1.5. .NET 框架核心

1.5.1. .NET 基类库（BCL）

BCL 提供所有 .NET 应用共享的核心功能。

1.5.1.1. System 命名空间

// 数学运算
Math.Max(10, 20);              // 20
Math.Min(10, 20);              // 10
Math.Abs(-5);                  // 5
Math.Pow(2, 10);               // 1024
Math.Round(3.14159, 2);        // 3.14
Math.Clamp(value, 0, 100);     // 限制在 [0, 100]

// 随机数
var rng = Random.Shared;
int dice = rng.Next(1, 7);     // 1~6
double pct = rng.NextDouble();  // 0.0~1.0

// 日期时间
DateTime now = DateTime.Now;
DateTime utc = DateTime.UtcNow;
DateTime today = DateTime.Today;
TimeSpan span = end - start;

// 时间运算
DateTime future = now.AddDays(7);
TimeSpan diff = future - now;
Console.WriteLine(diff.TotalHours);  // 168

// GUID
Guid id = Guid.NewGuid();
Console.WriteLine(id.ToString());    // 全局唯一 ID

1.5.2. 文件 I/O

// 读写文本
string content = File.ReadAllText("data.txt");
string[] lines = File.ReadAllLines("data.txt");
File.WriteAllText("output.txt", "Hello");

// 异步版本
string text = await File.ReadAllTextAsync("data.txt");
await File.WriteAllTextAsync("output.txt", "Hello");

// 流式处理
using var reader = new StreamReader("large.txt");
string? line;
while ((line = await reader.ReadLineAsync()) != null)
    ProcessLine(line);

// 文件/目录信息
var info = new FileInfo("data.txt");
Console.WriteLine(info.Length);    // 大小
Console.WriteLine(info.CreationTime);

var dir = new DirectoryInfo(@"C:\Data");
foreach (var file in dir.GetFiles("*.txt"))
    Console.WriteLine(file.Name);

1.5.3. 序列化

// System.Text.Json（.NET Core 3+ 内置）
var user = new User { Id = 1, Name = "Alice" };

// 序列化
string json = JsonSerializer.Serialize(user);
File.WriteAllText("user.json", json);

// 反序列化
string jsonText = File.ReadAllText("user.json");
var deserialized = JsonSerializer.Deserialize<User>(jsonText);

// 配置
var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    WriteIndented = true,
    PropertyNameCaseInsensitive = true,
};

// XML 序列化（旧版兼容）
var serializer = new XmlSerializer(typeof(User));
using var writer = new StreamWriter("user.xml");
serializer.Serialize(writer, user);

1.5.4. 进程与线程

// Task（推荐的方式）
Task task = Task.Run(() => DoWork());
await task;

Task<int> task2 = Task.Run(() => Compute());
int result = await task2;

// Parallel 循环
Parallel.For(0, 100, i =>
{
    Console.WriteLine($"并行处理 {i}");
});

// PLINQ
var result = data.AsParallel()
                 .Where(x => x > 5)
                 .Select(x => ExpensiveOperation(x))
                 .ToList();

// lock 同步
private readonly object _lock = new();
lock (_lock)
{
    sharedCounter++;
}

// 信号量
using var semaphore = new SemaphoreSlim(3);  // 最多 3 并发
await semaphore.WaitAsync();
try { await DoWork(); }
finally { semaphore.Release(); }

1.5.5. 配置管理

// appsettings.json
// {
//   "ConnectionStrings": {
//     "Default": "Server=.;Database=MyDb;..."
//   },
//   "Logging": { "Level": "Information" }
// }

// 读取配置
var config = new ConfigurationBuilder()
    .SetBasePath(Directory.GetCurrentDirectory())
    .AddJsonFile("appsettings.json")
    .Build();

string connStr = config.GetConnectionString("Default");
string logLevel = config["Logging:Level"];

// 强类型配置绑定
public class DatabaseOptions
{
    public string ConnectionString { get; set; } = "";
    public int TimeoutSeconds { get; set; } = 30;
}

var dbOptions = config
    .GetSection("Database")
    .Get<DatabaseOptions>();

1.5.6. 日志系统

// ILogger 接口（Microsoft.Extensions.Logging）
public class OrderService
{
    private readonly ILogger<OrderService> _logger;

    public OrderService(ILogger<OrderService> logger)
    {
        _logger = logger;
    }

    public async Task CreateOrderAsync(Order order)
    {
        _logger.LogInformation("创建订单：{OrderId}", order.Id);

        try
        {
            // 业务逻辑...
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "订单创建失败：{OrderId}", order.Id);
            throw;
        }
    }
}

// 日志级别：Trace < Debug < Information < Warning < Error < Critical