什么是Linq表達式？什么是Lambda表達式？
前一段時間用到這個只是，在網上也沒找到比較簡單明了的方法，今天就整理了一下相關知識，有空了再仔細研究研究

public Program() { List<Student> allStudent = new List<Student> { new Student("張三",23), new Student("李四",29), new Student("王二",25), new Student("趙六",26) }; //Ling表達式 var stus1 = from s in allStudent where s.Name == "王二" select new { s.Name, s.Age }; //Lanmbda表達式 var stus2 = allStudent.Where(t => t.Name == "王二").Select(t => new { t.Name, t.Age }); }public class Student { public string Name { set; get; } public int Age { set; get; } public Student(string name, int age) { this.Name = name; this.Age = age; } } Lambda確實比Linq表達式更加優雅 Linq表達式的select不能省略 //Linq var students1 = from t in db.Students where t.Name == "張三" select t; //Lambda var students2 = db.Students .Where(t => t.Name == "張三");Linq表達式必須需要括號包裹起來才能取結果集 //Linq var students1 = (from t in db.Students where t.Name == "張三" select t).ToList(); //Lambda var students2 = db.Students .Where(t => t.Name == "張三") .ToList();

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什么時候使用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：（本例適用于Linq to Object 和 沒有建主外鍵的EF查詢）
Lambda中的Join需要傳四個參數表達式，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 });

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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 });

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總覺得Linq更多的只是為了照顧那些寫慣了sql的程序員。

聯接查詢（內聯、左聯、交叉聯）

關于聯接查詢使用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代碼。（目的：可能有些人不會，同時在這里也給自己做個備忘）
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();

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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();

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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();

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

轉載于:https://www.cnblogs.com/chunhui212/p/5899158.html

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