什麼是Linq表示式？什麼是Lambda表示式？
前一段時間用到這個只是，在網上也沒找到比較簡單明瞭的方法，今天就整理了一下相關知識，有空了再仔細研究研究

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication2
{
    public class Person
    {
        public int Age { get; set; }
        public string Name { get; set; }

    }
    class Program
    {
        public static List<Person> PersonsList()//方法返回Person類的List集合
        {
            List<Person> persons = new List<Person>();
            for (int i = 1; i < 8; i++)
            {
                //Person p = new Person() { Name = i + "人物年齡", Age = 8 - i, };
                Person p = new Person() { Name ="張"+i, Age = i, }; 
                persons.Add(p);
            }
            return persons;
        }

        static void Main(string[] args)
        {
            List<Person> persons = PersonsList();
            //lanbuda表示式
            //List<Person> persons1 = persons.Where(p => p.Age > 6).ToList();   //所有Age>6的Person的集合
            //Person per = persons.SingleOrDefault(p => p.Age == 1);  //Age=1的單個people類
            //List<Person> persons2 = persons.Where(p => p.Name.Contains("2")).ToList();   //所有Name包含年齡的Person的集合
            //foreach (Person p in persons2)
            //{
            //    Console.WriteLine(p.Name);
            //}

            //Linq表示式 與lanmuda 的對比
            //Linq
            var person01 = from t in persons
                           where t.Name.Contains("張") &&  t.Age==2
                           select t;
            //Lambda
            var person02 = persons.Where(t => t.Name == "張3");

            foreach (Person p in person02)
            {
                Console.WriteLine(p.Age);
            }
        }

    }
}



 

什麼時候使用Linq？
通過上面的對比，好像Linq一文不值了。no，不是這樣的。
比如下面幾種情況我們就可以選擇使用Linq：
例一：（本例適用於Linq to Object 和 沒有建主外來鍵的EF查詢）
Lambda中的Join需要傳四個引數表示式，是不是有點暈了。。。

var list1 = new Dictionary<string, string> { { "1", "張三" }, { "2", "李四" }, { "3", "張三" }, { "4", "張三" } };
var list2 = new Dictionary<string, string> { { "1", "張三" }, { "2", "李四" }, { "3", "李四" }, { "4", "張三" } };

//Linq
var obj1 = from l1 in list1
join l2 in list2
on l1.Key equals l2.Key
select new { l1, l2 };
//Lambda
var obj = list1.Join(list2, l1 => l1.Key, l2 => l2.Key, (l1, l2) => new { l1, l2 });
 

例二：
Lambda需要區分OrderBy、ThenBy有沒有覺得麻煩

//Linq
var obj1 = from l1 in list1
join l2 in list2
on l1.Key equals l2.Key
orderby l1.Key, l2.Key descending
select new { l1, l2 };
//Lambda
var obj = list1.Join(list2, l1 => l1.Key, l2 => l2.Key, (l1, l2) => new { l1, l2 })
.OrderBy(li => li.l1.Key)
.ThenByDescending(li => li.l2.Key)
.Select(t => new { t.l1, t.l2 });
 

總覺得Linq更多的只是為了照顧那些寫慣了sql的程式設計師。

聯接查詢（內聯、左聯、交叉聯）

關於聯接查詢使用Linq會更合適一些這個上面已經說了。
接下來我們寫內聯、左聯、交叉聯的Linq和對應的Lambda程式碼。（目的：可能有些人不會，同時在這裡也給自己做個備忘）
內聯：

var list1 = new Dictionary<string, string> { { "1", "張三" }, { "2", "李四" }, { "3", "張三" }, { "4", "張三" } };
var list2 = new Dictionary<string, string> { { "1", "張三" }, { "2", "李四" }, { "3", "李四" }, { "4", "張三" } };
//Linq查詢
var ojb2 = (from l1 in list1
join l2 in list2
on l1.Key equals l2.Key
select new { l1, l2 }).ToList();
//Lambda查詢
var obj = list1.Join(list2, l1 => l1.Key, l2 => l2.Key, (l1, l2) => new { l1, l2 }).ToList();

左聯：

var list1 = new Dictionary<string, string> { { "1", "張三" }, { "2", "李四" }, { "3", "張三" }, { "4", "張三" } };
var list2 = new Dictionary<string, string> { { "1", "張三" }, { "2", "李四" }, { "3", "李四" }, { "4", "張三" } };
//Linq查詢
var ojb2 = (from l1 in list1
join l2 in list2
on l1.Key equals l2.Key into list
from l2 in list.DefaultIfEmpty()
select new { l1, l2 }).ToList();
//Lambda查詢
var obj = list1.GroupJoin(list2, l1 => l1.Key, l2 => l2.Key, (l1, l2) => new { l1, l2=l2.FirstOrDefault() }).ToList();

交叉聯：

var list1 = new Dictionary<string, string> { { "1", "張三" }, { "2", "李四" }, { "3", "張三" }, { "4", "張三" } };
var list2 = new Dictionary<string, string> { { "1", "張三" }, { "2", "李四" }, { "3", "李四" }, { "4", "張三" } };
//Linq查詢
var ojb2 = (from l1 in list1
from l2 in list2
select new { l1, l2 }).ToList();
//Lambda查詢
var obj = list1.SelectMany(l1 => list2.Select(l2 => new { l1,l2})).ToList();

參考：FROM :http://www.cnblogs.com/zhaopei/p/5746414.html