C#高级：利用LINQ进行实体列表的集合运算

问题引入： 

Teacher实体的唯一标识符是Name和Classes字段（或者说这两个字段唯一确定一条数据），如何对两个实体列表做交集、差集运算呢？（并集直接调用AddRange方法即可）

一、重写方法实现

1.原理

        重写 Equals() 和 GetHashCode() 方法，使用Intersect() 和 Except() 方法在比较时就会基于这些字段来判断对象是否相等

2.快速生成代码

3.代码

public class Teacher
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Classes { get; set; }

    public override bool Equals(object obj)
    {
        return obj is Teacher teacher &&
               Name == teacher.Name &&
               Classes == teacher.Classes;
    }

    public override int GetHashCode()
    {
        return HashCode.Combine(Name, Classes);
    }
}

class Program
{
    static void Main()
    {
        // 创建两个示例列表
        List<Teacher> listA = new List<Teacher>
        {
            new Teacher { Id = 1, Name = "Alice", Classes = "Math" },
            new Teacher { Id = 2, Name = "Bob", Classes = "Science" },
            new Teacher { Id = 3, Name = "Charlie", Classes = "English" }
        };

        List<Teacher> listB = new List<Teacher>
        {
            new Teacher { Id = 2, Name = "Bob", Classes = "Science" },
            new Teacher { Id = 3, Name = "Charlie", Classes = "English" },
            new Teacher { Id = 4, Name = "David", Classes = "History" }
        };

        // 求交集
        var intersection = listA.Intersect(listB).ToList();

        // 求差集 A - B
        var difference = listA.Except(listB).ToList();


        // 输出结果
        Console.WriteLine("交集:");
        foreach (var teacher in intersection)
        {
            Console.WriteLine($"Id: {teacher.Id}, Name: {teacher.Name}, Classes: {teacher.Classes}");
        }

        Console.WriteLine("\n差集 A - B:");
        foreach (var teacher in difference)
        {
            Console.WriteLine($"Id: {teacher.Id}, Name: {teacher.Name}, Classes: {teacher.Classes}");
        }
    }
}

二、嵌套查询实现

1.原理

var intersection = listA.Where(a => listB.Any(b => b.Name == a.Name && b.Classes == a.Classes)).ToList();
var difference = listA.Where(a => !listB.Any(b => b.Name == a.Name && b.Classes == a.Classes)).ToList();

        where-Any相当于A表内连接B表( A ∩ B )；

        where-!Any相当于A表左外连接B表并剔除匹配到的数据（A - B）。

2.代码

public class Teacher
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Classes { get; set; }
}

class Program
{
    static void Main()
    {
        // 创建两个示例列表
        List<Teacher> listA = new List<Teacher>
        {
            new Teacher { Id = 1, Name = "Alice", Classes = "Math" },
            new Teacher { Id = 2, Name = "Bob", Classes = "Science" },
            new Teacher { Id = 3, Name = "Charlie", Classes = "English" }
        };

        List<Teacher> listB = new List<Teacher>
        {
            new Teacher { Id = 2, Name = "Bob", Classes = "Science" },
            new Teacher { Id = 3, Name = "Charlie", Classes = "English" },
            new Teacher { Id = 4, Name = "David", Classes = "History" }
        };

        // 自定义方法来比较 Name 和 Classes
        var intersection = listA.Where(a => listB.Any(b => b.Name == a.Name && b.Classes == a.Classes)).ToList();
        var difference = listA.Where(a => !listB.Any(b => b.Name == a.Name && b.Classes == a.Classes)).ToList();

        // 输出交集
        Console.WriteLine("交集:");
        foreach (var teacher in intersection)
        {
            Console.WriteLine($"Id: {teacher.Id}, Name: {teacher.Name}, Classes: {teacher.Classes}");
        }

        // 输出差集
        Console.WriteLine("\n差集 A - B:");
        foreach (var teacher in difference)
        {
            Console.WriteLine($"Id: {teacher.Id}, Name: {teacher.Name}, Classes: {teacher.Classes}");
        }
    }
}