使用 DI 模拟调用.Net Core class 库

Question

I'm having trouble creating a console program to call the enrol method in referenced class library. I cannot get the dependencies injected except the config.

The Class library contains the code to call:

public interface IMyClient
{
    Task DoSomething(Request request);
}

public class MyClient : IMyClient
{
    private readonly MyClientConfig _config;
    private readonly ILogger<MClient> _logger;
    private readonly HttpClient _client;

    public MyClient()
    {

    }

    public MyClient(MyClientConfig config, ILogger<MyClient> logger, HttpClient client)
    {
        _config = config;
        _logger = logger;
        _client = client;
    }

    public async Task DoSomething(Request request)
    {
        // BREAKPOINT HERE HAS config (id = 1) but logger & client are null
    }
}

And the console program

class Program
{
    private static IServiceProvider _serviceProvider;
    static void Main(string[] args)
    {
        RegisterServices();
        IServiceScope scope = _serviceProvider.CreateScope();

        var request = new Request()
        {
            Id = 1
        };

        scope.ServiceProvider.GetRequiredService<IMyClient>().DoSomething(request);

        DisposeServices();
    }

    private static void RegisterServices()
    {
        var myClientConfig = new ConfigurationBuilder()
            .AddJsonFile("appsettings.json", true, true)
            .Build().GetSection("MyClient").Get<MyClientConfig>();


        var services = new ServiceCollection();
        services.AddSingleton<IMyClient, MyClient>();
        services.AddSingleton<IMyClientConfig>(myClientConfig);
        _serviceProvider = services.BuildServiceProvider(true);
    }

    private static void DisposeServices()
    {
        if (_serviceProvider == null)
        {
            return;
        }
        if (_serviceProvider is IDisposable)
        {
            ((IDisposable)_serviceProvider).Dispose();
        }
    }
}

I tried adding in

services.AddSingleton<HttpClient>(new HttpClient());

to get the HttpCLient injected but to no avail.

Any help appreciated or advice on testing the call to DoSomething in the referenced project

Answer 1

You're not configuring your Logger and HttpClient in services . You are also likely missing an additional Nuget package, Microsoft.Extensions.Logging.Console - install that if it's missing, since you mentioned this is a Console App

Modify your code as follows:

    private static void RegisterServices()
    {

        var myClientConfig = new ConfigurationBuilder()
                                .AddJsonFile("appsettings.json", true, true)
                                .Build().GetSection("MyClient").Get<MyClientConfig>();

        var services = new ServiceCollection();
        services.AddSingleton<IMyClient, MyClient>();

        // Configure logging service
        services.AddLogging(cfg => cfg.AddConsole())
                .AddTransient<MyClient>();

        // Configure HttpClient - might need some constructor params, depending on your use case
        services.AddSingleton<HttpClient>(new HttpClient());

        _serviceProvider = services.BuildServiceProvider(true);
    }

Also, very important : remove your empty constructor in MyClient ! You never want that one hit, In fact, it's far better for the program to crash in debugging than hit that constructor, as it will create appearance of working - like the situation you're in now.

The reason your code "didn't do anything different" when you added the services.AddSingleton<HttpClient>(new HttpClient());code is because of your empty constructor - since the Logger service was still not configured, DI framework couldn't match the correct constructor - so instead of getting a meaningful error, the empty constructor ran (since it always matches) - causing debugging pain and suffering .

As an aside, I recommend reading up on how DI works underneath (maybe even build out your own DI container, a very simple one, for practice. Do not use your own DI container in production.), This is a good article: by the guy that practically invented this stuff: https://martinfowler.com/articles/injection.html