C#的依赖注入(Dependency Injection)如何实现？

// 1. 定义服务接口
public interface IMessageSender
{
    void SendMessage(string message);
}

// 2. 实现服务接口
public class EmailSender : IMessageSender
{
    public void SendMessage(string message)
    {
        Console.WriteLine($"Sending email: {message}");
        // 实际中这里会有更复杂的邮件发送逻辑
    }
}

// 3. 定义一个需要依赖的服务（消费者）
public class NotificationService
{
    private readonly IMessageSender _messageSender;

    // 构造函数注入：通过构造函数接收依赖
    public NotificationService(IMessageSender messageSender)
    {
        _messageSender = messageSender ?? throw new ArgumentNullException(nameof(messageSender));
    }

    public void NotifyUser(string user, string message)
    {
        Console.WriteLine($"Notifying {user}...");
        _messageSender.SendMessage(message);
    }
}

using Microsoft.Extensions.DependencyInjection;
using System;

public class Program
{
    public static void Main(string[] args)
    {
        // 4. 配置DI容器
        var services = new ServiceCollection();

        // 注册服务：将IMessageSender接口映射到EmailSender实现
        // 这里使用AddTransient，表示每次请求都创建一个新的实例
        services.AddTransient<IMessageSender, EmailSender>();
        services.AddTransient<NotificationService>(); // NotificationService本身也可能被注入

        // 构建服务提供者
        var serviceProvider = services.BuildServiceProvider();

        // 5. 从容器中获取实例（消费者）
        // 容器会自动解析NotificationService所依赖的IMessageSender
        var notificationService = serviceProvider.GetService<NotificationService>();

        // 使用服务
        notificationService.NotifyUser("Alice", "Your order has been shipped!");

        // 尝试获取另一个实例，会发现EmailSender也是新的（因为是Transient）
        var anotherNotificationService = serviceProvider.GetService<NotificationService>();
        anotherNotificationService.NotifyUser("Bob", "Your account balance is low.");
    }
}

NotificationService

IMessageSender

new

new

Program.cs

Startup.cs

ConfigureServices

IServiceCollection

IMyService

MyServiceImplementation

// Program.cs
var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddControllersWithViews(); // 比如注册MVC相关的服务

// 注册你的自定义服务
builder.Services.AddTransient<IMessageSender, EmailSender>(); // 每次请求都创建新实例
builder.Services.AddScoped<IUserRepository, UserRepository>(); // 每个HTTP请求创建一个实例
builder.Services.AddSingleton<ICacheService, MemoryCacheService>(); // 整个应用生命周期只创建一个实例

var app = builder.Build();

// ... 其他配置
app.Run();

IMessageSender

EmailSender

new

public class HomeController : Controller
{
    private readonly IMessageSender _messageSender;
    private readonly IUserRepository _userRepository;

    // ASP.NET Core的DI容器会自动解析并注入这些依赖
    public HomeController(IMessageSender messageSender, IUserRepository userRepository)
    {
        _messageSender = messageSender;
        _userRepository = userRepository;
    }

    public IActionResult Index()
    {
        _messageSender.SendMessage("Hello from Home Controller!");
        var user = _userRepository.GetUserById(1);
        ViewBag.UserName = user?.Name;
        return View();
    }
}

Scoped

Singleton

Singleton

Scoped

Scoped