Cancat operator in LINQ (9)

The connection operators in LINQ mainly include Join () and GroupJoin ().

First, the Join () operator

 The Join () operator is very similar to the inner join in T-SQL, it connects the two data sources and matches based on the equal values ​​in the two data sources

Sample Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConnectOperation
{
    public class Category
    {
        public int Id { get; set; }
        public string CategoryName { get; set; }
        public DateTime CreateTime { get; set; }
    }

    public class Product
    {
        public int Id { get; set; }
        public int CategoryId { get; set; }
        public string Name { get; set; }
        public double Price { get; set; }
        public DateTime CreateTime { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            // 初始化数据
            List<Category> listCategory = new List<Category>()
            {
              new Category(){ Id=1,CategoryName="计算机",CreateTime=DateTime.Now.AddYears(-1)},
              new Category(){ Id=2,CategoryName="文学",CreateTime=DateTime.Now.AddYears(-2)},
              new Category(){ Id=3,CategoryName="高校教材",CreateTime=DateTime.Now.AddMonths(-34)},
              new Category(){ Id=4,CategoryName="心理学",CreateTime=DateTime.Now.AddMonths(-34)}
            };
            List<Product> listProduct = new List<Product>()
            {
               new Product(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},
               new Product(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},
               new Product(){Id=3,CategoryId=2, Name="活着", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},
               new Product(){Id=4,CategoryId=3, Name="高等数学", Price=97,CreateTime=DateTime.Now.AddMonths(-1)},
               new Product(){Id=5,CategoryId=6, Name="国家宝藏", Price=52.8 , CreateTime = DateTime.Now.AddMonths ( -1 )) 
            }; 

            // 1, query expression 
            var queryExpress = from c in listCategory 
                               join p in listProduct on c.Id equals p.CategoryId
                                select  new {Id = c.Id , CategoryName = c.CategoryName, ProductName = p.Name, PublishTime = p.CreateTime}; 
            Console.WriteLine ( " Query expression output: " );
             foreach ( var item in  queryExpress)
            {
                Console.WriteLine($"id:{item.Id},CategoryName:{item.CategoryName},ProductName:{item.ProductName},PublishTime:{item.PublishTime}");
            }

            Console.WriteLine("方法语法输出:");
            // 2、方法语法
            var queryFun = listCategory.Join(listProduct, c => c.Id, p => p.CategoryId, (c, p) => new { Id = c.Id, CategoryName = c.CategoryName, ProductName = p.Name, PublishTime = p.CreateTime });
            foreach (var item in queryFun)
            {
                Console.WriteLine($"id:{item.Id},CategoryName:{item.CategoryName},ProductName:{item.ProductName},PublishTime:{item.PublishTime}");
            }

            Console.ReadKey();
        }
    }
}

View Code