c# 两个list的交集问题！

Posted 2023-05-16

我写了个方法。

public void newList (List<object> a,List<object> b)


List<object> c= new List<object>();

for (int i = 0; i < a.Count; i++)

if(a[i].Equals(b[i]))

c.Add(a[i]);



我不知道是否有别的高效率的方法可以解决此问题，知道的分享下，谢谢！

有简单写法:
public IEnumerable<T> newList (IEnumerable<T> a, IEnumerable<T> b)
foreach (T item in a)
if(b.Contains(item))
yield return item;


参考技术A public IEnumerable<T> newList<T> (IEnumerable<T> a, IEnumerable<T> b)
foreach (T item in a)
if(b.Contains(item))
yield return item;


参考技术B 回楼主，你写的这个方法是错的，当b的数量小于a数量的时候就会报错

C# Linq 交集并集差集去重

其实只要明白 LINQ查询操作符的Distinct、Union、Concat、Intersect、Except、Skip、Take、SkipWhile、TakeWhile、Single、SingleOrDefault、Reverse、SelectMany,Aggregate()的使用，一些简单的操作就可以了。 
合并两个数组,并去掉重复元素,然后排序(C#)

List<int> numbers1 = new List<int>() { 5, 4, 1, 3, 9, 8, 6, 7, 12, 10 };
List<int> numbers2 = new List<int>() { 15, 14, 11, 13, 19, 18, 16, 17, 12, 10 };
var newQuerty = numbers1.Concat(
from n in numbers2
where !numbers1.Contains(n)
select n
).OrderBy(n => n);
string count = "";
foreach (int i in newQuerty)
{
    count += i + ",";
}
MessageBox.Show(count);

在这简单的介绍几个关键字，Distinct、Union、Concat、Intersect、Except、Skip、Take 
Distinct - 过滤集合中的相同项；

List<int> list= new List<int>() {1,2,3,4,4,5,6,6 };
var newlist=list.Distinct();

得到的结果就是；1,2,3,4,5,6
Union - 连接不同集合，自动过滤相同项

List<int> list= new List<int>() {1,2,3,4,4,5,6,6 };
List<int> list1= new List<int>() {5,6,6,7,8,9};
var newlist=list.Union (list1);

得到的结果就是；1,2,3,4,5,6,7,8,9
Concat - 连接不同集合，不会自动过滤相同项；

List<int> list= new List<int>() {1,2,3,4,4,5,6,6 };
List<int> list1= new List<int>() {5,6,6,7,8,9};
var newlist=list.Union (list1);

得到的结果就是；1,2,3,4,4,5,6,6,5,6,6,7,8,9
Intersect - 获取不同集合的相同项（交集）；

List<int> list= new List<int>() {1,2,3,4,4,5,6,6 };
List<int> list1= new List<int>() {5,6,6,7,8,9};
var newlist=list.Intersect (list1);

得到的结果就是；5,6
Except - 从某集合中删除其与另一个集合中相同的项；其实这个说简单点就是某集合中独有的元素

List<int> list= new List<int>() {1,2,3,4,4,5,6,6 };
List<int> list1= new List<int>() {5,6,6,7,8,9};
var newlist=list.Except (list1);

得到的结果就是；1,2,3,4
Skip - 跳过集合的前n个元素；

List<int> list= new List<int>() {1,2,3,4,4,5,6,6 };
var newlist=list.Skip (3);

得到的结果就是；4,4,5,6,6
Take - 获取集合的前n个元素；延迟

List<int> list= new List<int>() {1,2,2,3,4,4,5,6,6 };
var newlist=list.Take (3);

得到的结果就是；1,2,2

List<string> ListA = new List<string>();
List<string> ListB = new List<string>();
List<string> ListResult = new List<string>();

ListResult = ListA.Distinct().ToList();//去重
ListResult = ListA.Except(ListB).ToList();//差集
ListResult= ListA.Union(ListB).ToList();  //并集
ListResult = ListA.Intersect(ListB).ToList();//交集

重写比较方法

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data; 
namespace test
{
    class Program
    {
        static void Main(string[] args)
        {
            IList<Student> oneStudents = new List<Student>();
            oneStudents.Add(new Student(1,false,"小新1","徐汇"));
            oneStudents.Add(new Student(2,false,"小新2","闵行"));
            oneStudents.Add(new Student(3, false, "小新3", "嘉定"));
            oneStudents.Add(new Student(4, false, "小新4", "闸北"));

            IList<Student> twoStudents = new List<Student>();
            twoStudents.Add(new Student(5, false, "小新5", "贵州"));
            twoStudents.Add(new Student(6, false, "小新6", "湖北"));
            twoStudents.Add(new Student(7, false, "小新7", "山东"));
            twoStudents.Add(new Student(8, false, "小新8", "西藏"));
            
            IList<Student> threeStudents = new List<Student>();
            threeStudents.Add(new Student(1, false, "小新1", "徐汇"));
            threeStudents.Add(new Student(2, false, "小新2", "闵行"));
            var bingji = oneStudents.Union(twoStudents, new StudentListEquality()).ToList();//并（全）集 
              var jiaoji = oneStudents.Intersect(threeStudents, new StudentListEquality()).ToList();//交集 
              var chaji = oneStudents.Except(threeStudents, new StudentListEquality()).ToList();//差集

              Console.WriteLine();
            Console.WriteLine("以下是并集的结果");            
            bingji.ForEach(x =>
            {
                Console.WriteLine(x.StudentId.ToString() + "    " + x.Sex.ToString() + "   " + x.Name.ToString()+" "+x.Address.ToString());

            });
            Console.WriteLine();
            Console.WriteLine("以下是交集的结果");           
            jiaoji.ForEach(x =>
            {
                Console.WriteLine(x.StudentId.ToString() + "    " + x.Sex.ToString() + "   " + x.Name.ToString() + " " + x.Address.ToString());

            });
            Console.WriteLine();
            Console.WriteLine("以下是差集的结果");            
            chaji.ForEach(x =>
            {
                Console.WriteLine(x.StudentId.ToString() + "    " + x.Sex.ToString() + "   " + x.Name.ToString() + " " + x.Address.ToString());

            });
        }

    }
    public class Student
    {
        public Student(int studentId, bool sex, String name, String address)
        {
            this.StudentId = studentId;
            this.Sex = sex;
            this.Name = name;
            this.Address = address;
        }
        public int StudentId { get; set; }
        public bool Sex { get; set; }
        public String Name { get; set; }
        public String Address { get; set; }

    }
    public class StudentListEquality : IEqualityComparer<Student>
    {
        public bool Equals(Student x, Student y)
        {
            return x.StudentId == y.StudentId;
        }

        public int GetHashCode(Student obj)
        {
            if (obj == null)
            {
                return 0;
            }
            else
            {
                return obj.ToString().GetHashCode();
            }
        }
    }
}