How to add logs in asp.net vNext

Question

I need to set up logs in my asp.net application. It's easy to add output to the console, but I need to configure it in Azure. I don't know how to do it. I need to log all information that occurs with my app into some file and read it.

Answer 1

The ILoggerFactory allows an app to use any implementation of ILogger and ILoggerProvider.

For details on how to implement the interfaces properly, look at the framework's ConsoleLogger and ConsoleLoggerProvider. See also the ASP.NET Core documentation on logging.

Here is a minimal example of a custom ILogger to get started. This is not production code, rather, it demos enough technical depth either to write your own ILogger or to use one from the community.

project.json

"dependencies": {
    "Microsoft.AspNet.Mvc": "6.0.0-rc1-final",
    "Microsoft.AspNet.Server.Kestrel": "1.0.0-rc1-final",
    "Microsoft.Extensions.Logging": "1.0.0-rc1-final",
    "Microsoft.Extensions.Logging.Console": "1.0.0-rc1-final"
}

MyLoggingProvider.cs

namespace LoggingExample
{
    using Microsoft.Extensions.Logging;

    public class MyLoggingProvider : ILoggerProvider
    {
        public ILogger CreateLogger(string categoryName)
        {
            return new MyLogger();
        }

        public void Dispose()
        {
            // TODO Cleanup
        }
    }
}

MyLogger.cs

In Azure you will want to write to somewhere other than C:/temp/some-guid.txt. This is enough to get you started, though, with writing your own simple logger.

namespace LoggingExample
{
    using System;
    using Microsoft.Extensions.Logging;

    public class MyLogger : ILogger
    {
        public void Log(LogLevel logLevel, int eventId, object state,
            Exception exception, Func<object, Exception, string> formatter)
        {
            var builder = new StringBuilder();
            if (formatter != null) {
                builder.AppendLine(formatter(state, exception));
            }

            var values = state as ILogValues;
            if (values != null) {
                foreach (var v in values.GetValues()) {
                    builder.AppendLine(v.Key + ":" + v.Value);
                }
            }

            var logPath = string.Format("C:/temp/{0}.txt", Guid.NewGuid());
            File.WriteAllText(logPath, builder.ToString());
        }

        public bool IsEnabled(LogLevel logLevel) {
            return true;
        }

        public IDisposable BeginScopeImpl(object state) {
            return null;
        }
    }
}

Startup.cs

Now in startup you can use add your logger via loggerFactory.AddProvider(new MyLoggingProvider()). Every call to the ILogger will now log with your provider.

namespace LoggingExample
{
    using Microsoft.AspNet.Builder;
    using Microsoft.AspNet.Http;
    using Microsoft.Extensions.Logging;

    public class Startup
    {
        public void Configure(
            IApplicationBuilder app, 
            ILoggerFactory loggerFactory)
        {
            loggerFactory
                .AddConsole(minLevel: LogLevel.Verbose)
                .AddProvider(new MyLoggingProvider());

            app.Run(async (context) =>
            {
                var logger = loggerFactory.CreateLogger("CatchAll");
                logger.LogInformation("Hello logger!");

                await context.Response.WriteAsync("Hello world!");
            });
        }
    }
}

MyController.cs

Anywhere that supports dependency injection can now receive an ILogger that will log to all of the providers that we registered in the Startup.Configure method.

namespace LoggingExample
{
    using Microsoft.AspNet.Mvc;
    using Microsoft.Extensions.Logging;
    public class MyController : Controller
    {
        public MyController(ILogger logger)
        {
            logger.LogInformation("Logging from my controller");
        }
    }
}

Answer 2

Log4Net

Use Log4Net. Its a common framework for logging that everyone who follows up on your code will understand, and it lets you do things like attach a new log "destination" on the fly just by editing your config file. It already covers most of the things you'll want to do (like create a separate file for each "day"), and most of the log mining tools out there will be able to read the files l4n creates.

Setting it Up

There are tutorials online for how to get started, but they basically require a few simple steps:

1. Download the Log4Net nuget package.

2. Adjust the log settings in your web.config file

3. Create a static instance of the logger object

4. Log Stuff wherever you need to. If you decide you want your logger to write to a file, it will. If you add a database writer, it will write to the db too. Want your log entries to show up in console, just add that logger in your default (debug) config.

Once you get it setup, logging is as simple as this code:

...
} catch(SystemException ex) {
   logger.Error("This error was thrown by the XXX routine", ex);
}

Hope that's helpful.

Edit: Config File + Core

As @auga points out in his oh-so-helpful comment, config for ASP.Net 5 may require you to read carefully the link I added under step #2 above (configuring your logger). Instead of re-writing someone else's blog post, I'll just link to the article I used to set this up in our ASP.NET 5 environment. Works really well.

If you're reading this post to learn (instead of skimming it to critique), I'd suggest following the links...