGet package:

Install-Package Elmah.Io.AspNetCore.Log4Net
dotnet add package Elmah.Io.AspNetCore.Log4Net
<PackageReference Include="Elmah.Io.AspNetCore.Log4Net" Version="3.*" />
paket add Elmah.Io.AspNetCore.Log4Net

Finally, make sure to call the UseElmahIoLog4Net method in the Program.cs file:

// ... Exception handling middleware
// ... UseMvc etc.


Here are some things to try out if logging from log4net to doesn't work:

  • Run the diagnose command with the CLI as shown here: Diagnose potential problems with an installation.
  • Make sure that you have the newest Elmah.Io.Log4Net and Elmah.Io.Client packages installed.
  • Make sure to include all of the configuration from the example above. That includes both the <root> and <appender> element.
  • Make sure that the API key is valid and allow the Messages | Write permission.
  • Make sure to include a valid log ID.
  • Make sure that you have sufficient log messages in your subscription and that you didn't disable logging to the log or include any ignore filters/rules.
  • Enable and inspect log4net's internal debug log by including the following code in your web/app.config file to reveal any exceptions happening inside the log4net engine room or one of the appenders:

<?xml version="1.0" encoding="utf-8"?>
    <add key="log4net.Internal.Debug" value="true"/>
    <trace autoflush="true">
          initializeData="C:\temp\log4net-debug.log" />

System.IO.FileLoadException: Could not load file or assembly 'log4net ...

In case you get the following exception while trying to log messages to

System.IO.FileLoadException: Could not load file or assembly 'log4net, Version=, Culture=neutral, PublicKeyToken=669e0ddf0bb1aa2a' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

This indicates they either the log4net.dll file is missing or there's a problem with assembly bindings. You can include a binding redirect to the newest version of log4net by including the following code in your web/app.config file:

<?xml version="1.0" encoding="utf-8"?>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
        <assemblyIdentity name="log4net" publicKeyToken="669e0ddf0bb1aa2a" culture="neutral"/>
        <bindingRedirect oldVersion="" newVersion=""/>

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