📚 控制结构与流程控制

1. 条件语句

1.1 if 语句

using System;

public class IfStatements
{
    public static void Main()
    {
        // 基本 if 语句
        int age = 18;
        
        if (age >= 18)
        {
            Console.WriteLine("您已成年");
        }
        
        // if-else 语句
        int score = 85;
        
        if (score >= 90)
        {
            Console.WriteLine("优秀");
        }
        else if (score >= 80)
        {
            Console.WriteLine("良好");
        }
        else if (score >= 70)
        {
            Console.WriteLine("中等");
        }
        else if (score >= 60)
        {
            Console.WriteLine("及格");
        }
        else
        {
            Console.WriteLine("不及格");
        }
        
        // 嵌套 if 语句
        bool isWeekend = true;
        bool isRaining = false;
        
        if (isWeekend)
        {
            if (isRaining)
            {
                Console.WriteLine("周末下雨，在家休息");
            }
            else
            {
                Console.WriteLine("周末晴天，出去游玩");
            }
        }
        else
        {
            Console.WriteLine("工作日，正常上班");
        }
        
        // 逻辑运算符
        DemonstrateLogicalOperators();
        
        // 条件运算符（三元运算符）
        DemonstrateTernaryOperator();
    }
    
    private static void DemonstrateLogicalOperators()
    {
        Console.WriteLine("\n=== 逻辑运算符演示 ===");
        
        bool a = true;
        bool b = false;
        
        // 逻辑与 (&&) - 短路求值
        Console.WriteLine($"a && b = {a && b}");
        
        // 逻辑或 (||) - 短路求值
        Console.WriteLine($"a || b = {a || b}");
        
        // 逻辑非 (!)
        Console.WriteLine($"!a = {!a}");
        
        // 异或 (^)
        Console.WriteLine($"a ^ b = {a ^ b}");
        
        // 短路求值演示
        int x = 5;
        if (x > 0 && (x / x) > 0)  // 第二个条件不会导致除零错误
        {
            Console.WriteLine("短路求值保护了除法运算");
        }
        
        // 复杂条件
        int temperature = 25;
        bool isSunny = true;
        bool hasUmbrella = false;
        
        if ((temperature > 20 && isSunny) || (temperature <= 20 && hasUmbrella))
        {
            Console.WriteLine("适合外出");
        }
        else
        {
            Console.WriteLine("不适合外出");
        }
    }
    
    private static void DemonstrateTernaryOperator()
    {
        Console.WriteLine("\n=== 三元运算符演示 ===");
        
        int number = 42;
        
        // 基本三元运算符
        string result = number > 0 ? "正数" : "非正数";
        Console.WriteLine($"{number} 是 {result}");
        
        // 嵌套三元运算符
        string category = number > 0 ? (number % 2 == 0 ? "正偶数" : "正奇数") : "非正数";
        Console.WriteLine($"{number} 是 {category}");
        
        // 三元运算符 vs if-else
        int a = 10, b = 20;
        
        // 使用三元运算符
        int max1 = a > b ? a : b;
        
        // 使用 if-else
        int max2;
        if (a > b)
        {
            max2 = a;
        }
        else
        {
            max2 = b;
        }
        
        Console.WriteLine($"最大值: {max1} (三元运算符), {max2} (if-else)");
        
        // 空合并运算符 (??)
        string? nullableString = null;
        string defaultValue = nullableString ?? "默认值";
        Console.WriteLine($"空合并运算符结果: {defaultValue}");
        
        // 空合并赋值运算符 (??=) - C# 8.0+
        nullableString ??= "新赋值";
        Console.WriteLine($"空合并赋值后: {nullableString}");
    }
}

1.2 switch 语句

using System;

public class SwitchStatements
{
    public static void Main()
    {
        // 传统 switch 语句
        DemonstrateTraditionalSwitch();
        
        // C# 8.0+ switch 表达式
        DemonstrateSwitchExpressions();
        
        // 模式匹配
        DemonstratePatternMatching();
        
        // 复杂模式匹配
        DemonstrateAdvancedPatternMatching();
    }
    
    private static void DemonstrateTraditionalSwitch()
    {
        Console.WriteLine("=== 传统 Switch 语句 ===");
        
        int dayOfWeek = 3;
        
        switch (dayOfWeek)
        {
            case 1:
                Console.WriteLine("星期一");
                break;
            case 2:
                Console.WriteLine("星期二");
                break;
            case 3:
                Console.WriteLine("星期三");
                break;
            case 4:
                Console.WriteLine("星期四");
                break;
            case 5:
                Console.WriteLine("星期五");
                break;
            case 6:
            case 7:
                Console.WriteLine("周末");
                break;
            default:
                Console.WriteLine("无效的日期");
                break;
        }
        
        // 字符串 switch
        string grade = "A";
        
        switch (grade.ToUpper())
        {
            case "A":
                Console.WriteLine("优秀 (90-100)");
                break;
            case "B":
                Console.WriteLine("良好 (80-89)");
                break;
            case "C":
                Console.WriteLine("中等 (70-79)");
                break;
            case "D":
                Console.WriteLine("及格 (60-69)");
                break;
            case "F":
                Console.WriteLine("不及格 (0-59)");
                break;
            default:
                Console.WriteLine("无效等级");
                break;
        }
        
        // 枚举 switch
        DayOfWeek today = DayOfWeek.Wednesday;
        
        switch (today)
        {
            case DayOfWeek.Monday:
            case DayOfWeek.Tuesday:
            case DayOfWeek.Wednesday:
            case DayOfWeek.Thursday:
            case DayOfWeek.Friday:
                Console.WriteLine("工作日");
                break;
            case DayOfWeek.Saturday:
            case DayOfWeek.Sunday:
                Console.WriteLine("周末");
                break;
        }
    }
    
    private static void DemonstrateSwitchExpressions()
    {
        Console.WriteLine("\n=== Switch 表达式 (C# 8.0+) ===");
        
        // 基本 switch 表达式
        int dayNumber = 3;
        string dayName = dayNumber switch
        {
            1 => "星期一",
            2 => "星期二",
            3 => "星期三",
            4 => "星期四",
            5 => "星期五",
            6 or 7 => "周末",  // C# 9.0+ 逻辑模式
            _ => "无效日期"     // 默认情况
        };
        
        Console.WriteLine($"第 {dayNumber} 天是 {dayName}");
        
        // 复杂 switch 表达式
        var weather = (Temperature: 25, IsRaining: false, WindSpeed: 10);
        
        string activity = weather switch
        {
            { Temperature: > 30, IsRaining: false } => "游泳",
            { Temperature: > 20, IsRaining: false, WindSpeed: < 15 } => "户外运动",
            { Temperature: > 10, IsRaining: true } => "室内活动",
            { Temperature: <= 10 } => "在家休息",
            _ => "待定"
        };
        
        Console.WriteLine($"今天适合: {activity}");
        
        // 类型模式
        object value = "Hello";
        string description = value switch
        {
            int i => $"整数: {i}",
            string s => $"字符串: {s}",
            bool b => $"布尔值: {b}",
            null => "空值",
            _ => "未知类型"
        };
        
        Console.WriteLine(description);
    }
    
    private static void DemonstratePatternMatching()
    {
        Console.WriteLine("\n=== 模式匹配 ===");
        
        // is 模式匹配
        object obj = 42;
        
        if (obj is int number)
        {
            Console.WriteLine($"这是一个整数: {number}");
        }
        
        if (obj is int n && n > 0)
        {
            Console.WriteLine($"这是一个正整数: {n}");
        }
        
        // 类型模式和属性模式
        var person = new Person { Name = "张三", Age = 25 };
        
        string message = person switch
        {
            { Age: < 18 } => "未成年人",
            { Age: >= 18 and < 65 } => "成年人",
            { Age: >= 65 } => "老年人",
            null => "无效对象"
        };
        
        Console.WriteLine($"{person.Name} 是 {message}");
        
        // 位置模式（元组）
        var point = (X: 3, Y: 4);
        
        string quadrant = point switch
        {
            (0, 0) => "原点",
            (var x, var y) when x > 0 && y > 0 => "第一象限",
            (var x, var y) when x < 0 && y > 0 => "第二象限",
            (var x, var y) when x < 0 && y < 0 => "第三象限",
            (var x, var y) when x > 0 && y < 0 => "第四象限",
            (var x, 0) when x != 0 => "X轴",
            (0, var y) when y != 0 => "Y轴"
        };
        
        Console.WriteLine($"点 {point} 在 {quadrant}");
    }
    
    private static void DemonstrateAdvancedPatternMatching()
    {
        Console.WriteLine("\n=== 高级模式匹配 ===");
        
        // 关系模式 (C# 9.0+)
        int score = 85;
        string grade = score switch
        {
            >= 90 => "A",
            >= 80 => "B",
            >= 70 => "C",
            >= 60 => "D",
            _ => "F"
        };
        
        Console.WriteLine($"分数 {score} 对应等级 {grade}");
        
        // 逻辑模式 (C# 9.0+)
        bool IsWorkingHour(int hour) => hour switch
        {
            >= 9 and <= 17 => true,
            _ => false
        };
        
        Console.WriteLine($"10点是工作时间: {IsWorkingHour(10)}");
        Console.WriteLine($"20点是工作时间: {IsWorkingHour(20)}");
        
        // 否定模式 (C# 9.0+)
        object value = null;
        string result = value switch
        {
            not null => "有值",
            null => "空值"
        };
        
        Console.WriteLine($"值状态: {result}");
        
        // 嵌套模式
        var order = new Order
        {
            Items = new[] { new OrderItem { Price = 100 }, new OrderItem { Price = 200 } },
            Customer = new Customer { Type = CustomerType.Premium }
        };
        
        decimal discount = order switch
        {
            { Customer.Type: CustomerType.Premium, Items.Length: > 1 } => 0.2m,
            { Customer.Type: CustomerType.Premium } => 0.1m,
            { Items.Length: > 5 } => 0.05m,
            _ => 0m
        };
        
        Console.WriteLine($"订单折扣: {discount:P}");
    }
}

// 辅助类
public class Person
{
    public string Name { get; set; } = "";
    public int Age { get; set; }
}

public class Order
{
    public OrderItem[] Items { get; set; } = Array.Empty<OrderItem>();
    public Customer Customer { get; set; } = new Customer();
}

public class OrderItem
{
    public decimal Price { get; set; }
}

public class Customer
{
    public CustomerType Type { get; set; }
}

public enum CustomerType
{
    Regular,
    Premium,
    VIP
}

2. 循环语句

2.1 for 循环

using System;
using System.Collections.Generic;

public class ForLoops
{
    public static void Main()
    {
        // 基本 for 循环
        Console.WriteLine("=== 基本 for 循环 ===");
        
        for (int i = 0; i < 5; i++)
        {
            Console.WriteLine($"循环第 {i + 1} 次，i = {i}");
        }
        
        // 倒序循环
        Console.WriteLine("\n倒序循环:");
        for (int i = 5; i > 0; i--)
        {
            Console.WriteLine($"倒数第 {6 - i} 次，i = {i}");
        }
        
        // 步长不为1的循环
        Console.WriteLine("\n步长为2的循环:");
        for (int i = 0; i < 10; i += 2)
        {
            Console.WriteLine($"偶数索引: {i}");
        }
        
        // 多变量循环
        Console.WriteLine("\n多变量循环:");
        for (int i = 0, j = 10; i < j; i++, j--)
        {
            Console.WriteLine($"i = {i}, j = {j}");
        }
        
        // 无限循环（需要break跳出）
        Console.WriteLine("\n条件循环:");
        for (int i = 0; ; i++)
        {
            if (i >= 3)
                break;
            Console.WriteLine($"条件循环: {i}");
        }
        
        // 嵌套循环
        DemonstrateNestedLoops();
        
        // 循环控制语句
        DemonstrateLoopControl();
    }
    
    private static void DemonstrateNestedLoops()
    {
        Console.WriteLine("\n=== 嵌套循环 ===");
        
        // 打印乘法表
        Console.WriteLine("九九乘法表:");
        for (int i = 1; i <= 9; i++)
        {
            for (int j = 1; j <= i; j++)
            {
                Console.Write($"{j}×{i}={i * j,2}  ");
            }
            Console.WriteLine();
        }
        
        // 打印图案
        Console.WriteLine("\n星号三角形:");
        for (int i = 1; i <= 5; i++)
        {
            // 打印空格
            for (int j = 1; j <= 5 - i; j++)
            {
                Console.Write(" ");
            }
            // 打印星号
            for (int k = 1; k <= 2 * i - 1; k++)
            {
                Console.Write("*");
            }
            Console.WriteLine();
        }
    }
    
    private static void DemonstrateLoopControl()
    {
        Console.WriteLine("\n=== 循环控制语句 ===");
        
        // break 语句
        Console.WriteLine("break 示例:");
        for (int i = 0; i < 10; i++)
        {
            if (i == 5)
            {
                Console.WriteLine("遇到5，跳出循环");
                break;
            }
            Console.WriteLine($"i = {i}");
        }
        
        // continue 语句
        Console.WriteLine("\ncontinue 示例:");
        for (int i = 0; i < 10; i++)
        {
            if (i % 2 == 0)
            {
                continue;  // 跳过偶数
            }
            Console.WriteLine($"奇数: {i}");
        }
        
        // 标签和 goto（不推荐使用）
        Console.WriteLine("\n嵌套循环中的 break:");
        bool found = false;
        for (int i = 0; i < 3 && !found; i++)
        {
            for (int j = 0; j < 3; j++)
            {
                if (i == 1 && j == 1)
                {
                    Console.WriteLine($"找到目标位置: ({i}, {j})");
                    found = true;
                    break;  // 只跳出内层循环
                }
                Console.WriteLine($"检查位置: ({i}, {j})");
            }
        }
    }
}

2.2 while 和 do-while 循环

using System;

public class WhileLoops
{
    public static void Main()
    {
        // while 循环
        DemonstrateWhileLoop();
        
        // do-while 循环
        DemonstrateDoWhileLoop();
        
        // 实际应用示例
        DemonstrateRealWorldExamples();
    }
    
    private static void DemonstrateWhileLoop()
    {
        Console.WriteLine("=== while 循环 ===");
        
        // 基本 while 循环
        int count = 0;
        while (count < 5)
        {
            Console.WriteLine($"while 循环: count = {count}");
            count++;
        }
        
        // 条件复杂的 while 循环
        int number = 1;
        int sum = 0;
        while (sum < 100)
        {
            sum += number;
            Console.WriteLine($"累加 {number}，当前和为 {sum}");
            number++;
        }
        
        // 无限循环（需要break）
        Console.WriteLine("\n输入数字（输入0退出）:");
        while (true)
        {
            Console.Write("请输入: ");
            string input = Console.ReadLine() ?? "";
            
            if (int.TryParse(input, out int value))
            {
                if (value == 0)
                {
                    Console.WriteLine("退出循环");
                    break;
                }
                Console.WriteLine($"您输入的数字是: {value}");
            }
            else
            {
                Console.WriteLine("请输入有效数字");
            }
        }
    }
    
    private static void DemonstrateDoWhileLoop()
    {
        Console.WriteLine("\n=== do-while 循环 ===");
        
        // 基本 do-while 循环
        int count = 0;
        do
        {
            Console.WriteLine($"do-while 循环: count = {count}");
            count++;
        } while (count < 5);
        
        // do-while 至少执行一次
        Console.WriteLine("\ndo-while 至少执行一次的特性:");
        int x = 10;
        do
        {
            Console.WriteLine($"x = {x}，条件为假但仍执行一次");
            x++;
        } while (x < 10);  // 条件为假，但循环体已执行一次
        
        // 菜单系统示例
        DemonstrateMenuSystem();
    }
    
    private static void DemonstrateMenuSystem()
    {
        Console.WriteLine("\n=== 菜单系统示例 ===");
        
        int choice;
        do
        {
            Console.WriteLine("\n--- 主菜单 ---");
            Console.WriteLine("1. 选项一");
            Console.WriteLine("2. 选项二");
            Console.WriteLine("3. 选项三");
            Console.WriteLine("0. 退出");
            Console.Write("请选择: ");
            
            string input = Console.ReadLine() ?? "";
            
            if (int.TryParse(input, out choice))
            {
                switch (choice)
                {
                    case 1:
                        Console.WriteLine("执行选项一");
                        break;
                    case 2:
                        Console.WriteLine("执行选项二");
                        break;
                    case 3:
                        Console.WriteLine("执行选项三");
                        break;
                    case 0:
                        Console.WriteLine("再见！");
                        break;
                    default:
                        Console.WriteLine("无效选择，请重新输入");
                        choice = -1;  // 继续循环
                        break;
                }
            }
            else
            {
                Console.WriteLine("请输入数字");
                choice = -1;  // 继续循环
            }
            
        } while (choice != 0);
    }
    
    private static void DemonstrateRealWorldExamples()
    {
        Console.WriteLine("\n=== 实际应用示例 ===");
        
        // 数字猜测游戏
        PlayGuessingGame();
        
        // 输入验证
        ValidateInput();
    }
    
    private static void PlayGuessingGame()
    {
        Console.WriteLine("\n--- 数字猜测游戏 ---");
        
        Random random = new Random();
        int targetNumber = random.Next(1, 101);
        int attempts = 0;
        bool guessed = false;
        
        Console.WriteLine("我想了一个1到100之间的数字，请猜猜是多少？");
        
        while (!guessed)
        {
            Console.Write($"第 {attempts + 1} 次猜测: ");
            string input = Console.ReadLine() ?? "";
            
            if (int.TryParse(input, out int guess))
            {
                attempts++;
                
                if (guess == targetNumber)
                {
                    Console.WriteLine($"恭喜！您用 {attempts} 次猜中了数字 {targetNumber}！");
                    guessed = true;
                }
                else if (guess < targetNumber)
                {
                    Console.WriteLine("太小了，请猜大一点");
                }
                else
                {
                    Console.WriteLine("太大了，请猜小一点");
                }
            }
            else
            {
                Console.WriteLine("请输入有效数字");
            }
        }
    }
    
    private static void ValidateInput()
    {
        Console.WriteLine("\n--- 输入验证示例 ---");
        
        int age;
        do
        {
            Console.Write("请输入您的年龄 (1-120): ");
            string input = Console.ReadLine() ?? "";
            
            if (int.TryParse(input, out age))
            {
                if (age >= 1 && age <= 120)
                {
                    Console.WriteLine($"您的年龄是 {age} 岁");
                    break;
                }
                else
                {
                    Console.WriteLine("年龄必须在1到120之间");
                }
            }
            else
            {
                Console.WriteLine("请输入有效的数字");
            }
            
        } while (true);
    }
}

2.3 foreach 循环

using System;
using System.Collections.Generic;
using System.Linq;

public class ForeachLoops
{
    public static void Main()
    {
        // 数组遍历
        DemonstrateArrayIteration();
        
        // 集合遍历
        DemonstrateCollectionIteration();
        
        // 字符串遍历
        DemonstrateStringIteration();
        
        // 自定义可枚举类型
        DemonstrateCustomEnumerable();
        
        // foreach vs for 性能比较
        ComparePerformance();
    }
    
    private static void DemonstrateArrayIteration()
    {
        Console.WriteLine("=== 数组遍历 ===");
        
        int[] numbers = { 1, 2, 3, 4, 5 };
        
        // foreach 遍历
        Console.WriteLine("使用 foreach:");
        foreach (int number in numbers)
        {
            Console.WriteLine($"数字: {number}");
        }
        
        // 多维数组遍历
        int[,] matrix = {
            { 1, 2, 3 },
            { 4, 5, 6 },
            { 7, 8, 9 }
        };
        
        Console.WriteLine("\n多维数组遍历:");
        foreach (int value in matrix)
        {
            Console.Write($"{value} ");
        }
        Console.WriteLine();
        
        // 锯齿数组遍历
        int[][] jaggedArray = {
            new int[] { 1, 2 },
            new int[] { 3, 4, 5 },
            new int[] { 6, 7, 8, 9 }
        };
        
        Console.WriteLine("\n锯齿数组遍历:");
        foreach (int[] row in jaggedArray)
        {
            Console.Write("行: ");
            foreach (int value in row)
            {
                Console.Write($"{value} ");
            }
            Console.WriteLine();
        }
    }
    
    private static void DemonstrateCollectionIteration()
    {
        Console.WriteLine("\n=== 集合遍历 ===");
        
        // List 遍历
        var fruits = new List<string> { "苹果", "香蕉", "橙子", "葡萄" };
        
        Console.WriteLine("水果列表:");
        foreach (string fruit in fruits)
        {
            Console.WriteLine($"- {fruit}");
        }
        
        // Dictionary 遍历
        var scores = new Dictionary<string, int>
        {
            { "张三", 85 },
            { "李四", 92 },
            { "王五", 78 }
        };
        
        Console.WriteLine("\n成绩单:");
        foreach (KeyValuePair<string, int> kvp in scores)
        {
            Console.WriteLine($"{kvp.Key}: {kvp.Value} 分");
        }
        
        // 使用 var 简化
        Console.WriteLine("\n使用 var 简化:");
        foreach (var item in scores)
        {
            Console.WriteLine($"{item.Key}: {item.Value} 分");
        }
        
        // 只遍历键或值
        Console.WriteLine("\n只遍历键:");
        foreach (string name in scores.Keys)
        {
            Console.WriteLine($"学生: {name}");
        }
        
        Console.WriteLine("\n只遍历值:");
        foreach (int score in scores.Values)
        {
            Console.WriteLine($"分数: {score}");
        }
    }
    
    private static void DemonstrateStringIteration()
    {
        Console.WriteLine("\n=== 字符串遍历 ===");
        
        string text = "Hello, World!";
        
        Console.WriteLine($"字符串: {text}");
        Console.WriteLine("字符分解:");
        
        foreach (char c in text)
        {
            if (char.IsLetter(c))
            {
                Console.WriteLine($"字母: {c}");
            }
            else if (char.IsDigit(c))
            {
                Console.WriteLine($"数字: {c}");
            }
            else if (char.IsPunctuation(c))
            {
                Console.WriteLine($"标点: {c}");
            }
            else
            {
                Console.WriteLine($"其他: '{c}'");
            }
        }
        
        // 统计字符
        var charCount = new Dictionary<char, int>();
        foreach (char c in text.ToLower())
        {
            if (char.IsLetter(c))
            {
                if (charCount.ContainsKey(c))
                {
                    charCount[c]++;
                }
                else
                {
                    charCount[c] = 1;
                }
            }
        }
        
        Console.WriteLine("\n字符统计:");
        foreach (var item in charCount.OrderBy(x => x.Key))
        {
            Console.WriteLine($"'{item.Key}': {item.Value} 次");
        }
    }
    
    private static void DemonstrateCustomEnumerable()
    {
        Console.WriteLine("\n=== 自定义可枚举类型 ===");
        
        var range = new NumberRange(1, 5);
        
        Console.WriteLine("自定义范围遍历:");
        foreach (int number in range)
        {
            Console.WriteLine($"数字: {number}");
        }
        
        // 使用 yield 的简单示例
        Console.WriteLine("\n斐波那契数列前10项:");
        foreach (int fib in GetFibonacci(10))
        {
            Console.Write($"{fib} ");
        }
        Console.WriteLine();
    }
    
    private static void ComparePerformance()
    {
        Console.WriteLine("\n=== 性能比较 ===");
        
        int[] largeArray = Enumerable.Range(1, 1000000).ToArray();
        
        // foreach 性能测试
        var stopwatch = System.Diagnostics.Stopwatch.StartNew();
        long sum1 = 0;
        foreach (int number in largeArray)
        {
            sum1 += number;
        }
        stopwatch.Stop();
        Console.WriteLine($"foreach 耗时: {stopwatch.ElapsedMilliseconds} ms, 和: {sum1}");
        
        // for 性能测试
        stopwatch.Restart();
        long sum2 = 0;
        for (int i = 0; i < largeArray.Length; i++)
        {
            sum2 += largeArray[i];
        }
        stopwatch.Stop();
        Console.WriteLine($"for 耗时: {stopwatch.ElapsedMilliseconds} ms, 和: {sum2}");
        
        // LINQ Sum 性能测试
        stopwatch.Restart();
        long sum3 = largeArray.Sum(x => (long)x);
        stopwatch.Stop();
        Console.WriteLine($"LINQ Sum 耗时: {stopwatch.ElapsedMilliseconds} ms, 和: {sum3}");
    }
    
    // 自定义可枚举类型
    private static IEnumerable<int> GetFibonacci(int count)
    {
        int a = 0, b = 1;
        for (int i = 0; i < count; i++)
        {
            yield return a;
            (a, b) = (b, a + b);
        }
    }
}

// 自定义数字范围类
public class NumberRange : IEnumerable<int>
{
    private readonly int _start;
    private readonly int _end;
    
    public NumberRange(int start, int end)
    {
        _start = start;
        _end = end;
    }
    
    public IEnumerator<int> GetEnumerator()
    {
        for (int i = _start; i <= _end; i++)
        {
            yield return i;
        }
    }
    
    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

3. 跳转语句

3.1 break 和 continue

using System;
using System.Collections.Generic;

public class JumpStatements
{
    public static void Main()
    {
        // break 语句详解
        DemonstrateBreak();
        
        // continue 语句详解
        DemonstrateContinue();
        
        // 实际应用场景
        DemonstrateRealWorldUsage();
    }
    
    private static void DemonstrateBreak()
    {
        Console.WriteLine("=== break 语句 ===");
        
        // 在 for 循环中使用 break
        Console.WriteLine("查找第一个偶数:");
        int[] numbers = { 1, 3, 5, 8, 9, 12, 15 };
        
        for (int i = 0; i < numbers.Length; i++)
        {
            if (numbers[i] % 2 == 0)
            {
                Console.WriteLine($"找到第一个偶数: {numbers[i]}，位置: {i}");
                break;  // 找到后立即退出循环
            }
            Console.WriteLine($"检查数字: {numbers[i]}");
        }
        
        // 在 while 循环中使用 break
        Console.WriteLine("\n累加到超过100:");
        int sum = 0;
        int counter = 1;
        
        while (true)  // 无限循环
        {
            sum += counter;
            Console.WriteLine($"累加 {counter}，当前和: {sum}");
            
            if (sum > 100)
            {
                Console.WriteLine("和超过100，退出循环");
                break;
            }
            
            counter++;
        }
        
        // 在嵌套循环中使用 break
        Console.WriteLine("\n嵌套循环中的 break:");
        bool found = false;
        
        for (int i = 1; i <= 3 && !found; i++)
        {
            Console.WriteLine($"外层循环: i = {i}");
            
            for (int j = 1; j <= 3; j++)
            {
                Console.WriteLine($"  内层循环: j = {j}");
                
                if (i * j == 6)
                {
                    Console.WriteLine($"  找到乘积为6的组合: {i} × {j} = 6");
                    found = true;
                    break;  // 只跳出内层循环
                }
            }
        }
    }
    
    private static void DemonstrateContinue()
    {
        Console.WriteLine("\n=== continue 语句 ===");
        
        // 在 for 循环中使用 continue
        Console.WriteLine("打印1到10的奇数:");
        for (int i = 1; i <= 10; i++)
        {
            if (i % 2 == 0)
            {
                continue;  // 跳过偶数
            }
            Console.WriteLine($"奇数: {i}");
        }
        
        // 在 foreach 循环中使用 continue
        Console.WriteLine("\n处理字符串列表（跳过空字符串）:");
        string[] words = { "Hello", "", "World", "   ", "C#", null, "Programming" };
        
        foreach (string word in words)
        {
            if (string.IsNullOrWhiteSpace(word))
            {
                Console.WriteLine("跳过空字符串或null");
                continue;
            }
            
            Console.WriteLine($"处理单词: {word}");
        }
        
        // 在 while 循环中使用 continue
        Console.WriteLine("\n输入处理（输入'skip'跳过，'quit'退出）:");
        
        while (true)
        {
            Console.Write("请输入: ");
            string input = Console.ReadLine() ?? "";
            
            if (input.ToLower() == "quit")
            {
                Console.WriteLine("退出程序");
                break;
            }
            
            if (input.ToLower() == "skip")
            {
                Console.WriteLine("跳过此次输入");
                continue;
            }
            
            Console.WriteLine($"您输入了: {input}");
        }
    }
    
    private static void DemonstrateRealWorldUsage()
    {
        Console.WriteLine("\n=== 实际应用场景 ===");
        
        // 数据验证和处理
        ProcessStudentScores();
        
        // 文件处理模拟
        ProcessFiles();
        
        // 游戏循环模拟
        GameLoop();
    }
    
    private static void ProcessStudentScores()
    {
        Console.WriteLine("\n--- 学生成绩处理 ---");
        
        var studentScores = new Dictionary<string, int[]>
        {
            { "张三", new int[] { 85, 92, 78, 96 } },
            { "李四", new int[] { 88, 76, 94, 82 } },
            { "王五", new int[] { 92, 88, 90, 94 } },
            { "赵六", new int[] { 65, 70, 68, 72 } }
        };
        
        foreach (var student in studentScores)
        {
            string name = student.Key;
            int[] scores = student.Value;
            
            Console.WriteLine($"\n处理学生: {name}");
            
            double total = 0;
            int validScores = 0;
            
            foreach (int score in scores)
            {
                // 跳过无效成绩
                if (score < 0 || score > 100)
                {
                    Console.WriteLine($"  跳过无效成绩: {score}");
                    continue;
                }
                
                total += score;
                validScores++;
                Console.WriteLine($"  有效成绩: {score}");
            }
            
            if (validScores == 0)
            {
                Console.WriteLine($"  {name} 没有有效成绩");
                continue;
            }
            
            double average = total / validScores;
            Console.WriteLine($"  {name} 的平均成绩: {average:F2}");
            
            // 如果平均分低于70，标记为需要关注
            if (average < 70)
            {
                Console.WriteLine($"  ⚠️ {name} 需要额外关注（平均分低于70）");
            }
        }
    }
    
    private static void ProcessFiles()
    {
        Console.WriteLine("\n--- 文件处理模拟 ---");
        
        string[] files = {
            "document.txt",
            "image.jpg",
            "corrupted.dat",
            "report.pdf",
            "empty.txt",
            "large_file.zip"
        };
        
        foreach (string file in files)
        {
            Console.WriteLine($"\n处理文件: {file}");
            
            // 模拟文件检查
            if (file.Contains("corrupted"))
            {
                Console.WriteLine("  文件已损坏，跳过处理");
                continue;
            }
            
            if (file.Contains("empty"))
            {
                Console.WriteLine("  文件为空，跳过处理");
                continue;
            }
            
            if (file.Contains("large"))
            {
                Console.WriteLine("  文件过大，需要特殊处理");
                // 这里可以调用特殊处理逻辑
                Console.WriteLine("  开始特殊处理...");
                
                // 模拟处理时间
                for (int i = 0; i < 3; i++)
                {
                    Console.WriteLine($"    处理进度: {(i + 1) * 33}%");
                    if (i == 1)  // 模拟处理中断
                    {
                        Console.WriteLine("    处理中断，跳过剩余步骤");
                        break;
                    }
                }
                continue;
            }
            
            // 正常文件处理
            Console.WriteLine("  文件处理成功");
        }
    }
    
    private static void GameLoop()
    {
        Console.WriteLine("\n--- 游戏循环模拟 ---");
        
        int playerHealth = 100;
        int level = 1;
        bool gameRunning = true;
        
        while (gameRunning)
        {
            Console.WriteLine($"\n=== 第 {level} 关 ===");
            Console.WriteLine($"玩家生命值: {playerHealth}");
            
            // 模拟关卡事件
            Random random = new Random();
            int eventType = random.Next(1, 5);
            
            switch (eventType)
            {
                case 1:
                    Console.WriteLine("遇到敌人！");
                    int damage = random.Next(10, 31);
                    playerHealth -= damage;
                    Console.WriteLine($"受到 {damage} 点伤害");
                    
                    if (playerHealth <= 0)
                    {
                        Console.WriteLine("游戏结束！");
                        gameRunning = false;
                        break;  // 跳出 switch，然后退出 while 循环
                    }
                    break;
                    
                case 2:
                    Console.WriteLine("找到治疗药水！");
                    int healing = random.Next(15, 26);
                    playerHealth = Math.Min(100, playerHealth + healing);
                    Console.WriteLine($"恢复 {healing} 点生命值");
                    break;
                    
                case 3:
                    Console.WriteLine("发现陷阱，但成功避开了");
                    break;
                    
                case 4:
                    Console.WriteLine("什么都没发生");
                    break;
            }
            
            // 检查是否继续游戏
            if (!gameRunning)
            {
                break;
            }
            
            level++;
            
            // 模拟玩家选择
            if (level > 5)
            {
                Console.WriteLine("恭喜通关！");
                break;
            }
            
            Console.WriteLine("按任意键继续下一关...");
            Console.ReadKey();
        }
        
        Console.WriteLine($"\n最终结果: 到达第 {level} 关，剩余生命值: {Math.Max(0, playerHealth)}");
    }
}

3.2 return 语句

using System;
using System.Collections.Generic;

public class ReturnStatements
{
    public static void Main()
    {
        // 基本 return 用法
        DemonstrateBasicReturn();
        
        // 多个返回点
        DemonstrateMultipleReturns();
        
        // 早期返回模式
        DemonstrateEarlyReturn();
        
        // 返回值类型
        DemonstrateReturnTypes();
    }
    
    private static void DemonstrateBasicReturn()
    {
        Console.WriteLine("=== 基本 return 用法 ===");
        
        // void 方法中的 return
        PrintNumbers(5);
        
        // 有返回值的方法
        int sum = CalculateSum(1, 10);
        Console.WriteLine($"1到10的和: {sum}");
        
        // 条件返回
        string result = GetGrade(85);
        Console.WriteLine($"85分对应等级: {result}");
    }
    
    private static void PrintNumbers(int count)
    {
        Console.WriteLine($"打印前 {count} 个数字:");
        
        for (int i = 1; i <= count; i++)
        {
            if (i > 10)  // 限制最多打印10个
            {
                Console.WriteLine("达到最大限制，停止打印");
                return;  // 提前退出方法
            }
            
            Console.Write($"{i} ");
        }
        
        Console.WriteLine("\n打印完成");
    }
    
    private static int CalculateSum(int start, int end)
    {
        if (start > end)
        {
            Console.WriteLine("起始值不能大于结束值");
            return 0;  // 错误情况下返回默认值
        }
        
        int sum = 0;
        for (int i = start; i <= end; i++)
        {
            sum += i;
        }
        
        return sum;
    }
    
    private static string GetGrade(int score)
    {
        if (score >= 90) return "A";
        if (score >= 80) return "B";
        if (score >= 70) return "C";
        if (score >= 60) return "D";
        return "F";
    }
    
    private static (int Min, int Max, double Average) GetStatistics(int[] numbers)
    {
        if (numbers == null || numbers.Length == 0)
        {
            return (0, 0, 0);
        }
        
        int min = numbers[0];
        int max = numbers[0];
        long sum = 0;
        
        foreach (int number in numbers)
        {
            if (number < min) min = number;
            if (number > max) max = number;
            sum += number;
        }
        
        return (min, max, (double)sum / numbers.Length);
    }
}

3.3 goto 语句（不推荐使用）

using System;

public class GotoStatements
{
    public static void Main()
    {
        Console.WriteLine("=== goto 语句示例（不推荐使用） ===");
        
        // 基本 goto 用法
        DemonstrateBasicGoto();
        
        // goto 的替代方案
        DemonstrateAlternatives();
    }
    
    private static void DemonstrateBasicGoto()
    {
        Console.WriteLine("\n--- 基本 goto 示例 ---");
        
        int i = 0;
        
        start:
        Console.WriteLine($"i = {i}");
        i++;
        
        if (i < 3)
        {
            goto start;  // 跳转到标签
        }
        
        Console.WriteLine("循环结束");
        
        // goto 跳出嵌套循环
        Console.WriteLine("\n--- 跳出嵌套循环 ---");
        
        for (int x = 0; x < 3; x++)
        {
            for (int y = 0; y < 3; y++)
            {
                Console.WriteLine($"({x}, {y})");
                
                if (x == 1 && y == 1)
                {
                    Console.WriteLine("跳出所有循环");
                    goto end;
                }
            }
        }
        
        end:
        Console.WriteLine("嵌套循环结束");
    }
    
    private static void DemonstrateAlternatives()
    {
        Console.WriteLine("\n--- goto 的替代方案 ---");
        
        // 使用 break 和标志变量替代 goto
        Console.WriteLine("使用标志变量:");
        bool shouldExit = false;
        
        for (int x = 0; x < 3 && !shouldExit; x++)
        {
            for (int y = 0; y < 3; y++)
            {
                Console.WriteLine($"({x}, {y})");
                
                if (x == 1 && y == 1)
                {
                    Console.WriteLine("跳出所有循环");
                    shouldExit = true;
                    break;
                }
            }
        }
        
        // 使用方法提取替代 goto
        Console.WriteLine("\n使用方法提取:");
        ProcessMatrix();
    }
    
    private static void ProcessMatrix()
    {
        for (int x = 0; x < 3; x++)
        {
            for (int y = 0; y < 3; y++)
            {
                Console.WriteLine($"({x}, {y})");
                
                if (x == 1 && y == 1)
                {
                    Console.WriteLine("找到目标，退出处理");
                    return;  // 直接退出方法
                }
            }
        }
    }
}

4. 实践练习

4.1 综合练习：学生成绩管理系统

using System;
using System.Collections.Generic;
using System.Linq;

public class StudentGradeManager
{
    private static Dictionary<string, List<int>> studentGrades = new Dictionary<string, List<int>>();
    
    public static void Main()
    {
        Console.WriteLine("=== 学生成绩管理系统 ===");
        
        bool running = true;
        
        while (running)
        {
            ShowMenu();
            
            string choice = Console.ReadLine() ?? "";
            
            switch (choice)
            {
                case "1":
                    AddStudent();
                    break;
                case "2":
                    AddGrade();
                    break;
                case "3":
                    ShowStudentGrades();
                    break;
                case "4":
                    ShowAllStudents();
                    break;
                case "5":
                    ShowStatistics();
                    break;
                case "6":
                    FindTopStudents();
                    break;
                case "0":
                    Console.WriteLine("再见！");
                    running = false;
                    break;
                default:
                    Console.WriteLine("无效选择，请重新输入");
                    break;
            }
            
            if (running)
            {
                Console.WriteLine("\n按任意键继续...");
                Console.ReadKey();
                Console.Clear();
            }
        }
    }
    
    private static void ShowMenu()
    {
        Console.WriteLine("\n--- 主菜单 ---");
        Console.WriteLine("1. 添加学生");
        Console.WriteLine("2. 添加成绩");
        Console.WriteLine("3. 查看学生成绩");
        Console.WriteLine("4. 显示所有学生");
        Console.WriteLine("5. 显示统计信息");
        Console.WriteLine("6. 查找优秀学生");
        Console.WriteLine("0. 退出");
        Console.Write("请选择: ");
    }
    
    private static void AddStudent()
    {
        Console.WriteLine("\n--- 添加学生 ---");
        
        while (true)
        {
            Console.Write("请输入学生姓名（输入'back'返回）: ");
            string name = Console.ReadLine() ?? "";
            
            if (name.ToLower() == "back")
            {
                return;
            }
            
            if (string.IsNullOrWhiteSpace(name))
            {
                Console.WriteLine("姓名不能为空，请重新输入");
                continue;
            }
            
            if (studentGrades.ContainsKey(name))
            {
                Console.WriteLine($"学生 {name} 已存在");
                continue;
            }
            
            studentGrades[name] = new List<int>();
            Console.WriteLine($"学生 {name} 添加成功");
            break;
        }
    }
    
    private static void AddGrade()
    {
        Console.WriteLine("\n--- 添加成绩 ---");
        
        if (studentGrades.Count == 0)
        {
            Console.WriteLine("没有学生记录，请先添加学生");
            return;
        }
        
        // 显示所有学生
        Console.WriteLine("现有学生:");
        int index = 1;
        foreach (string name in studentGrades.Keys)
        {
            Console.WriteLine($"{index}. {name}");
            index++;
        }
        
        Console.Write("请输入学生姓名: ");
        string studentName = Console.ReadLine() ?? "";
        
        if (!studentGrades.ContainsKey(studentName))
        {
            Console.WriteLine("学生不存在");
            return;
        }
        
        while (true)
        {
            Console.Write("请输入成绩 (0-100，输入-1结束): ");
            string input = Console.ReadLine() ?? "";
            
            if (!int.TryParse(input, out int grade))
            {
                Console.WriteLine("请输入有效数字");
                continue;
            }
            
            if (grade == -1)
            {
                break;
            }
            
            if (grade < 0 || grade > 100)
            {
                Console.WriteLine("成绩必须在0-100之间");
                continue;
            }
            
            studentGrades[studentName].Add(grade);
            Console.WriteLine($"成绩 {grade} 添加成功");
        }
    }
    
    private static void ShowStudentGrades()
    {
        Console.WriteLine("\n--- 查看学生成绩 ---");
        
        if (studentGrades.Count == 0)
        {
            Console.WriteLine("没有学生记录");
            return;
        }
        
        Console.Write("请输入学生姓名: ");
        string name = Console.ReadLine() ?? "";
        
        if (!studentGrades.ContainsKey(name))
        {
            Console.WriteLine("学生不存在");
            return;
        }
        
        var grades = studentGrades[name];
        
        if (grades.Count == 0)
        {
            Console.WriteLine($"{name} 还没有成绩记录");
            return;
        }
        
        Console.WriteLine($"\n{name} 的成绩:");
        for (int i = 0; i < grades.Count; i++)
        {
            Console.WriteLine($"第 {i + 1} 次: {grades[i]} 分");
        }
        
        double average = grades.Average();
        int highest = grades.Max();
        int lowest = grades.Min();
        
        Console.WriteLine($"\n统计信息:");
        Console.WriteLine($"平均分: {average:F2}");
        Console.WriteLine($"最高分: {highest}");
        Console.WriteLine($"最低分: {lowest}");
        Console.WriteLine($"总次数: {grades.Count}");
    }
    
    private static void ShowAllStudents()
    {
        Console.WriteLine("\n--- 所有学生信息 ---");
        
        if (studentGrades.Count == 0)
        {
            Console.WriteLine("没有学生记录");
            return;
        }
        
        foreach (var student in studentGrades)
        {
            string name = student.Key;
            var grades = student.Value;
            
            Console.WriteLine($"\n学生: {name}");
            
            if (grades.Count == 0)
            {
                Console.WriteLine("  暂无成绩");
                continue;
            }
            
            Console.WriteLine($"  成绩: [{string.Join(", ", grades)}]");
            Console.WriteLine($"  平均分: {grades.Average():F2}");
            Console.WriteLine($"  考试次数: {grades.Count}");
        }
    }
    
    private static void ShowStatistics()
    {
        Console.WriteLine("\n--- 统计信息 ---");
        
        if (studentGrades.Count == 0)
        {
            Console.WriteLine("没有学生记录");
            return;
        }
        
        int totalStudents = studentGrades.Count;
        int studentsWithGrades = 0;
        var allGrades = new List<int>();
        
        foreach (var grades in studentGrades.Values)
        {
            if (grades.Count > 0)
            {
                studentsWithGrades++;
                allGrades.AddRange(grades);
            }
        }
        
        Console.WriteLine($"总学生数: {totalStudents}");
        Console.WriteLine($"有成绩的学生数: {studentsWithGrades}");
        
        if (allGrades.Count == 0)
        {
            Console.WriteLine("暂无成绩数据");
            return;
        }
        
        Console.WriteLine($"总成绩记录数: {allGrades.Count}");
        Console.WriteLine($"全体平均分: {allGrades.Average():F2}");
        Console.WriteLine($"全体最高分: {allGrades.Max()}");
        Console.WriteLine($"全体最低分: {allGrades.Min()}");
        
        // 成绩分布
        int excellent = allGrades.Count(g => g >= 90);
        int good = allGrades.Count(g => g >= 80 && g < 90);
        int average = allGrades.Count(g => g >= 70 && g < 80);
        int pass = allGrades.Count(g => g >= 60 && g < 70);
        int fail = allGrades.Count(g => g < 60);
        
        Console.WriteLine("\n成绩分布:");
        Console.WriteLine($"优秀 (90-100): {excellent} 次 ({(double)excellent / allGrades.Count * 100:F1}%)");
        Console.WriteLine($"良好 (80-89):  {good} 次 ({(double)good / allGrades.Count * 100:F1}%)");
        Console.WriteLine($"中等 (70-79):  {average} 次 ({(double)average / allGrades.Count * 100:F1}%)");
        Console.WriteLine($"及格 (60-69):  {pass} 次 ({(double)pass / allGrades.Count * 100:F1}%)");
        Console.WriteLine($"不及格 (<60):  {fail} 次 ({(double)fail / allGrades.Count * 100:F1}%)");
    }
    
    private static void FindTopStudents()
    {
        Console.WriteLine("\n--- 优秀学生查找 ---");
        
        if (studentGrades.Count == 0)
        {
            Console.WriteLine("没有学生记录");
            return;
        }
        
        var studentsWithAverage = new List<(string Name, double Average)>();
        
        foreach (var student in studentGrades)
        {
            if (student.Value.Count > 0)
            {
                double avg = student.Value.Average();
                studentsWithAverage.Add((student.Key, avg));
            }
        }
        
        if (studentsWithAverage.Count == 0)
        {
            Console.WriteLine("没有学生有成绩记录");
            return;
        }
        
        // 按平均分排序
        studentsWithAverage.Sort((a, b) => b.Average.CompareTo(a.Average));
        
        Console.WriteLine("学生排名（按平均分）:");
        
        for (int i = 0; i < studentsWithAverage.Count; i++)
        {
            var student = studentsWithAverage[i];
            string rank = GetRankSuffix(i + 1);
            string grade = GetGradeLevel(student.Average);
            
            Console.WriteLine($"{i + 1}{rank}: {student.Name} - 平均分: {student.Average:F2} ({grade})");
        }
        
        // 显示优秀学生（平均分>=85）
        var excellentStudents = studentsWithAverage.Where(s => s.Average >= 85).ToList();
        
        if (excellentStudents.Count > 0)
        {
            Console.WriteLine("\n🏆 优秀学生（平均分≥85）:");
            foreach (var student in excellentStudents)
            {
                Console.WriteLine($"  {student.Name}: {student.Average:F2} 分");
            }
        }
        else
        {
            Console.WriteLine("\n暂无优秀学生（平均分≥85）");
        }
    }
    
    private static string GetRankSuffix(int rank)
    {
        return rank switch
        {
            1 => "st",
            2 => "nd",
            3 => "rd",
            _ => "th"
        };
    }
    
    private static string GetGradeLevel(double average)
    {
        return average switch
        {
            >= 90 => "优秀",
            >= 80 => "良好",
            >= 70 => "中等",
            >= 60 => "及格",
            _ => "不及格"
        };
    }
}

4.2 练习：数字游戏集合

using System;
using System.Collections.Generic;
using System.Linq;

public class NumberGames
{
    public static void Main()
    {
        Console.WriteLine("=== 数字游戏集合 ===");
        
        bool playing = true;
        
        while (playing)
        {
            ShowGameMenu();
            
            string choice = Console.ReadLine() ?? "";
            
            switch (choice)
            {
                case "1":
                    PlayGuessingGame();
                    break;
                case "2":
                    PlayNumberPattern();
                    break;
                case "3":
                    PlayPrimeChecker();
                    break;
                case "4":
                    PlayFibonacci();
                    break;
                case "5":
                    PlayCalculator();
                    break;
                case "0":
                    Console.WriteLine("感谢游玩！");
                    playing = false;
                    break;
                default:
                    Console.WriteLine("无效选择，请重新输入");
                    break;
            }
            
            if (playing)
            {
                Console.WriteLine("\n按任意键返回主菜单...");
                Console.ReadKey();
                Console.Clear();
            }
        }
    }
    
    private static void ShowGameMenu()
    {
        Console.WriteLine("\n--- 游戏菜单 ---");
        Console.WriteLine("1. 数字猜测游戏");
        Console.WriteLine("2. 数字模式识别");
        Console.WriteLine("3. 质数检查器");
        Console.WriteLine("4. 斐波那契数列");
        Console.WriteLine("5. 简单计算器");
        Console.WriteLine("0. 退出");
        Console.Write("请选择游戏: ");
    }
    
    private static void PlayGuessingGame()
    {
        Console.WriteLine("\n=== 数字猜测游戏 ===");
        Console.WriteLine("我想了一个1到100之间的数字，请猜猜是多少？");
        
        Random random = new Random();
        int targetNumber = random.Next(1, 101);
        int attempts = 0;
        int maxAttempts = 7;
        bool won = false;
        
        while (attempts < maxAttempts && !won)
        {
            Console.Write($"第 {attempts + 1}/{maxAttempts} 次猜测: ");
            string input = Console.ReadLine() ?? "";
            
            if (!int.TryParse(input, out int guess))
            {
                Console.WriteLine("请输入有效数字");
                continue;
            }
            
            attempts++;
            
            if (guess == targetNumber)
            {
                Console.WriteLine($"🎉 恭喜！您用 {attempts} 次猜中了数字 {targetNumber}！");
                won = true;
            }
            else if (guess < targetNumber)
            {
                int diff = targetNumber - guess;
                if (diff <= 5)
                {
                    Console.WriteLine("很接近了！太小了，请猜大一点");
                }
                else
                {
                    Console.WriteLine("太小了，请猜大一点");
                }
            }
            else
            {
                int diff = guess - targetNumber;
                if (diff <= 5)
                {
                    Console.WriteLine("很接近了！太大了，请猜小一点");
                }
                else
                {
                    Console.WriteLine("太大了，请猜小一点");
                }
            }
        }
        
        if (!won)
        {
            Console.WriteLine($"😞 游戏结束！正确答案是 {targetNumber}");
        }
    }
    
    private static void PlayNumberPattern()
    {
        Console.WriteLine("\n=== 数字模式识别 ===");
        
        var patterns = new[]
        {
            new { Sequence = new[] { 2, 4, 6, 8 }, Next = 10, Description = "偶数序列" },
            new { Sequence = new[] { 1, 4, 9, 16 }, Next = 25, Description = "平方数序列" },
            new { Sequence = new[] { 1, 1, 2, 3, 5 }, Next = 8, Description = "斐波那契序列" },
            new { Sequence = new[] { 3, 6, 12, 24 }, Next = 48, Description = "倍数序列" },
            new { Sequence = new[] { 1, 3, 6, 10 }, Next = 15, Description = "三角数序列" }
        };
        
        Random random = new Random();
        var pattern = patterns[random.Next(patterns.Length)];
        
        Console.WriteLine("请找出下面数字序列的规律，并说出下一个数字:");
        Console.WriteLine($"序列: {string.Join(", ", pattern.Sequence)}, ?");
        
        int attempts = 0;
        bool correct = false;
        
        while (attempts < 3 && !correct)
        {
            Console.Write($"第 {attempts + 1}/3 次尝试: ");
            string input = Console.ReadLine() ?? "";
            
            if (!int.TryParse(input, out int guess))
            {
                Console.WriteLine("请输入有效数字");
                continue;
            }
            
            attempts++;
            
            if (guess == pattern.Next)
            {
                Console.WriteLine($"🎉 正确！这是 {pattern.Description}");
                correct = true;
            }
            else
            {
                if (attempts < 3)
                {
                    Console.WriteLine("不正确，请再试一次");
                }
                else
                {
                    Console.WriteLine($"😞 正确答案是 {pattern.Next}，这是 {pattern.Description}");
                }
            }
        }
    }
    
    private static void PlayPrimeChecker()
    {
        Console.WriteLine("\n=== 质数检查器 ===");
        Console.WriteLine("输入数字，我来判断是否为质数（输入0退出）");
        
        while (true)
        {
            Console.Write("请输入数字: ");
            string input = Console.ReadLine() ?? "";
            
            if (!int.TryParse(input, out int number))
            {
                Console.WriteLine("请输入有效数字");
                continue;
            }
            
            if (number == 0)
            {
                break;
            }
            
            if (number < 2)
            {
                Console.WriteLine($"{number} 不是质数（质数必须大于1）");
                continue;
            }
            
            bool isPrime = IsPrime(number);
            
            if (isPrime)
            {
                Console.WriteLine($"✅ {number} 是质数");
                
                // 显示附近的质数
                var nearbyPrimes = FindNearbyPrimes(number, 5);
                if (nearbyPrimes.Count > 1)
                {
                    Console.WriteLine($"附近的质数: {string.Join(", ", nearbyPrimes)}");
                }
            }
            else
            {
                Console.WriteLine($"❌ {number} 不是质数");
                
                // 显示因数分解
                var factors = GetFactors(number);
                Console.WriteLine($"因数: {string.Join(", ", factors)}");
            }
        }
    }
    
    private static bool IsPrime(int number)
    {
        if (number < 2) return false;
        if (number == 2) return true;
        if (number % 2 == 0) return false;
        
        for (int i = 3; i * i <= number; i += 2)
        {
            if (number % i == 0)
            {
                return false;
            }
        }
        
        return true;
    }
    
    private static List<int> FindNearbyPrimes(int center, int count)
    {
        var primes = new List<int>();
        
        // 向前查找
        for (int i = center - 1; i >= 2 && primes.Count < count / 2; i--)
        {
            if (IsPrime(i))
            {
                primes.Insert(0, i);
            }
        }
        
        // 添加中心数字（如果是质数）
        if (IsPrime(center))
        {
            primes.Add(center);
        }
        
        // 向后查找
        for (int i = center + 1; primes.Count < count; i++)
        {
            if (IsPrime(i))
            {
                primes.Add(i);
            }
        }
        
        return primes;
    }
    
    private static List<int> GetFactors(int number)
    {
        var factors = new List<int>();
        
        for (int i = 1; i <= number; i++)
        {
            if (number % i == 0)
            {
                factors.Add(i);
            }
        }
        
        return factors;
    }
    
    private static void PlayFibonacci()
    {
        Console.WriteLine("\n=== 斐波那契数列 ===");
        
        while (true)
        {
            Console.Write("请输入要生成的斐波那契数列长度（输入0退出）: ");
            string input = Console.ReadLine() ?? "";
            
            if (!int.TryParse(input, out int count))
            {
                Console.WriteLine("请输入有效数字");
                continue;
            }
            
            if (count == 0)
            {
                break;
            }
            
            if (count < 1)
            {
                Console.WriteLine("长度必须大于0");
                continue;
            }
            
            Console.WriteLine($"\n前 {count} 个斐波那契数:");
            
            long a = 0, b = 1;
            
            for (int i = 0; i < count; i++)
            {
                if (i == 0)
                {
                    Console.Write($"{a}");
                }
                else if (i == 1)
                {
                    Console.Write($", {b}");
                }
                else
                {
                    long next = a + b;
                    Console.Write($", {next}");
                    a = b;
                    b = next;
                }
                
                // 每10个数字换行
                if ((i + 1) % 10 == 0)
                {
                    Console.WriteLine();
                }
            }
            
            Console.WriteLine();
            
            // 显示黄金比例近似值
            if (count >= 10)
            {
                double ratio = (double)b / a;
                Console.WriteLine($"\n黄金比例近似值: {ratio:F6}");
                Console.WriteLine($"实际黄金比例: {(1 + Math.Sqrt(5)) / 2:F6}");
            }
        }
    }
    
    private static void PlayCalculator()
    {
        Console.WriteLine("\n=== 简单计算器 ===");
        Console.WriteLine("支持的运算: +, -, *, /, %, ^ (幂运算)");
        Console.WriteLine("输入格式: 数字1 运算符 数字2");
        Console.WriteLine("输入 'quit' 退出");
        
        while (true)
        {
            Console.Write("\n请输入表达式: ");
            string input = Console.ReadLine() ?? "";
            
            if (input.ToLower() == "quit")
            {
                break;
            }
            
            string[] parts = input.Split(' ', StringSplitOptions.RemoveEmptyEntries);
            
            if (parts.Length != 3)
            {
                Console.WriteLine("格式错误，请使用: 数字1 运算符 数字2");
                continue;
            }
            
            if (!double.TryParse(parts[0], out double num1))
            {
                Console.WriteLine("第一个数字格式错误");
                continue;
            }
            
            if (!double.TryParse(parts[2], out double num2))
            {
                Console.WriteLine("第二个数字格式错误");
                continue;
            }
            
            string op = parts[1];
            double result = 0;
            bool validOperation = true;
            
            switch (op)
            {
                case "+":
                    result = num1 + num2;
                    break;
                case "-":
                    result = num1 - num2;
                    break;
                case "*":
                    result = num1 * num2;
                    break;
                case "/":
                    if (num2 == 0)
                    {
                        Console.WriteLine("错误: 除数不能为零");
                        validOperation = false;
                    }
                    else
                    {
                        result = num1 / num2;
                    }
                    break;
                case "%":
                    if (num2 == 0)
                    {
                        Console.WriteLine("错误: 除数不能为零");
                        validOperation = false;
                    }
                    else
                    {
                        result = num1 % num2;
                    }
                    break;
                case "^":
                    result = Math.Pow(num1, num2);
                    break;
                default:
                    Console.WriteLine($"不支持的运算符: {op}");
                    validOperation = false;
                    break;
            }
            
            if (validOperation)
            {
                Console.WriteLine($"结果: {num1} {op} {num2} = {result}");
                
                // 显示额外信息
                if (op == "/" && num1 % num2 != 0)
                {
                    Console.WriteLine($"整数部分: {(int)(num1 / num2)}");
                    Console.WriteLine($"余数: {num1 % num2}");
                }
            }
        }
    }
}

5. 总结

本章详细介绍了C#中的控制结构与流程控制，包括：

5.1 条件语句

• if语句: 基本条件判断，支持嵌套和复杂逻辑
• switch语句: 多分支选择，包括传统switch和C# 8.0+的switch表达式
• 模式匹配: C# 7.0+引入的强大特性，支持类型模式、属性模式等

5.2 循环语句

• for循环: 适用于已知循环次数的情况
• while循环: 适用于条件驱动的循环
• do-while循环: 至少执行一次的循环
• foreach循环: 用于遍历集合和可枚举对象

5.3 跳转语句

• break: 跳出循环或switch语句
• continue: 跳过当前循环迭代
• return: 从方法返回值或控制权
• goto: 无条件跳转（不推荐使用）

5.4 最佳实践

1. 优先使用早期返回模式，减少嵌套层次
2. 合理使用switch表达式，提高代码可读性
3. 避免使用goto语句，使用结构化控制流
4. 在循环中合理使用break和continue，提高代码效率
5. 利用模式匹配，编写更简洁的条件判断代码

5.5 性能考虑

• foreach通常比for循环更安全，性能差异很小
• switch语句在多分支情况下比多个if-else更高效
• 避免在循环中进行复杂计算，考虑提前计算或缓存

下一章我们将学习数组与集合，了解如何在C#中存储和操作数据集合。

private static void DemonstrateMultipleReturns()
{
    Console.WriteLine("\n=== 多个返回点 ===");

    // 测试不同的输入
    TestDivision(10, 2);
    TestDivision(10, 0);
    TestDivision(-5, 2);

    // 测试查找功能
    int[] numbers = { 1, 3, 5, 7, 9, 11, 13 };
    Console.WriteLine($"查找5: 索引 {FindNumber(numbers, 5)}");
    Console.WriteLine($"查找8: 索引 {FindNumber(numbers, 8)}");
}

private static void TestDivision(double a, double b)
{
    Console.WriteLine($"\n计算 {a} ÷ {b}:");

    double result = SafeDivide(a, b);
    if (!double.IsNaN(result))
    {
        Console.WriteLine($"结果: {result}");
    }
}

private static double SafeDivide(double dividend, double divisor)
{
    // 检查除数是否为零
    if (divisor == 0)
    {
        Console.WriteLine("错误: 除数不能为零");
        return double.NaN;
    }

    // 检查被除数是否为负数（假设业务规则不允许）
    if (dividend < 0)
    {
        Console.WriteLine("警告: 被除数为负数");
        return double.NaN;
    }

    // 正常计算
    return dividend / divisor;
}

private static int FindNumber(int[] array, int target)
{
    if (array == null)
    {
        Console.WriteLine("数组为空");
        return -1;
    }

    for (int i = 0; i < array.Length; i++)
    {
        if (array[i] == target)
        {
            return i;  // 找到目标，立即返回索引
        }
    }

    return -1;  // 未找到
}

private static void DemonstrateEarlyReturn()
{
    Console.WriteLine("\n=== 早期返回模式 ===");

    // 测试用户验证
    ValidateUser("admin", "password123");
    ValidateUser("", "password123");
    ValidateUser("admin", "");
    ValidateUser("user", "wrongpassword");

    // 测试文件处理
    ProcessFile("document.txt");
    ProcessFile("large_file.zip");
    ProcessFile("corrupted.dat");
}

private static bool ValidateUser(string username, string password)
{
    Console.WriteLine($"\n验证用户: {username}");

    // 早期返回 - 避免深层嵌套
    if (string.IsNullOrEmpty(username))
    {
        Console.WriteLine("用户名不能为空");
        return false;
    }

    if (string.IsNullOrEmpty(password))
    {
        Console.WriteLine("密码不能为空");
        return false;
    }

    if (username.Length < 3)
    {
        Console.WriteLine("用户名长度至少3个字符");
        return false;
    }

    if (password.Length < 8)
    {
        Console.WriteLine("密码长度至少8个字符");
        return false;
    }

    // 模拟数据库验证
    if (username != "admin" || password != "password123")
    {
        Console.WriteLine("用户名或密码错误");
        return false;
    }

    Console.WriteLine("验证成功");
    return true;
}

private static void ProcessFile(string filename)
{
    Console.WriteLine($"\n处理文件: {filename}");

    // 早期返回避免复杂的条件嵌套
    if (string.IsNullOrEmpty(filename))
    {
        Console.WriteLine("文件名不能为空");
        return;
    }

    if (filename.Contains("corrupted"))
    {
        Console.WriteLine("文件已损坏，无法处理");
        return;
    }

    if (filename.Contains("large"))
    {
        Console.WriteLine("文件过大，需要特殊处理");
        // 这里可以调用特殊处理逻辑
        return;
    }

    // 正常处理逻辑
    Console.WriteLine("文件处理成功");
}

private static void DemonstrateReturnTypes()
{
    Console.WriteLine("\n=== 返回值类型 ===");

    // 值类型返回
    int number = GetRandomNumber();
    Console.WriteLine($"随机数: {number}");

    // 引用类型返回
    string message = GetWelcomeMessage("张三");
    Console.WriteLine(message);

    // 集合返回
    List<int> evenNumbers = GetEvenNumbers(1, 10);
    Console.WriteLine($"偶数: [{string.Join(", ", evenNumbers)}]");

    // 可空类型返回
    int? result = TryParseInt("123");
    Console.WriteLine($"解析结果: {result}");

    result = TryParseInt("abc");
    Console.WriteLine($"解析结果: {result ?? -1} (失败时显示-1)");

    // 元组返回 (C# 7.0+)
    var (min, max, avg) = GetStatistics(new[] { 1, 5, 3, 9, 2, 7 });
    Console.WriteLine($"统计: 最小={min}, 最大={max}, 平均={avg:F2}");
}

private static int GetRandomNumber()
{
    Random random = new Random();
    return random.Next(1, 101);
}

private static string GetWelcomeMessage(string name)
{
    if (string.IsNullOrEmpty(name))
    {
        return "欢迎访问！";
    }

    return $"欢迎您，{name}！";
}

private static List<int> GetEvenNumbers(int start, int end)
{
    var result = new List<int>();

    for (int i = start; i <= end; i++)
    {
        if (i % 2 == 0)
        {
            result.Add(i);
        }
    }

    return result;
}

private static int? TryParseInt(string input)
{
    if (int.TryParse(input, out int result))
    {
        return result;
    }

    return null;
}

private static (int Min, int Max, double Average) GetStatistics(int[] numbers)
{
    if (numbers == null || numbers.Length == 0)
    {
        return (0, 0, 0);
    }

    int min = numbers[0];
    int max = numbers[0];
    long sum = 0;