linq中的分組操作符
分組是根據一個特定的值將序列中的元素進行分組。LINQ隻包含一個分組操作符：GroupBy。GroupBy操作符類似於T-SQL語言中的Group By語句。來看看GroupBy的方法定義：
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);
從方法定義中可以看出：GroupBy的返回值類型是：IEnumerable<IGrouping<TKey, TSource>>。其元素類型是IGrouping<TKey, TSource>。TKey屬性代表瞭分組時使用的關鍵值，TSource屬性代表瞭分組之後的元素集合。遍歷IGrouping<TKey, TSource>元素可以讀取到每一個TSource類型。看下面的例子：
1、定義Product類，其定義如下：
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()方法中調用
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#高級編程第10版", Price=100.67,CreateTime=DateTime.Now},
new Product(){Id=2,CategoryId=1, Name="Redis開發和運維", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},
new Product(){Id=3,CategoryId=2, Name="活著", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},
new Product(){Id=4,CategoryId=3, Name="高等數學", Price=97,CreateTime=DateTime.Now.AddMonths(-1)},
new Product(){Id=5,CategoryId=6, Name="國傢寶藏", Price=52.8,CreateTime=DateTime.Now.AddMonths(-1)}
};
// 查詢表達式
var listExpress = from p in listProduct group p by p.CategoryId;
Console.WriteLine("輸出查詢表達式結果");
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("***************************************");
// 查詢方法
var listFun = listProduct.GroupBy(p => p.CategoryId);
Console.WriteLine("輸出方法語法結果");
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();
}
}
}
結果：
下面在來看看多個分組條件的例子。
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#高級編程第10版", Price=100.67,CreateTime=DateTime.Now},
new Product(){Id=2,CategoryId=1, Name="Redis開發和運維", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},
new Product(){Id=3,CategoryId=2, Name="活著", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},
new Product(){Id=4,CategoryId=3, Name="高等數學", Price=97,CreateTime=DateTime.Now.AddMonths(-1)},
new Product(){Id=5,CategoryId=6, Name="國傢寶藏", Price=52.8,CreateTime=DateTime.Now.AddMonths(-1)}
};
// 查詢表達式
var list = from p in listProduct group p by new { p.CategoryId, p.Price };
Console.WriteLine("查詢表達式方式1輸出：");
foreach (var item in list)
{
Console.WriteLine("key:" + item.Key);
foreach (var subItem in item)
{
Console.WriteLine($"ProduceName:{subItem.Name},ProductPrice:{subItem.Price},PublishTime:{subItem.CreateTime}");
}
}
var listExpress = from p in listProduct
group p by new { p.CategoryId, p.Price } into r // 使用into把數據填充到局部變量r中，然後select篩選數據
select new { key = r.Key, ListGroup = r.ToList() };
Console.WriteLine("查詢表達式方式2輸出：");
foreach(var item in listExpress)
{
Console.WriteLine("key:"+item.key);
foreach (var subItem in item.ListGroup)
{
Console.WriteLine($"ProduceName:{subItem.Name},ProductPrice:{subItem.Price},PublishTime:{subItem.CreateTime}");
}
}
// 方法語法
var listFun = listProduct.GroupBy(p => new { p.CategoryId, p.Price }).Select(g => new { key = g.Key, ListGroup = g.ToList() });
Console.WriteLine("方法語法輸出：");
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();
}
}
}
結果：
以上就是本文的全部內容，希望對大傢的學習有所幫助，也希望大傢多多支持WalkonNet。