linq中的分组操作符

分组是根据一个特定的值将序列中的元素进行分组。LINQ只包含一个分组操作符：GroupBy。GroupBy操作符类似于T-SQL语言中的Group By语句。来看看GroupBy的方法定义：

public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector);public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer);

从方法定义中可以看出：GroupBy的返回值类型是：IEnumerable<IGrouping<TKey, TSource>>。其元素类型是IGrouping<TKey, TSource>。TKey属性代表了分组时使用的关键值，TSource属性代表了分组之后的元素集合。遍历IGrouping<TKey, TSource>元素可以读取到每一个TSource类型。看下面的例子：

1、定义Product类，其定义如下：

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace GroupOperation{    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; }    }}

2、在Main()方法中调用

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.Threading.Tasks;namespace GroupOperation{    class Program    {        static void Main(string[] args)        {            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)}            };            // 查询表达式            var listExpress = from p in listProduct group p by p.CategoryId;            Console.WriteLine("输出查询表达式结果");            foreach (var item in listExpress)            {                Console.WriteLine($"CategoryId:{item.Key}");                foreach(var p in item)                {                    Console.WriteLine($"ProduceName:{p.Name},ProductPrice:{p.Price},PublishTime:{p.CreateTime}");                }            }            Console.WriteLine("***************************************");            // 查询方法            var listFun = listProduct.GroupBy(p => p.CategoryId);            Console.WriteLine("输出方法语法结果");            foreach (var item in listFun)            {                Console.WriteLine($"CategoryId:{item.Key}");                foreach (var p in item)                {                    Console.WriteLine($"ProduceName:{p.Name},ProductPrice:{p.Price},PublishTime:{p.CreateTime}");                }            }            Console.ReadKey();        }    }}

结果：

(string[] args) { 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)} }; // 查询表达式 var list = from p in listProduct group p by new { p.CategoryId, p.Price }; Console.WriteLine("查询表达式方式1输出："); foreach (var item in list) { Console.WriteLine("key:" + item.Key); foreach (var subItem in item) { Console.WriteLine($"ProduceName:{subItem.Name},ProductPrice:{subItem.Price},PublishTime:{subItem.CreateTime}"); } } var listExpress = from p in listProduct group p by new { p.CategoryId, p.Price } into r // 使用into把数据填充到局部变量r中，然后select筛选数据 select new { key = r.Key, ListGroup = r.ToList() }; Console.WriteLine("查询表达式方式2输出："); foreach(var item in listExpress) { Console.WriteLine("key:"+item.key); foreach (var subItem in item.ListGroup) { Console.WriteLine($"ProduceName:{subItem.Name},ProductPrice:{subItem.Price},PublishTime:{subItem.CreateTime}"); } } // 方法语法 var listFun = listProduct.GroupBy(p => new { p.CategoryId, p.Price }).Select(g => new { key = g.Key, ListGroup = g.ToList() }); Console.WriteLine("方法语法输出："); foreach (var item in listFun) { Console.WriteLine("key:" + item.key); foreach (var subItem in item.ListGroup) { Console.WriteLine($"ProduceName:{subItem.Name},ProductPrice:{subItem.Price},PublishTime:{subItem.CreateTime}"); } } Console.ReadKey(); } }}

结果：

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