Log4Net：如何将程序内部的日志结果输出到变量中？

Question

I am trying to learn using Log4Net to be able to improve my logging technique in my program. I have read the documents provided in the Log4Net website, however I have these questions in mind.

This is the most simple output shown in the example program on the Log4Net website produced by BasicConfigurator.Configure() .

0    [main] INFO  MyApp        - Entering application.
36   [main] DEBUG Com.Foo.Bar  - Did it again!
51   [main] INFO  MyApp        - Exiting application.

My program already has a pane (in debug mode) to show the flow of the program which cycles through numerous asynchronous attempts. So when I run it I can see the flow of the program (typical mass debugging method).

In this level, as far as I know by using Log4Net I can collect logs in those moments and these logs can be appended into so many different output methods such as console , external file, db and etc. This leads to double ups in the code. One line to output the logs to the UI and the other to log for Log4Net, in each test point.

Is there any method that I can output the contents of one of the appenders, such as the one shown above, to a variable of a type string or similar to be able to show it at runtime?

I am not sure how you can see the output of the console in the WPF application . And it seems quite funny to output the Log4Net into a file and then print the contests of it back to a string.

---

Update

The output of the Console Appender can be seen in the Output window of Visual Studio when Show output from: drop-down is set to "Debug". Many thanks to @Aron for pointing it out.

Answer 1

Log4net comes with the UDPAppender , which is a low profile way of transmitting the content of your logs to any listener on the network. I would recommend using it as a way to push your logs to your UI.

<appender name="UdpAppender" type="log4net.Appender.UdpAppender">
  <remoteAddress value="127.0.0.1" /> <!-- local for the demo on my machine --> 
  <remotePort value="10000" />
  <layout type="log4net.Layout.PatternLayout" value="%-5level %logger [%ndc] - %message%newline" />
</appender>

In the UI, hooking a listener in a background thread of your WPF app to these messages lets you display them however you like, for example as a sliding window on the x latest logs, or pushing all the logs to the pane you mentioned in your question.

// launch this in a background thread
private static void UDPListen()
{
    IPEndPoint remoteEndPoint = new IPEndPoint(IPAddress.Any, 0);
    var udpClient = new UdpClient(10000);

    while (true)
    {
        var buffer = udpClient.Receive(ref remoteEndPoint);
        var loggingEvent = System.Text.Encoding.Unicode.GetString(buffer);
        // write the log to your pane
    }
}

Interestingly, this is an approach many "dashboards" for log4net use in order to monitor applications, so if you want you can remove the pane in a release build of your application and look at your app from another machine on the network.

---

EDIT : Writing to the console in any application writes to the standard output stream of the program, which you can pipe into other processes; for example launching your wpf app that writes to the console with the following syntax would log the messages to the log.txt file

myApp.exe > log.txt

(kudos to @Aron for the following example) As a much more useful example Visual studio pipes the stdout of the program you are attached to into its Output window, so anything sent to the ConsoleAppender will be present in the Output window.

So a console appender can be useful if you reuse the default output of the program, but nothing forces you to use it.

Answer 2

Maybe offtopic a little, but I found this topic when tried to find a way to write logs both to file and to message bar in the app. I managed to find a more elegant solution.

So, we're in 2020, and there is .Net Core and Microsoft.Extensions.Logging with its universal logger factory and interfaces:

        var messageBar = new MessageBar();

        using var loggerFactory = LoggerFactory.Create(builder => builder
            .AddLog4Net()
            .AddProvider(new MessageBarLoggerProvider(messageBar))
            .SetMinimumLevel(LogLevel.Debug));

        ILogger programLogger = loggerFactory.CreateLogger(nameof(App));
        programLogger.LogDebug("hello world!");

The "hello world!" text appeared both in configured in the log4net.config log file and message bar. It's not that difficult to implement ILoggerProvider and ILogger , here's the concept:

internal sealed class MessageBarLoggerProvider : ILoggerProvider
{
    private readonly MessageBar _messageBar;
    public MessageBarLoggerProvider(MessageBar messageBar) => _messageBar = messageBar;
    public ILogger CreateLogger(string categoryName) => new MessageBarLogger(_messageBar);
    public void Dispose() { }
}


internal class MessageBarLogger : ILogger
{
    private readonly MessageBar _messageBar;
    public MessageBarLogger(MessageBar messageBar) => _messageBar = messageBar;
    public IDisposable BeginScope<TState>(TState state) => throw new NotImplementedException();
    public bool IsEnabled(LogLevel logLevel) => true;

    public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception,
        Func<TState, Exception, string> formatter)
    {
        _messageBar.MessagesList.Add(new Message(formatter(state, exception)));
    }
}