标签：cat 高级 write set source new ext reading 表达
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：
1 public static IEnumerable<IGrouping<TKey, TSource>> GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector); 2 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：
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace GroupOperation 8 { 9 public class Product 10 { 11 public int Id { get; set; } 12 public int CategoryId { get; set; } 13 public string Name { get; set; } 14 public double Price { get; set; } 15 public DateTime CreateTime { get; set; } 16 } 17 }
2、在Main()方法中调用
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace GroupOperation 8 { 9 class Program 10 { 11 static void Main(string[] args) 12 { 13 List<Product> listProduct = new List<Product>() 14 { 15 new Product(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now}, 16 new Product(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)}, 17 new Product(){Id=3,CategoryId=2, Name="活着", Price=57,CreateTime=DateTime.Now.AddMonths(-3)}, 18 new Product(){Id=4,CategoryId=3, Name="高等数学", Price=97,CreateTime=DateTime.Now.AddMonths(-1)}, 19 new Product(){Id=5,CategoryId=6, Name="国家宝藏", Price=52.8,CreateTime=DateTime.Now.AddMonths(-1)} 20 }; 21 22 // 查询表达式 23 var listExpress = from p in listProduct group p by p.CategoryId; 24 Console.WriteLine("输出查询表达式结果"); 25 foreach (var item in listExpress) 26 { 27 Console.WriteLine($"CategoryId:{item.Key}"); 28 foreach(var p in item) 29 { 30 Console.WriteLine($"ProduceName:{p.Name},ProductPrice:{p.Price},PublishTime:{p.CreateTime}"); 31 } 32 } 33 Console.WriteLine("***************************************"); 34 // 查询方法 35 var listFun = listProduct.GroupBy(p => p.CategoryId); 36 Console.WriteLine("输出方法语法结果"); 37 foreach (var item in listFun) 38 { 39 Console.WriteLine($"CategoryId:{item.Key}"); 40 foreach (var p in item) 41 { 42 Console.WriteLine($"ProduceName:{p.Name},ProductPrice:{p.Price},PublishTime:{p.CreateTime}"); 43 } 44 } 45 Console.ReadKey(); 46 } 47 } 48 }
结果：
下面在来看看多个分组条件的例子。
1 using System; 2 using System.Collections.Generic; 3 using System.Linq; 4 using System.Text; 5 using System.Threading.Tasks; 6 7 namespace GroupOperation 8 { 9 class Program 10 { 11 static void Main(string[] args) 12 { 13 List<Product> listProduct = new List<Product>() 14 { 15 new Product(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now}, 16 new Product(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)}, 17 new Product(){Id=3,CategoryId=2, Name="活着", Price=57,CreateTime=DateTime.Now.AddMonths(-3)}, 18 new Product(){Id=4,CategoryId=3, Name="高等数学", Price=97,CreateTime=DateTime.Now.AddMonths(-1)}, 19 new Product(){Id=5,CategoryId=6, Name="国家宝藏", Price=52.8,CreateTime=DateTime.Now.AddMonths(-1)} 20 }; 21 22 // 查询表达式 23 var list = from p in listProduct group p by new { p.CategoryId, p.Price }; 24 Console.WriteLine("查询表达式方式1输出："); 25 foreach (var item in list) 26 { 27 Console.WriteLine("key:" + item.Key); 28 foreach (var subItem in item) 29 { 30 Console.WriteLine($"ProduceName:{subItem.Name},ProductPrice:{subItem.Price},PublishTime:{subItem.CreateTime}"); 31 } 32 } 33 var listExpress = from p in listProduct 34 group p by new { p.CategoryId, p.Price } into r // 使用into把数据填充到局部变量r中，然后select筛选数据 35 select new { key = r.Key, ListGroup = r.ToList() }; 36 Console.WriteLine("查询表达式方式2输出："); 37 foreach(var item in listExpress) 38 { 39 Console.WriteLine("key:"+item.key); 40 foreach (var subItem in item.ListGroup) 41 { 42 Console.WriteLine($"ProduceName:{subItem.Name},ProductPrice:{subItem.Price},PublishTime:{subItem.CreateTime}"); 43 } 44 } 45 46 // 方法语法 47 var listFun = listProduct.GroupBy(p => new { p.CategoryId, p.Price }).Select(g => new { key = g.Key, ListGroup = g.ToList() }); 48 Console.WriteLine("方法语法输出："); 49 foreach (var item in listFun) 50 { 51 Console.WriteLine("key:" + item.key); 52 foreach (var subItem in item.ListGroup) 53 { 54 Console.WriteLine($"ProduceName:{subItem.Name},ProductPrice:{subItem.Price},PublishTime:{subItem.CreateTime}"); 55 } 56 } 57 58 Console.ReadKey(); 59 } 60 } 61 }
结果：
标签：cat 高级 write set source new ext reading 表达
原文地址：https://www.cnblogs.com/dotnet261010/p/9308911.html
