linq中的分组操作符 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> GroupBy(this IEnumerable source, Func keySelector); public static IEnumerable> GroupBy(this IEnumerable source, Func keySelector, IEqualityComparer comparer);

从方法定义中可以看出：GroupBy的返回值类型是：IEnumerable>。其元素类型是IGrouping。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 listProduct = new List(){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：

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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 listProduct = new List(){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($"ProduceName:{subItem.Name},ProductPrice:{subItem.Price},PublishTime:{subItem.CreateTime}"); }}var listExpress = from p in listProductgroup p by new { p.CategoryId, p.Price } into r// 使用into把数据填充到局部变量r中，然后select筛选数据select new { key = r.Key, ListGroup = r.ToList() }; Console.WriteLine("查询表达式方式2输出："); foreach(var item in listExpress){Console.WriteLine("key:"+item.key); foreach (var subItem in item.ListGroup){Console.WriteLine($"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：