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：
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=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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