Dependency Injection in .NET

// Dependency Injection lifetimes and IOptions<T>

using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Options;

// --- Configuration options class ---
public class EmailSettings
{
    public string  Host     { get; set; } = "smtp.example.com";
    public int     Port     { get; set; } = 587;
    public bool    UseSsl   { get; set; } = true;
    public string  FromName { get; set; } = "App";
}

// --- Service abstractions ---
public interface IEmailSender { Task SendAsync(string to, string subject, string body); }
public interface INotifier    { Task NotifyAsync(string userId, string msg); }

// --- Concrete services ---
public class SmtpEmailSender : IEmailSender
{
    private readonly EmailSettings _cfg;
    public SmtpEmailSender(IOptions<EmailSettings> opts) => _cfg = opts.Value;

    public Task SendAsync(string to, string subject, string body)
    {
        Console.WriteLine($"SMTP [{_cfg.Host}:{_cfg.Port}] To={to} Subject={subject}");
        return Task.CompletedTask;
    }
}

public class UserNotifier : INotifier
{
    private readonly IEmailSender _email;
    public UserNotifier(IEmailSender email) => _email = email; // injected!

    public Task NotifyAsync(string userId, string msg) =>
        _email.SendAsync($"{userId}@example.com", "Notification", msg);
}

// --- Composition root ---
var services = new ServiceCollection();

services.Configure<EmailSettings>(opts => { opts.Host = "mail.myapp.com"; opts.Port = 465; });
services.AddSingleton<IEmailSender, SmtpEmailSender>();
services.AddTransient<INotifier,    UserNotifier>();

using var sp      = services.BuildServiceProvider();
using var scope   = sp.CreateScope();
var notifier = scope.ServiceProvider.GetRequiredService<INotifier>();
await notifier.NotifyAsync("alice", "Your order has shipped!");