.NET Console Application with injectable commands

Console applications are alive and kicking. Setting them up might be a bit hard. In this article I'll explore how to create a .NET console application that provides commands using the new System.CommandLine package. This will provide arguments to command mapping out of the box. I'll be showing how to combine it with dependency injection for even more power ⚡.

Goals
NuGet Packages
Project structure
Fake weather service
Commands
1. Current Temperature Command
2. Forecast Command
Dependency injection
Final thoughts

Goals

We want to create a CLI application with the following goals:

System.CommandLine — this is a fairly new project by .NET that helps to create better CLI applications. It offers the ability to add commands, arguments and options to your application. It comes with a --help feature and it will do the command line argument mapping for you.
Dependency Injection — why go anywhere without it? Dependency injection has made ASP.NET way more composable. I wrote an entire article on how to add it to console applications as well. We'll be reusing some of the code.
Environment variable injection support — some of the configuration should be overridable using environment variables.

We're making a CLI, so what's a better way to describe it than showing what the --help should look like?

Description:
  Weather information using a fake weather service.

Usage:
  MyCli [command] [options]

Options:
  --version       Show version information
  -?, -h, --help  Show help and usage information

Commands:
  current   Gets the current temperature.
  forecast  Get the forecast. Almost always wrong.

Note: if you want to use command line argument when executing a dotnet run, you can use -- to feed the arguments to the application instead of the .NET CLI (so dotnet run -- --help in this case).

NuGet Packages

If you say .NET, you say NuGet packages. We'll be using the following packages:

Install-Package System.CommandLine -Version 2.0.0-beta4.22272.1  
Install-Package Microsoft.Extensions.Configuration -Version 7.0.0  
Install-Package Microsoft.Extensions.Configuration.EnvironmentVariables -Version 7.0.0  
Install-Package Microsoft.Extensions.DependencyInjection -Version 7.0.0  
Install-Package Microsoft.Extensions.DependencyInjection.Abstractions -Version 7.0.0  
Install-Package Microsoft.Extensions.Options -Version 7.0.1  
Install-Package Microsoft.Extensions.Options.ConfigurationExtensions -Version 7.0.0

The System.CommandLine package is still in ~~beta~~ preview. I expect it to be released soon, but things might still change. It is used by the .NET dotnet CLI.

Project structure

I'm using the following project structure:

.
├── src/
│   └── MyCli/
│       ├── Commands/
│       │   ├── CurrentCommand.cs
│       │   └── ForcastCommand.cs
│       ├── Services/
│       │   ├── FakeWeatherService.cs
│       │   └── FakeWeatherServiceSettings.cs
│       └── Program.cs
└── MyCli.sln

Fake weather service

What is injection without a good service? Let's create a fake weather service that returns the temperature based on a randomizer:

namespace MyCli.Services;

public class FakeWeatherServiceSettings
{
    public string DefaultCity { get; set; } = "Zwolle, NLD";

    public int DefaultForecastDays { get; set; } = 5;
}

public class FakeWeatherService
{
    public FakeWeatherService(IOptions<FakeWeatherServiceSettings> settings)
    {
        Settings = settings?.Value ?? throw new ArgumentNullException(nameof(settings));
    }

    public FakeWeatherServiceSettings Settings { get; }

    public Task<string> GetTemperature(string? city = null)
    {
        if (city == null) city = Settings.DefaultCity;

        var report = $"In {city} it is now {Random.Shared.Next(-20, 40)} degrees celcius.";
        return Task.FromResult(report);
    }

    public Task<string[]> Forecast(int days, string? city = null)
    {
        if (city == null) city = Settings.DefaultCity;

        var reports = new List<string>
        {
            $"Report for {city} for the next {days} days:"
        };

        for (var i = 0; i<days; i++)
        {
            var date = DateTime.Now.AddDays(i + 1).ToString("yyyy-MM-dd");
            var report = $"- {date}: {Random.Shared.Next(-20, 40),3} degrees celcius.";
            reports.Add(report);
        }

        return Task.FromResult(reports.ToArray());
    }
}

Commands

Commands are implementations of the System.CommandLine.Command class. To make them injectable, we create classes that are derived from the Command class (see dependency injection section).

Current Temperature Command

To get our current temperature command, we'll need to do the following:

Call the base constructor with the name and description of the command. This will be used by the --help feature.
Inject the FakeWeatherService, as it does the actual work.
Use the FakeWeatherService.Settings to get the default value for the --city option.
Map it all together using a SetHandler. The option in automatically mapped to the city parameter of the Execute method.

Now the implementation is very easy:

using MyCli.Services;
using System.CommandLine;

namespace MyCli.Commands;

class CurrentCommand : Command
{
    private readonly FakeWeatherService _weather;

    public CurrentCommand(FakeWeatherService weather) : base("current", "Gets the current temperature.")
    {
        _weather = weather ?? throw new ArgumentNullException(nameof(weather));

        var cityOption = new Option<string>("--city", () => _weather.Settings.DefaultCity, "The city.");

        AddOption(cityOption);

        this.SetHandler(Execute, cityOption);
    }

    private async Task Execute(string city)
    {
        var report = await _weather.GetTemperature(city);
        Console.WriteLine(report);
    }
}

What I like about the setup is that we can add optional arguments with defaults. Here we get the default value from an object from our dependency injection. When we do a current --help, we can a nice description and the actual injected value:

Description:
  Gets the current temperature.

Usage:
  MyCli current [options]

Options:
  --city <city>   The city. [default: Amsterdam, NLD]
  -?, -h, --help  Show help and usage information

Forecast Command

The same goes for the forecast command, but now we have 2 options: --city and --days.

using Microsoft.Extensions.Options;
using MyCli.Services;
using System.CommandLine;

namespace MyCli.Commands;

class ForecastCommand : Command
{
    private readonly FakeWeatherService _weather;

    public ForecastCommand(FakeWeatherService weather) : base("forecast", "Get the forecast. Almost always wrong.")
    {
        _weather = weather ?? throw new ArgumentNullException(nameof(weather));

        var cityOption = new Option<string>("--city", ()=> _weather.Settings.DefaultCity, "The city.");
        var daysOption = new Option<int>("--days", () => _weather.Settings.DefaultForecastDays, "Number of days.");

        AddOption(cityOption);
        AddOption(daysOption);

        this.SetHandler(Execute, cityOption, daysOption);
    }

    private async Task Execute(string city, int days)
    {
        var report = await _weather.Forecast(days, city);
        foreach (var item in report)
        {
            Console.WriteLine(item);
        }
    }
}

Dependency injection

Now, let's tie it all together using dependency injection. We need to do the following:

Setup a ServiceCollection to store our dependencies.
Setup the configuration to use environment variables and read them into our WeatherServiceSettings object.
Add the commands CurrentCommand and ForecastCommand to the service collection.
Add the WeatherService to the service collection.
Create a System.CommandLine.RootCommand and tie it to the registered Command implementation.
Invoke the root command with the given command line arguments.

This leads to the followingProgram.cs code:

using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using MyCli.Commands;
using MyCli.Services;
using System.CommandLine;

static void ConfigureServices(IServiceCollection services)
{
    // build config
    var configuration = new ConfigurationBuilder()
        .AddEnvironmentVariables()
        .Build();

    // settings
    services.Configure<FakeWeatherServiceSettings>(configuration.GetSection("Weather"));

    // add commands:
    services.AddTransient<Command, CurrentCommand>();
    services.AddTransient<Command, ForecastCommand>();

    // add services:
    services.AddTransient<FakeWeatherService>();
}

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

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

// entry to run app
var commands = serviceProvider.GetServices<Command>();
var rootCommand = new RootCommand("Weather information using a fake weather service.");
commands.ToList().ForEach(command => rootCommand.AddCommand(command));

await rootCommand.InvokeAsync(args);