# How ro read and register configuration on app startup in .Net

This is a task that I seldom do. It is usually only done at the start of a project or when adding subtansial features. And I always have to look it up.

In the standard webapp-template for .Net, the configuration class is injected into the Startup.cs file. However, some projects live quite well without Startup.cs. Think worker services or the new slim and hip new style of top level Program.cs. We will see how to get a hold of configuration, read the content into classes and register them with the Dependency Injection Engine in .Net. We will also see it in use.

Our example is a very useful 🐱-based background service.

Consider the following configuration, rather crutial info we need to run our application.

// appsettings.json
  "Cat": {
    "Name": "Lotta"
  "CatWorkerConfiguration": {
    "DelayInMs": 3000

It maps to the following in c#

// Cat.cs
public record Cat
    public string Name { get; init; } = string.Empty;
// ICatWorkerConfiguration.cs
public interface ICatWorkerConfiguration
    int DelayInMs { get; }

// CatWorkerConfiguration.cs
public record CatWorkerConfiguration : ICatWorkerConfiguration
    public int DelayInMs { get; init; }

And the app entrypoint is as follows

// Program.cs
await Host
    .ConfigureServices(services => services.ReadConfigAndAddConfigAndWorker())

And now to the main point. What is .ReadConfigAndAddConfigAndWorker() and what does it do. It is a builder-pattern extention method we will write and use to do all the config related stuff.

// ConfigLoaderHelper.cs
public static class ConfigLoaderHelper
    public static IServiceCollection ReadConfigAndAddConfigAndWorker(
    this IServiceCollection services)
        // get a hold of the services container
        ServiceProvider serviceProvider = services.BuildServiceProvider();
        // When bootstrpping the application .Net auto loades a config class for us. 
        // .Net also will add whetever it finds in appsettings.json.
        // This is how we ask to see that config class.
        IConfiguration config = serviceProvider
            ?? throw new Exception("Missing congiguration");
        // Read and DI-register based on class
        // Let's do that for the cat part
        Cat catConfig = config.GetSection(nameof(Cat)).Get<Cat>();
        // We can also read and DI-register based on an interface
        // Let's do that with the CatWorkerConfiguration
        CatWorkerConfiguration workerConfig = config
        // Finally we load our mighty service and return happily
        return services;

Now we can inject our config at will where ever we need it. Let's see that in action and finally see what use this mighty application does.

// CatWorker.cs
public class CatWorker : BackgroundService
    private readonly ILogger<CatWorker> _logger;
    private readonly ICatWorkerConfiguration _workerConfig;
    private readonly Cat _cat;

    public CatWorker(
        Cat cat,
        ICatWorkerConfiguration workerConfig,
        ILogger<CatWorker> logger)
        _logger = logger;
        _workerConfig = workerConfig;
        _cat = cat;

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
        while (!stoppingToken.IsCancellationRequested)
            var logtext = $"{_cat.Name} is the cat's name 🐱 Time: {DateTimeOffset.Now}";
                await Task.Delay(_workerConfig.DelayInMs, stoppingToken);
            catch (OperationCanceledException)

The output is catastic 😽

info: CatWorkerService.CatWorker[0]
      Lotta is the cat's name 🐱 Time: 09/04/2021 16:55:12 +02:00
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: /Users/petter/Code/WorkerService/WorkerService
info: CatWorkerService.CatWorker[0]
      Lotta is the cat's name 🐱 Time: 09/04/2021 16:55:15 +02:00
info: CatWorkerService.CatWorker[0]
      Lotta is the cat's name 🐱 Time: 09/04/2021 16:55:18 +02:00
info: CatWorkerService.CatWorker[0]
      Lotta is the cat's name 🐱 Time: 09/04/2021 16:55:21 +02:00

Some people like to use the options pattern. Others like a global static instance. Do it whatever way makes you happy. I happen to like this technique 🦄 🛸 🌈

Last updated: September 5th 2021