RealtimeLogger - An NLog Companion for Real-Time Logging

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A few months ago I found myself wanting to be able to display messages logged by NLog in an application, specifically on a Windows Form first, and later via a SignalR hub in a web application. I couldn’t find any suitable events within NLog, however I did discover the MethodCall logging target.

This target allows you to specify a static method to be invoked whenever a message is logged to a logger configured to write to it. While this is pretty straightforward, I found the static method a little difficult to integrate into my forms application, and I wanted the ability to retrieve logs that were captured prior to my SignalR hub being started so that I could load them later.

To answer both of these needs I created the RealtimeLogger class. The code can be found on GitHub here and the binaries can be downloaded from NuGet here.


The design of the RealtimeLogger class is extremely simple, containing only three primary elements.

The first element is a public static method named AppendLog. This method accepts five string parameters representing various details about log messages and uses them to create a new instance of RealtimeLoggerEventArgs, which parses the strings into the target data types for each field. AppendLog then invokes the LogAppended event and passes the new RealtimeLoggerEventArgs to it.

The second element is the LogAppended event. This event is raised each time the AppendLog method is invoked by NLog. Anything in the application that needs to be informed of new logs simply needs to subscribe to this event and parse the supplied RealtimeLoggerEventArgs to retrieve the log details.

The third element is a queue of Type Queue<RealtimeLoggerEventArgs>. The queue contains the last N RealtimeLoggerEventArgs instances, where N is user-configurable by specifying a variable in the NLog configuration with the name RealtimeLogger.LogHistoryLimit. The nature of a queue is such that when new items are added, if it has reached it’s limit, the oldest items are discarded. The queue is exposed as the static property

A MethodLogger target corresponding to the RealtimeLogger's ```AppendLog``` method must be added, along with a logger configured to log to the target.  The configuration is described below.

# Installation

Install from the NuGet gallery GUI or with the Package Manager Console using the following command:

```Install-Package NLog.RealtimeLogger```

# Usage

## NLog Configuration

Note that the following instructions are applicable if you aren't using the NuGet package (e.g. you compiled the code yourself, or inserted the source in your project).

Copy the RealtimeLogger.cs file into your project and rename the namespace to fit your application.  In the example below we'll pretend we named it ```MyNamespace```.

In your NLog configuration, add the following target:

<target name="method" xsi:type="MethodCall" className="MyNamespace.RealtimeLogger, MyNamespace" methodName="AppendLog">
  <parameter layout="${threadid}"/>
  <parameter layout="${longdate}"/>
  <parameter layout="${level}"/>
  <parameter layout="${logger}"/>
  <parameter layout="${message} ${exception:format=tostring}"/>

Note that in the className attribute, the fully qualified name of the RealtimeLogger class is used. This includes the namespace, which in this case is MyNamespace. The second part of the attribute must be set to the name of your assembly. This will be defined in Properties\AssemblyInfo.cs within the AssemblyTitle attribute.

Next, add a logger for the method call to your NLog configuration:

<logger name="*" minlevel="Info" writeTo="method"/>

Change the minlevel attribute to whichever logging level you’d like to catch.

When you are finished, your NLog configuration should look something like this:

<nlog xmlns="" 
    <target name="method" xsi:type="MethodCall" className="Example.RealtimeLogger, Example" methodName="AppendLog">
	  <parameter layout="${threadid}"/>
      <parameter layout="${longdate}"/>
      <parameter layout="${level}"/>
      <parameter layout="${logger}"/>
      <parameter layout="${message} ${exception:format=tostring}"/>
    <logger name="*" minlevel="Info" writeTo="method"/>

NuGet Package Configuration

If you downloaded and installed the library from NuGet, your target should match the following:

    <target name="method" xsi:type="MethodCall" className="NLog.RealtimeLogger.RealtimeLogger, NLog.RealtimeLogger" methodName="AppendLog">
	  <parameter layout="${threadid}"/>
      <parameter layout="${longdate}"/>
      <parameter layout="${level}"/>
      <parameter layout="${logger}"/>
      <parameter layout="${message} ${exception:format=tostring}"/>

Event Handler

Next, create a method that can be bound to the RealtimeLogger’s LogAppended event, like so:

private void AppendLog(object sender, RealtimeLoggerEventArgs e)
  Console.WriteLine("New log: " + e.Message);

Bind this event handler to the LogAppended event:

RealtimeLogger.LogAppended += AppendLog;

Generally you’ll want to put the statement above in Main(). Note that any number of handlers can be added.

That’s it! Your handler should be invoked each time a new log is added to the logger.


The properties of the event arguments RealtimeLoggerEventArgs for the LogAppended event are as follows:

ThreadIDintThe ID of the tread from which the log message originated.
DateTimeDateTimeThe DateTime corresponding to the instant the log was added to the logger.
LevelLogLevelThe logging level used to add the log to the logger.
LoggerstringThe name of the logger.
MessagestringThe log message.

Note that the constructor for RealtimeLoggerEventArgs parses the ThreadID, DateTime and LogLevel passed from NLog from strings. If parsing fails the code will prefix the log message with an error before adding it to the log queue.

If the specified ThreadID value from NLog fails to be parsed, the ThreadID property will be set to 0 and the following message will be prepended to the log message:

[Invalid ThreadID; substituted with default]

If the specified DateTime value from NLog fails to be parsed, the DateTime property will be set to DateTime.Now and the following message will be prepended to the log message:

[Invalid DateTime; substituted with DateTime.Now]

The DateTime property for the log message will also be set to DateTime.Now.

If the specified LogLevel fails to be parsed, the LogLevel property will be set to LogLevel.Info and the following message will be prepended to the log message:

[Invalid LogLevel; substituted with LogLevel.Info]

Log Queue

A queue of log messages is provided and may be accessed using the LogHistory property of type Queue<RealtimeLoggerEventArgs>. Logs will accumulate in the queue until it reaches the size specified in the LogHistoryLimit property, after which point logs will be dequeued when new logs are enqueued.

The variable logHistoryLimit is used to initialize the value of the LogHistoryLimit property. This can be changed by editing the class, or LogHistoryLimit can be changed programmatically at runtime.


If the async option is used for the logging target the log event will not fire precisely in real time. It would be a good idea to use the AsyncWrapper functionality described here to limit the asynchronous execution to only those targets that need it, rather than adding the async attribute to the <targets> node of the config file.


Create and modify the NLog configuration variable “RealtimeLogger.LogHistoryLimit” to change the number of logs saved in the log history. The default is 300.

  <variable name="RealtimeLogger.LogHistoryLimit" value="300"/>


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