C# 兩個集合比較差值 Linq的Except的用法

值類型的集合比較差值

    List<string> strList1 = new List<string>(){"a", "b", "c", "d"};
    List<string> strList2 = new List<string>() { "a", "b", "f", "e"};

    var strList3 = strList1.Except(strList2).ToList();
    for (int i = 0; i < strList3.Count; i++)
    {
        Console.WriteLine(strList3[i]);
    }

輸出的結果是 c d

var strList3 = strList1.Except(strList2).ToList();

這里的意思是strList1中哪些是strList2中沒有的,並將獲得的差值存放在strList3 (即:strList1中有,strList2中沒有)

var strList3 = strList2.Except(strList1).ToList();

這樣輸出的結果就是 f e

這里將strList1與strList2位置對調下,就是strList2中哪些是strList1中沒有的存放在strList3 (即:strList2中有,strList1中沒有)

引用類型的集合比較差值

首先創建Student類

    public class Student
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

然后創建2個List 泛型 並取差值

    List<Student> studentList1=new List<Student>()
    {
        new Student(){Id = 1,Name = "小明"},
        new Student(){Id = 2,Name = "小剛"},
        new Student(){Id = 3,Name = "小紅"},
    };

    List<Student> studentList2 = new List<Student>()
    {
        new Student(){Id = 1,Name = "小明"}
    };

    var studentList3 = studentList1.Except(studentList2).ToList();
    for (int i = 0; i < studentList3.Count; i++)
    {
        Console.WriteLine($"學號: {studentList3[i].Id} 姓名: {studentList3[i].Name}");
    }

結果輸出

學號: 1 姓名: 小明
學號: 2 姓名: 小剛
學號: 3 姓名: 小紅

這是因為Except通過使用默認的相等比較器對值進行比較，生成兩個序列的差集,需要重寫 Equals和GetHashCode 方法

如下:

1. 方法一

    public class Student
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public override bool Equals(object obj)
        {
            if (obj is Student)
            {
                Student student = obj as Student;
                return Id == student.Id && Name == student.Name;
            }
            return false;
        }
        public override int GetHashCode()
        {
            return Id.GetHashCode() ^ Name.GetHashCode();
        }
    }

2. 方法二重寫一個類讓它繼承IEqualityComparer

    public class StudentComparer: IEqualityComparer<Student>
    {
        public bool Equals(Student x, Student y)
        {
            return x.Id == y.Id && x.Name == y.Name;
        }

        public int GetHashCode(Student obj)
        {
            return obj.Id.GetHashCode() ^ obj.Name.GetHashCode();
        }
    }

***** 特別注意

var studentList3 = studentList1.Except(studentList2,new StudentComparer()).ToList();

這里需要加上參數 new StudentComparer()