Dependency injection (with IOptions) in Console Apps in .NET

Dependency injection (with IOptions) in Console Apps in .NET

When you are used to building web applications, you kind of get hooked to the ease of Dependency Injection (DI) and the way settings can be specified in a JSON file and accessed through DI (IOptions). It's only logical to want the same feature in your Console app.

After reading many - many! - outdated blogs, I decided to add my 50 cents specifically on a Console app. It all comes down to packages and separation of concerns.

Console Applications evolve and Microsoft is working on a System.CommandLine package that you might want to include in your CLI. Read more at .NET Console Application with injectable commands.

The first version of this article was written in 2018. It has gotten lots of views since and it should stay green. That's why I've rewritten the article to include instructions for .NET 8 (with is LTS).

Nuget me!

First, we need the right packages. And that can be quite a hassle - because the ASP.NET Core All package will include them for you when you are building an ASP.NET application - but there's no such thing for a Console app. They might be hard to google find:

dotnet add package Microsoft.Extensions.Configuration
dotnet add package Microsoft.Extensions.Configuration.EnvironmentVariables
dotnet add package Microsoft.Extensions.Configuration.FileExtensions
dotnet add package Microsoft.Extensions.Configuration.Json
dotnet add package Microsoft.Extensions.Logging
dotnet add package Microsoft.Extensions.Logging.Abstractions
dotnet add package Microsoft.Extensions.Logging.Configuration
dotnet add package Microsoft.Extensions.Logging.Console
dotnet add package Microsoft.Extensions.Logging.Debug
dotnet add package Microsoft.Extensions.Options
dotnet add package Microsoft.Extensions.Options.DataAnnotations

The project file

Edit your console project file and replace its contents with this.

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.Extensions.Configuration" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Configuration.FileExtensions" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Logging" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Logging.Configuration" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Logging.Console" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Logging.Debug" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Options" Version="9.0.0" />
    <PackageReference Include="Microsoft.Extensions.Options.DataAnnotations" Version="9.0.0" />
  </ItemGroup>

  <ItemGroup>
    <None Update="appsettings.json">
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </None>
  </ItemGroup>

</Project>

This project uses implicit usings, a C# 10 feature, so we don't have to declare all usings.

Now you can upgrade the packages to the latest version in your Nuget manager. If you're not on .NET 5 or higher, you might need to add a latest to the first PropertyGroup. This will enable the latest version of C# including the async main (needed for .NET Core < 3).

AppOptions class + json

This blog has been around for a while now and people asked for an example of the actual options to inject. Let's create a class that uses data annotation to validate the configuration:

using System.ComponentModel.DataAnnotations;

public class AppOptions
{
    public const string SectionName = "App";

    [Required(AllowEmptyStrings = false)]
    public string Greeting { get; set; } = String.Empty;
}

Next, you can add an appsettings.json to the root of your application:

{
  "App": {
    "Greeting": "Hello {0}! Welcome to the tool."
  }
}

Make sure the file has properties: Build Action must be None and Copy to Output Directory must be Copy always. It should like this in your project file:

<ItemGroup>
    <None Update="appsettings.json">
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </None>
  </ItemGroup>

The JSON file works with sections, and we'll see later in the article how we bind the App section to an AppOptions instance. Want to know more about the setup with JSON files? Please read Setup multiple setting-files with a .NET Console Application.

Program vs. App

Normally the program class contains the main method and all the program-logic. If you want to do DI, it might be handy to separate the application from the setup that's required for your DI. So I propose to build an App class that does the actual running. For argument's sake, I've created such a class with a logger and the AppOptions. The class is derived from RootCommand that will do the argument parsing.

using Microsoft.Extensions.Logging;

public class App(ILogger<App> _logger, AppOptions _options)
{
    public async Task Execute(string[] args)
    {
        var name = args.Length == 0 ? "World" : args[0];

        _logger.LogInformation("Starting...");
        var greeting = string.Format(_options.Greeting, name);
        _logger.LogDebug($"Greeting: {greeting}");

        Console.WriteLine(greeting);

        _logger.LogInformation("Finished!");

        await Task.CompletedTask;
    }
}

This basically gives us an abstracted application class that is even testable. It also gives us the opportunity to focus on setting up the application in the program class (separating the actual application logic).

Setting up the program

In the program, we'll focus on DI. The App will also be added through DI. We're using the C# 10 top level statement, so we don't need a Program class:

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;

static void ConfigureServices(IServiceCollection services)
{
    // configure logging
    services.AddLogging(builder =>
    {
        builder.AddConsole();
        builder.AddDebug();
    });

    // build config
    services.AddSingleton<IConfiguration>(_ => 
        new ConfigurationBuilder()
            .SetBasePath(Directory.GetCurrentDirectory())
            .AddJsonFile("appsettings.json", optional: false)
            .AddEnvironmentVariables()
            .Build());

    void Configure<TConfig>(string sectionName) where TConfig : class
    {
        services
            .AddSingleton(p => p.GetRequiredService<IOptions<TConfig>>().Value)
            .AddOptionsWithValidateOnStart<TConfig>()
            .BindConfiguration(sectionName);
            .Validate(options =>
            {
                var results = new List<ValidationResult>();
                var context = new ValidationContext(options);
                if (!Validator.TryValidateObject(options, context, results, validateAllProperties: true))
                {
                    throw new OptionsValidationException(
                        sectionName,
                        typeof(TOptions),
                        results.Select(r => $"[{sectionName}] {r.ErrorMessage}")
                    );
                }
                return true;
            });
    }

    Configure<AppOptions>(AppOptions.SectionName);

    // add services:
    // services.AddTransient<IMyRespository, MyConcreteRepository>();

    // add app
    services.AddTransient<App>();
}

// create service collection
var services = new ServiceCollection();
ConfigureServices(services);

// create service provider
using var serviceProvider = services.BuildServiceProvider();

// entry to run app
await serviceProvider.GetRequiredService<App>().Execute(args);

Since C# 8, the using statement does not need braces anymore.

Invoke the program

When you do a dotnet run -- "peeps from KeesTalksTech" you'll see:

info: App[0]
      Starting...
Hello peeps from KeesTalksTech! Welcome to the tool.
info: App[0]
      Finished!

Final thoughts

We've injected a few things:

The program will resolve the App through DI and start your program. So that's it. Easy to use.

The code is on GitHub, so check it out: code gallery / 1. Dependency injection (with IOptions) in Console Apps in .NET.

Changelog

View original post · View raw Markdown