SelectMany in LINQ (Select many Operator in LINQ)

The SelectMany function can expand the returned list to list. The best way to understand this sentence is to pass an example:

 Public classperson{ Public stringFirstName;  Public stringLastName;  Public stringGender;  Public intanualsalary;  Publiclist<string>PhoneNumber;  Public StaticList<person>Getallperson () {List<Person> personinlist =NewList<person>()         {            NewPerson () {FirstName="Jonh", LastName="Liu", Gender="Male", Anualsalary=70000, PhoneNumber=Newlist<string>()                 {                     "0101235566",                     "12056878445"                 }            },            NewPerson () {FirstName="Sara", LastName="Guo", Gender="Famale", Anualsalary=100000, PhoneNumber=Newlist<string>()                 {                     "13356985469",                     "15536215459"                 }            },            NewPerson () {FirstName="Marttin", LastName="Zeng", Gender="Male", Anualsalary=80000, PhoneNumber=Newlist<string>()                 {                     "15862354569",                     "13756542698",                     "15936545269"                 }            },            NewPerson () {FirstName="Sccot", LastName="Jiang", Gender="Male", Anualsalary=90000, PhoneNumber=Newlist<string>()                 {                     "02388695645"                 }            }        }; returnpersoninlist; }}

In the code above, the person class has a property named PhoneNumber with the list<string> type. If we're going to get all the Phonenumer for the list<person> of sex as "male" we can use the SELECT statement:

List<person> personinlist =Person.getallperson (); IEnumerable<List<string>> marttinsphonenumberlist = personinlist.where (p = P.gender = ="Male"). Select (p =p.phonenumber); foreach(list<string> eachphonenumberlistinchmarttinsphonenumberlist) {     foreach(stringEachphonenumberincheachphonenumberlist)     {Console.WriteLine (eachphonenumber); } Console.WriteLine (New string('-', -)); }

As you can see, the result we filter with the Select function is the ienumerable<list<string>> type. This is well understood, because PhoneNumber is inherently a list<string> type in the person class. But what happens if we use the SelectMany function? Take a look at the following code:

list<person> personinlist = Person.getallperson () IEnumerable<string"Male "). SelectMany (p = p.phonenumber); foreach (string in allphonenumberinlist) {    Console.WriteLine (eachphonenumber);}

In fact, we must use ienumerable<string> to receive the return value of SelectMany! Because SelectMany will further expand the returned type of list<string> PhoneNumber to the ienumerable<string> type. This also explains the meaning of the "SelectMany function can expand the returned list to list" At the beginning of the article. Give another example if we run the following code:

list<person> personinlist = Person.getallperson () IEnumerable<char"Sccot "). SelectMany (p = p.lastname); foreach (char in allcharoflastname) {    Console.WriteLine (Eachchar);}

The above code first selects the person with the FirstName as "Sccot" and returns its LastName with SelectMany. Note that the result of the LastName type is String,selectmany is to expand the string to a list of char types.

The SelectMany also has additional overloads. For example, if we want to display fullname and corresponding phonenumber at the same time, we can use the following code to achieve the goal:

List<person> personinlist =Person.getallperson ();varresult = Personinlist.where (p = = P.gender = ="Male"). SelectMany (P = p.phonenumber, (person, phonenumber) =New{FullName = person. FirstName +" "+ person. LastName, PhoneNumber =PhoneNumber});foreach(varIteminchresult) {Console.WriteLine (string. Format ("Fullname:{0}\r\nphonenumber:{1}", item. FullName, item. PhoneNumber));}

Reference: Http://stackoverflow.com/questions/958949/difference-between-select-and-selectmany

SelectMany in LINQ (Select many Operator in LINQ)