C# 技术备忘 - LINQ

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// 1. 基本类型 Contains
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var ids = new List<int> { 1, 2, 3, 4, 5 };
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Console.WriteLine($"包含 3: {ids.Contains(3)}");    // True
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Console.WriteLine($"包含 10: {ids.Contains(10)}");  // False
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var names = new List<string> { "张三", "李四", "王五" };
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Console.WriteLine($"包含 '张三': {names.Contains("张三")}");  // True
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Console.WriteLine($"包含 '赵六': {names.Contains("赵六")}");  // False
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// 2. 引用类型 Contains（需要实现 IEquatable 或重写 Equals）
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var targetStudent = students.First(s => s.Id == 2);
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bool containsTarget = students.Contains(targetStudent);
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Console.WriteLine($"包含李四吗: {containsTarget}");  // True
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// 3. Contains 用于字符串列表
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var classNames = new List<string> { "软件工程1班", "软件工程2班", "软件工程3班" };
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bool hasClass = classNames.Contains("软件工程1班");
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Console.WriteLine($"有软件工程1班吗: {hasClass}");  // True

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var classes = new List<Class>
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{
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    new Class { Id = 1, ClassName = "软件工程1班", Teacher = "王老师" },
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    new Class { Id = 2, ClassName = "软件工程2班", Teacher = "李老师" },
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    new Class { Id = 3, ClassName = "软件工程3班", Teacher = "张老师" },
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};
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public class Class
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{
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    public int Id { get; set; }
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    public string ClassName { get; set; }
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    public string Teacher { get; set; }
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}
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// 修改 Student 类，添加 ClassId
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public class Student
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{
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    public int Id { get; set; }
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    public string Name { get; set; }
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    public int Age { get; set; }
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    public int ClassId { get; set; }  // 外键
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    public string Class { get; set; } // 保留原字段
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    public int Score { get; set; }
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    public decimal? Balance { get; set; }
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}

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var students = new List<Student>
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{
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    new Student { Id = 1, Name = "张三", Age = 20, Score = 85, Balance = 50.5m },
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    new Student { Id = 2, Name = "李四", Age = 22, Score = 92, Balance = 120.0m },
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    new Student { Id = 3, Name = "王五", Age = 21, Score = 78, Balance = null },
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};
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// ToList - 立即执行，生成新的 List
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var highScoreList = students.Where(s => s.Score >= 80).ToList();
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Console.WriteLine($"高分学生数: {highScoreList.Count}");  // 输出：2
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// 关键：ToList 会立即执行并固化结果
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var filtered = students.Where(s => s.Age > 20);
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students.Add(new Student { Id = 4, Name = "赵六", Age = 25, Score = 88 });  // 后添加
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// 延迟执行：filtered 会包含赵六
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Console.WriteLine($"延迟执行数量: {filtered.Count()}");  // 输出：3（李四、王五、赵六）
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var filteredToList = students.Where(s => s.Age > 20).ToList();
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students.Add(new Student { Id = 5, Name = "孙七", Age = 24, Score = 90 });  // 再添加
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// ToList 已固化：不包含孙七
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Console.WriteLine($"ToList固化数量: {filteredToList.Count}");  // 输出：3（李四、王五、赵六）

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// ToDictionary - 把集合转成字典，必须指定唯一的 Key
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try
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{
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    // 用 Id 作为 Key（唯一）
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    var dictById = students.ToDictionary(s => s.Id);
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    Console.WriteLine($"字典数量: {dictById.Count}");
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    // 通过 Key 快速查找（O(1) 时间复杂度）
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    var student = dictById[2];
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    Console.WriteLine($"ID=2的学生: {student.Name}");  // 输出：李四
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    // 遍历字典
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    foreach (var kvp in dictById)
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    {
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        Console.WriteLine($"Key: {kvp.Key}, Name: {kvp.Value.Name}");
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    }
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}
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catch (ArgumentException ex)
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{
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    Console.WriteLine($"转字典报错: {ex.Message}");  // 如果 Key 重复会报错
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}
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// 带值选择器的 ToDictionary（可以只取部分字段）
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var dictNameAndScore = students.ToDictionary(
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    s => s.Id,           // Key 选择器
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    s => new { s.Name, s.Score }  // Value 选择器
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);
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Console.WriteLine($"学生2: {dictNameAndScore[2].Name} - {dictNameAndScore[2].Score}分");