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SelectMany函数可以将返回的List展开为List。理解这句话的最好方式是通过一个例子:
public class Person { public string FirstName; public string LastName; public string Gender; public int AnualSalary; public List<string> PhoneNumber; public static List<Person> GetAllPerson() { List<Person> personInList = new List<Person>() { new Person() { FirstName = "Jonh", LastName = "Liu", Gender = "Male", AnualSalary = 70000, PhoneNumber = new List<string>() { "0101235566", "12056878445" } }, new Person() { FirstName = "Sara", LastName = "Guo", Gender = "Famale", AnualSalary = 100000, PhoneNumber = new List<string>() { "13356985469", "15536215459" } }, new Person() { FirstName="Marttin", LastName = "Zeng", Gender = "Male", AnualSalary = 80000, PhoneNumber = new List<string>() { "15862354569", "13756542698", "15936545269" } }, new Person() { FirstName="Sccot", LastName= "Jiang", Gender = "Male", AnualSalary = 90000, PhoneNumber = new List<string>() { "02388695645" } } }; return personInList; } }
上面的代码中，Person类中有一个List<string>类型的名为PhoneNumber的属性。如果我们要获取性别为“男”的List<Person>的所有PhoneNumer我们可以用Select语句：
List<Person> personInList = Person.GetAllPerson(); IEnumerable<List<string>> marttinsPhoneNumberList = personInList.Where(p => p.Gender == "Male").Select(p => p.PhoneNumber); foreach (List<string> eachPhoneNumberList in marttinsPhoneNumberList) { foreach (string eachPhoneNumber in eachPhoneNumberList) { Console.WriteLine(eachPhoneNumber); } Console.WriteLine(new string(‘-‘, 20)); }
可以看到，我们用Select函数筛选的结果是IEnumerable<List<string>>类型。这很好理解，因为PhoneNumber在Person类中本来就是一个List<string>类型。但如果我们用SelectMany函数会发生什么呢？请看下面的代码：
List<Person> personInList = Person.GetAllPerson(); IEnumerable<string> allPhoneNumberInList = personInList.Where(p => p.Gender == "Male").SelectMany(p => p.PhoneNumber); foreach (string eachPhoneNumber in allPhoneNumberInList) { Console.WriteLine(eachPhoneNumber); }
事实上，我们必须用IEnumerable<string>来接收SelectMany的返回值！因为SelectMany会将返回的类型为List<string>的PhoneNumber进一步展开为IEnumerable<string>类型。这也就解释了文章开头的“SelectMany函数可以将返回的List展开为List”的意思。再举一个例子，如果我们运行下面的代码:
List<Person> personInList = Person.GetAllPerson(); IEnumerable<char> allCharOfLastName = personInList.Where(p => p.FirstName == "Sccot").SelectMany(p => p.LastName); foreach (char eachChar in allCharOfLastName) { Console.WriteLine(eachChar); }
上面的代码首先选出了FirstName为"Sccot"的person，然后用SelectMany返回其LastName。注意LastName的类型为string，SelectMany返回的结果是把string展开为一个char类型的List了。
SelectMany还有另外的重载。例如，如果我们想同时显示FullName和对应的PhoneNumber的话，可以使用下面的代码达到目的:
List<Person> personInList = Person.GetAllPerson(); var result = personInList.Where(p => p.Gender == "Male").SelectMany(p => p.PhoneNumber, (person, phoneNumber) => new { FullName = person.FirstName + " " + person.LastName, PhoneNumber = phoneNumber }); foreach (var item in result) { Console.WriteLine(string.Format("FullName:{0}\r\nPhoneNumber:{1}", item.FullName, item.PhoneNumber)); }
参考:http://stackoverflow.com/questions/958949/difference-between-select-and-selectmany
08 LINQ中的SelectMany(Select Many Operator in LINQ)
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原文地址：http://www.cnblogs.com/kuillldan/p/5137045.html
