C#的依赖注入(DI)是什么？如何在ASP.NET Core中配置和使用？

public class OrderService
{
    private readonly ILogger _logger;

    // 依赖通过构造函数注入
    public OrderService(ILogger logger)
    {
        _logger = logger;
    }
}

var builder = WebApplication.CreateBuilder(args);

// 注册服务
builder.Services.AddScoped();
builder.Services.AddSingleton();
builder.Services.AddTransient();

var app = builder.Build();

[ApiController]
[Route("[controller]")]
public class OrderController : ControllerBase
{
    private readonly IOrderService _orderService;

    // 框架会自动注入已注册的服务
    public OrderController(IOrderService orderService)
    {
        _orderService = orderService;
    }

    [HttpGet]
    public IActionResult Get()
    {
        var orders = _orderService.GetAllOrders();
        return Ok(orders);
    }
}

app.Use(async (context, next) =>
{
    var logger = context.RequestServices.GetService>();
    logger?.LogInformation("Request incoming");
    await next();
});

public interface IEmailSender
{
    void Send(string to, string subject, string body);
}

public class SmtpEmailSender : IEmailSender { ... }

builder.Services.AddScoped();