LINQ 101——约束、投影、排序

什么是LINQ：LINQ 是一组 .NET Framework 扩展模块集合，内含语言集成查询、集合以及转换操作。它使用查询的本机语言语法来扩展 C# 和 Visual Basic，并提供利用这些功能的类库。

什么是LINQ 101：是学习LINQ的不错的资源（下载地址：http://pan.baidu.com/s/1ntE74NJ），当初我也是看LINQ101来熟悉LINQ的。现把我当时的学习笔记整理出来

Restriction （约束)
Projection （投影）
Ordering （排序）
Partitioning （分区）
Join
Aggregate(聚合)
Grouping（分组）
Set(集合)
Conversion（转换）
Element（元素操作）

本篇文章介绍前三个

一、 约束 ——where

var query = from item in [集合] where [item满足的条件] select item;

int[] numbers = { 5, 4, 3, 7, 9, -12 };
var query = from n in numbers
            where n % 3 == 0
            select n;
Console.WriteLine("被3整除的数：");
foreach (var num in numbers)
{
    Console.WriteLine(num);
}

多个Where 条件且筛选

var query = from item in sources
            where (字段1的条件1) || (字段1的条件2)
            where (字段2的条件1) || (字段2的条件2)
            select new Entity ....

如 
var query = from item in sources
            where (string.IsNullOrEmpty(txtSearch.Text.Trim())) || (item.Filed1.Contains(txtSearch.Text.Trim()))
            where (string.IsNullOrEmpty(txtFilter.Text.Trim())) || (item.Filed2.Contains(txtFilter.Text.Trim()))
            ...
            select new Entity ....

二、投影——select

例1：让数组中每个数都加1

int[] numbers = { 5, 4, 1, 3, 9, 8,-13 };

var numsPlusOne =
    from n in numbers
    select n + 1;

Console.WriteLine("整数+1:");
foreach (var num in numsPlusOne)
{
    Console.WriteLine(num);
}

例2：选取某类中一个属性

public class Config
{
    public Guid ID { get; set; }
    public string Name { get; set; }
    public string Remark { get; set; }

    Config() { }

    public static List<Config> GetConfigs()
    {
        return new List<Config> 
        { 
            new Config{ ID=Guid.NewGuid(), Name="XX1号",Remark="Remark1号"},
            new Config{ ID=Guid.NewGuid(), Name="XX2号",Remark="Remark2号"},
            new Config{ ID=Guid.NewGuid(), Name="XX3号",Remark="Remark3号"}
        };
    }
}


static void Linq3()
{
    var source = Config.GetConfigs();
    var query = from item in source
                select item.Name;
    foreach (var c in query)
    {
        Console.WriteLine(c);
    }
}

例3：数组下标转换

static void Linq4()
{
    int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
    string[] strings = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

    var textNums =
        from n in numbers
        select strings[n];

    Console.WriteLine("数组下标转换");
    foreach (var s in textNums)
    {
        Console.WriteLine(s);
    }
}

例3：匿名类型，大小写

static void Linq5()
{
    string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };
    var query = from word in words
                            select new { Upper = word.ToUpper(), Lower = word.ToLower() };
    foreach (var item in query)
    {
        Console.WriteLine("大写: {0} 小写: {1}",item.Upper,item.Lower);
    }
}

例4：将投影的属性放到一个类中

public class Dto
{
    public string Field1 { get; set; }
    public string Field2 { get; set; }
    public string Field3 { get; set; }
}

static void Linq6()
{
    var source = Config.GetConfigs();
    var query = from item in source
                select new Dto { Field1 = item.ID.ToString(), Field2 = item.Name, Field3 = item.Remark };
    foreach (var item in query)
    {
        Console.WriteLine("{0} {1} {2}", item.Field1, item.Field2, item.Field3);
    }
}

例5：筛选 加where条件参见 where 例1

例6：复合from子句（选取座机号码区号为021的客户信息）

public class Guest
{
    /// <summary>
    /// 姓名
    /// </summary>
    public string Name { get; set; }

    /// <summary>
    /// 年龄
    /// </summary>
    public int Age { get; set; }

    /// <summary>
    /// 电话表
    /// </summary>
    public List<string> TelTable { get; set; }
}


static void Linq8()
{
    var source = new List<Guest>
    {
        new Guest{ Name="张三",Age=21,TelTable=new List<string>{"010-123456","13000000"}},
        new Guest{ Name="李四",Age=22,TelTable=new List<string>{"021-123456","13100000"}},
        new Guest{ Name="王五",Age=23,TelTable=new List<string>{"022-123456","13200000"}}
    };
    // 找电话簿中是021区号的客户
    var query = from guest in source
                from tel in guest.TelTable
                where tel.Contains("021-")
                select guest;

    foreach (var guest in query)
    {
        Console.WriteLine("{0} 年龄：{1}", guest.Name, guest.Age);
        foreach (var tel in guest.TelTable) { Console.WriteLine("     电话：{0}",tel); }
    }
}

例7：多个 form 子句（交叉连接）

static void Linq7()
{
    int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
    int[] numbersB = { 1, 3, 5, 7, 8 };

    var pairs =
        from a in numbersA
        from b in numbersB
        where a < b
        select new { a, b };

    Console.WriteLine("交叉连接 a < b:");
    foreach (var pair in pairs)
    {
        Console.WriteLine("{0} 小于 {1}", pair.a, pair.b);
    }
}

三、排序

orderby 排序字段

orderby 排序字段 descending

Thenby(orderby 多个排序字段)

ThenByDescending  (orderby 多个排序字段 descending)

例1：orderby 排序字段

static void Linq9()
{
    string[] words = { "cherry", "apple", "blueberry" };

    var sortedWords =
        from w in words
        orderby w
        select w;

    Console.WriteLine("单词排序 order by 单个排序字段:");
    foreach (var w in sortedWords)
    {
        Console.WriteLine(w);
    }
}

例2：orderby 排序字段 descending

static void Linq10()
{
    string[] words = { "cherry", "apple", "blueberry" };

    var sortedWords =
        from w in words
        orderby w descending
        select w;

    Console.WriteLine("单词排序 order by 单个排序字段 从大到小:");
    foreach (var w in sortedWords)
    {
        Console.WriteLine(w);
    }
}

例3：Thenby(orderby 多个排序字段)

static void Linq11()
{
    string[] words = { "cherry", "appppp", "cherryo", "appppppple", "blueberry" };

    // 先按名称排，再按长度排序
    var sortedWords =
        from w in words
        orderby w,w.Length
        select w;

    Console.WriteLine("单词排序 ThenBy 多个排序字段");
    foreach (var w in sortedWords)
    {
        Console.WriteLine(w);
    }
}

例4：ThenByDescending  (orderby 多个排序字段 descending)

注意主要条件和次要的升降序不传递

static void Linq12()
        {
            string[] words = { "cherry", "appppp", "cherryo", "appppppple", "blueberry" };

            // 先按名称排，再按长度排序
            var sortedWords =
                from w in words
                orderby w descending, w.Length descending
                select w;

            Console.WriteLine("单词排序 ThenBy 多个排序字段");
            foreach (var w in sortedWords)
            {
                Console.WriteLine(w);
            }
        }

本文代码： http://pan.baidu.com/s/1dDnJ0tV