• LINQ 101——分区、Join、聚合


    一、Partitioning 分区

    Take

    例1:取前3个数

     1 static void Linq1()
     2 {
     3     int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
     4     var first3Numbers = numbers.Take(3);
     5     Console.WriteLine("前3个数:");
     6     foreach (var n in first3Numbers)
     7     {
     8         Console.WriteLine(n);
     9     }
    10 }
    View Code

    Skip

    例2:跳过前3个数

     1 static void Linq2()
     2 {
     3     int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
     4     var skip3Numbers = numbers.Skip(3);
     5     Console.WriteLine("除去前3个数后数字:");
     6     foreach (var n in skip3Numbers)
     7     {
     8         Console.WriteLine(n);
     9     }
    10 }
    View Code

    Take + Skip 实现分页

    1 int currentPageIndex;  // 当前页号
    2 int pageSize;      // 每页有多少条记录
    3 source 是总数据源集合
    4 
    5 则用LINQ分页的代码为
    6 
    7 var query = source.Skip((currentPageIndex-1) * pageSize).Take(pageSize);
    View Code

    二、Join

    例1: Cross Join

     1 static void Linq3()
     2 {
     3     string[] cates = {"酒类","烟类","肉类" };
     4     var products = Product.GetDefaultData();
     5 
     6     // CategoryName 没有与 cate 匹配的将被剔除
     7     var query = from c in cates
     8                 join p in products on c equals p.CategoryName
     9                 select new { ShowName = c + " : " + p.ProductName };
    10 
    11     foreach (var item in query)
    12     {
    13         Console.WriteLine(item.ShowName);
    14     }
    15 }
    16 
    17 
    18 public class Product
    19 {
    20     public string CategoryName { get; set; }
    21     public string ProductName { get; set; }
    22 
    23     public static List<Product> GetDefaultData()
    24     {
    25         return new List<Product>
    26         {
    27             new Product{ CategoryName="烟类", ProductName="中华"},
    28             new Product{ CategoryName="酒类", ProductName="白酒"},
    29             new Product{ CategoryName="酒类", ProductName="红酒"},
    30             new Product{ CategoryName="肉类", ProductName="猪肉"},
    31             new Product{ CategoryName="肉类", ProductName="牛肉"},
    32             new Product{ CategoryName="零食", ProductName="饼干"},
    33             new Product{ ProductName="暂未分类产品1"},
    34             new Product{ ProductName="暂未分类产品2"},
    35         };
    36     }
    37 }
    View Code

    例2: Group Join

     1 static void Linq4()
     2 {
     3     string[] cates = { "酒类", "烟类", "肉类" };
     4     var products = Product.GetDefaultData();
     5 
     6     var query = from c in cates
     7                 join p in products on c equals p.CategoryName
     8                 into tgroup
     9                 select new { Name = c, Group = tgroup };
    10 
    11     foreach (var item in query)
    12     {
    13         Console.Write("{0}: ",item.Name);
    14         foreach (var g in item.Group)
    15         {
    16             Console.Write("{0} ",g.ProductName);
    17         }
    18         Console.WriteLine();
    19     }
    20 }
    View Code

    例3:Cross Join + Group Join

    分组后再查询符合条件的

     1 static void Linq5()
     2 {
     3     string[] cates = { "酒类", "烟类", "肉类" };
     4     var products = Product.GetDefaultData();
     5 
     6     var query = from c in cates
     7                 join p in products on c equals p.CategoryName
     8                 into tgroup
     9                 from g in tgroup
    10                 where g.ProductName.Contains("牛腩")  // 仅要产品名字含有牛腩的分组
    11                 select new { Name = c, Group = tgroup };
    12 
    13     foreach (var item in query)
    14     {
    15         Console.Write("{0}: ", item.Name);
    16         foreach (var g in item.Group)
    17         {
    18             Console.Write("{0} ", g.ProductName);
    19         }
    20         Console.WriteLine();
    21     }
    22 }
    View Code

    例4 Left Outer Join

     1 static void Linq6()
     2 {
     3     string[] cates = { "酒类", "烟类", "肉类" ,"调料类"};
     4     var products = Product.GetDefaultData();
     5 
     6     var query = from c in cates
     7                 join p in products on c equals p.CategoryName
     8                 into tgroup
     9                 from g in tgroup.DefaultIfEmpty()
    10                 select new { Name = c, ProductName = g==null ? "该分组下没有内容":g.ProductName };
    11             
    12     foreach (var item in query)
    13     {
    14         Console.WriteLine("{0} {1}",item.Name,item.ProductName);
    15     }
    16 }
    View Code

    三、 聚合

    Count();

    Count(n => 条件); 后面都类似

    Sum();

    Min()

    Max()

    Average()

    本文代码:http://files.cnblogs.com/Aphasia/ConsoleApplication2.rar

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  • 原文地址:https://www.cnblogs.com/Aphasia/p/4145351.html
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