Centralized ASP.NET Core Logging in One Line of Code

ASP.NET Core comes with some great built-in logging. Framework components such as Routing, MVC and EntityFramework have been updated to support structured logging throughout - for example, when MVC selects a controller and action it includes ActionName in a log event so that later, you can drill down easily to requests hitting a specific action. The framework also adds convenient properties like RequestId to log events by default, making it trivial to zoom in on just the events raised during handling of a particular HTTP request. Setting up truly great application logging in an ASP.NET app has never been easier.

ASP.NET Core log messages in Seq

Seq has had first-class support for ASP.NET Core apps through Serilog since the early beta releases.

Just recently, we've taken this a step further. On File > New Project precious time spent configuring libraries can really add up. We want Seq to be so simple to include that there's no reason to put it off until later. That's why we've created a new package, Seq.Extensions.Logging, that gets centralized logging configuration down to just one line of code.


Here's all it takes to get a new ASP.NET Core app hooked up to Seq. First, add the package:

"dependencies": {
  "Seq.Extensions.Logging": "1.0.1"

Then in your Startup class's Configure() method, call AddSeq():

public void Configure(IApplicationBuilder app,
                      IHostingEnvironment env,
                      ILoggerFactory loggerFactory)

The AddSeq() method supports a few more parameters including the logging level and API key if one is needed. It can also pull configuration from appsettings.json for simple deployment-time configuration. You can check out the README for any other details you need.

Once the logger is configured, you will immediately see some events from the framework on each request. You can add logging to your own code by taking a dependency on Microsoft.Extensions.Logging's ILogger<T>:

class HomeController : Controller
    readonly ILogger<HomeController> _log;

    public HomeController(ILogger<HomeController> log)
        _log = log;

    public IActionResult Index()
        var secret = 42;
        _log.LogInformation("The secret number is {Secret}");

Notice that ASP.NET Core logging has full support for message templates, meaning tokens like {Secret} in the log message will be translated into fully-searchable properties in Seq.

Under the hood

The API of Seq.Extensions.Logging is complete: you can comfortably use it all the way through to production without thinking about how any of it works under the hood. But, if you find you need more control over how log events are collected, or if you'd like to use more advanced Serilog features to enrich or filter events, it's easy to migrate over to Serilog.

Under the hood, the package wraps Serilog, the Serilog provider for Microsoft.Extensions.Logging, and the other bits and pieces of plumbing that make Seq and Serilog work together. Replacing AddSeq() with AddSerilog() is straightforward and mechanical, and all of your logging will continue working in exactly the same way.

Levelling up

There's a whole host of interesting details on ASP.NET Core's logging in the official documentation. Taking some time to learn how to use the API can make your application much easier to debug once it's out there in production.

Don't forget to Install-Package Seq.Extensions.Logging and AddSeq() next time you're starting out on ASP.NET Core!

Nicholas Blumhardt

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