linq操作符：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=,CategoryId=, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},
                new Product(){Id=,CategoryId=, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-)},
                new Product(){Id=,CategoryId=, Name="活着", Price=,CreateTime=DateTime.Now.AddMonths(-)},
                new Product(){Id=,CategoryId=, Name="高等数学", Price=,CreateTime=DateTime.Now.AddMonths(-)},
                new Product(){Id=,CategoryId=, Name="国家宝藏", Price=52.8,CreateTime=DateTime.Now.AddMonths(-)}
             };

             // 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：

下面在来看看多个分组条件的例子。

 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=,CategoryId=, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},
                new Product(){Id=,CategoryId=, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-)},
                new Product(){Id=,CategoryId=, Name="活着", Price=,CreateTime=DateTime.Now.AddMonths(-)},
                new Product(){Id=,CategoryId=, Name="高等数学", Price=,CreateTime=DateTime.Now.AddMonths(-)},
                new Product(){Id=,CategoryId=, Name="国家宝藏", Price=52.8,CreateTime=DateTime.Now.AddMonths(-)}
             };

             // 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：