.NET TraceListener is not working with Azure Functions

We’re trying write a TraceListener within Azure Function that can intercept the traces and send to somewhere else, ElasticSearch for example. However, the listener below can’t intercept any traces.

using System;
using System.Diagnostics;
using System.Diagnostics.Tracing;

public static void Run(string input, TraceWriter log)
{
    log.Info($"C# manually triggered function called with input: {input}");
    var listener = new MyListener(log);

    Trace.TraceError("This is a message from trace . . . Error Level.");
    Trace.WriteLine("This is a message from trace . . . The write line method.");
}

public class MyListener : TraceListener
{
    private TraceWriter _log;

    public MyListener(TraceWriter log)
    {
        Trace.Listeners.Add(this);
        this._log = log;
        _log.Info("MyListener is initialized");
    }

    public override void Write(string message)
    {
        _log.Info("MyListener message received: " + message);
    }

    public override void WriteLine(string message)
    {
        _log.Info("MyListener message received: " + message);
    }
}

Expected behavior

“MyListener” can intercept traces and print the message in the log-streaming service

Actual behavior

Nothing shows up

[davidebbo]

Turns out there is a very simple way to enable this: just add this line at the top of your function:

#define TRACE

[davidebbo]

I think there are maybe different issues here. The one tracked here is what OP @yantang-msft raised, where he was registering his own trace listener and it wasn’t getting called. I tested that exact scenario with the #define TRACE and it worked for me. Can you try that?

If it’s a different scenario that’s not working, let’s use a separate issue.