C#语法中的select

原文地址为： C#语法中的select

1、简单的例子

代码

class  SelectSample1
{   
     static   void  Main()
    {           
         // Create the data source
        List < int >  Scores  =   new  List < int > () {  97 ,  92 ,  81 ,  60  };

         //  Create the query.
        IEnumerable < int >  queryHighScores  =
            from score  in  Scores
             where  score  >   80
            select score;

         //  Execute the query.
         foreach  ( int  i  in  queryHighScores)
        {
            Console.Write(i  +   "   " );
        }            
    }
}

2、集合序列化：

代码

         private  XElement SerializeDesignerItems(IEnumerable < DesignerItem >  designerItems)
        {
            XElement serializedItems  =   new  XElement( " DesignerItems " ,
                                       from item  in  designerItems
                                       let contentXaml  =  XamlWriter.Save(((DesignerItem)item).Content)
                                       select  new  XElement( " DesignerItem " ,
                                                   new  XElement( " Left " , Canvas.GetLeft(item)),
                                                   new  XElement( " Top " , Canvas.GetTop(item)),
                                                   new  XElement( " Width " , item.Width),
                                                   new  XElement( " Height " , item.Height),
                                                   new  XElement( " ID " , item.ID),
                                                   new  XElement( " zIndex " , Canvas.GetZIndex(item)),
                                                   new  XElement( " IsGroup " , item.IsGroup),
                                                   new  XElement( " ParentID " , item.ParentID),
                                                   new  XElement( " Content " , contentXaml)
                                              )
                                   );

             return  serializedItems;
        }

3、下面的示例演示了 select 子句可能采用的所有不同形式。在每个查询中，请注意 select 子句和查询变量（studentQuery1、studentQuery2 等）的类型之间的关系。

代码

     class  SelectSample2
    {
         //  Define some classes
         public   class  Student
        {
             public   string  First {  get ;  set ; }
             public   string  Last {  get ;  set ; }
             public   int  ID {  get ;  set ; }
             public  List < int >  Scores;
             public  ContactInfo GetContactInfo(SelectSample2 app,  int  id)
            {
                ContactInfo cInfo  =
                    (from ci  in  app.contactList
                     where  ci.ID  ==  id
                    select ci)
                    .FirstOrDefault();

                 return  cInfo;
            }

             public   override   string  ToString()
            {
                 return  First  +   "   "   +  Last  +   " : "   +  ID;
            }
        }

         public   class  ContactInfo
        {
             public   int  ID {  get ;  set ; }
             public   string  Email {  get ;  set ; }
             public   string  Phone {  get ;  set ; }
             public   override   string  ToString() {  return  Email  +   " , "   +  Phone; }
        }

         public   class  ScoreInfo
        {
             public   double  Average {  get ;  set ; }
             public   int  ID {  get ;  set ; }
        }

         //  The primary data source
        List < Student >  students  =   new  List < Student > ()
        {
              new  Student {First = " Svetlana " , Last = " Omelchenko " , ID = 111 , Scores =   new  List < int > () { 97 ,  92 ,  81 ,  60 }},
              new  Student {First = " Claire " , Last = " O'Donnell " , ID = 112 , Scores =   new  List < int > () { 75 ,  84 ,  91 ,  39 }},
              new  Student {First = " Sven " , Last = " Mortensen " , ID = 113 , Scores =   new  List < int > () { 88 ,  94 ,  65 ,  91 }},
              new  Student {First = " Cesar " , Last = " Garcia " , ID = 114 , Scores =   new  List < int > () { 97 ,  89 ,  85 ,  82 }},
        };

         //  Separate data source for contact info.
        List < ContactInfo >  contactList  =   new  List < ContactInfo > ()
        {
             new  ContactInfo {ID = 111 , Email = " [email protected] " , Phone = " 206-555-0108 " },
             new  ContactInfo {ID = 112 , Email = " [email protected] " , Phone = " 206-555-0298 " },
             new  ContactInfo {ID = 113 , Email = " [email protected] " , Phone = " 206-555-1130 " },
             new  ContactInfo {ID = 114 , Email = " [email protected] " , Phone = " 206-555-0521 " }
        };


         static   void  Main( string [] args)
        {
            SelectSample2 app  =   new  SelectSample2();

             //  Produce a filtered sequence of unmodified Students.
            IEnumerable < Student >  studentQuery1  =
                from student  in  app.students
                 where  student.ID  >   111
                select student;

            Console.WriteLine( " Query1: select range_variable " );
             foreach  (Student s  in  studentQuery1)
            {
                Console.WriteLine(s.ToString());
            }

             //  Produce a filtered sequence of elements that contain
             //  only one property of each Student.
            IEnumerable < String >  studentQuery2  =
                from student  in  app.students
                 where  student.ID  >   111
                select student.Last;

            Console.WriteLine( " \r\n studentQuery2: select range_variable.Property " );
             foreach  ( string  s  in  studentQuery2)
            {
                Console.WriteLine(s);
            }

             //  Produce a filtered sequence of objects created by
             //  a method call on each Student.
            IEnumerable < ContactInfo >  studentQuery3  =
                from student  in  app.students
                 where  student.ID  >   111
                select student.GetContactInfo(app, student.ID);

            Console.WriteLine( " \r\n studentQuery3: select range_variable.Method " );
             foreach  (ContactInfo ci  in  studentQuery3)
            {
                Console.WriteLine(ci.ToString());
            }

             //  Produce a filtered sequence of ints from
             //  the internal array inside each Student.
            IEnumerable < int >  studentQuery4  =
                from student  in  app.students
                 where  student.ID  >   111
                select student.Scores[ 0 ];

            Console.WriteLine( " \r\n studentQuery4: select range_variable[index] " );
             foreach  ( int  i  in  studentQuery4)
            {
                Console.WriteLine( " First score = {0} " , i);
            }

             //  Produce a filtered sequence of doubles 
             //  that are the result of an expression.
            IEnumerable < double >  studentQuery5  =
                from student  in  app.students
                 where  student.ID  >   111
                select student.Scores[ 0 ]  *   1.1 ;

            Console.WriteLine( " \r\n studentQuery5: select expression " );
             foreach  ( double  d  in  studentQuery5)
            {
                Console.WriteLine( " Adjusted first score = {0} " , d);
            }

             //  Produce a filtered sequence of doubles that are
             //  the result of a method call.
            IEnumerable < double >  studentQuery6  =
                from student  in  app.students
                 where  student.ID  >   111
                select student.Scores.Average();

            Console.WriteLine( " \r\n studentQuery6: select expression2 " );
             foreach  ( double  d  in  studentQuery6)
            {
                Console.WriteLine( " Average = {0} " , d);
            }

             //  Produce a filtered sequence of anonymous types
             //  that contain only two properties from each Student.
            var studentQuery7  =
                from student  in  app.students
                 where  student.ID  >   111
                select  new  { student.First, student.Last };

            Console.WriteLine( " \r\n studentQuery7: select new anonymous type " );
             foreach  (var item  in  studentQuery7)
            {
                Console.WriteLine( " {0}, {1} " , item.Last, item.First);
            }

             //  Produce a filtered sequence of named objects that contain
             //  a method return value and a property from each Student.
             //  Use named types if you need to pass the query variable 
             //  across a method boundary.
            IEnumerable < ScoreInfo >  studentQuery8  =
                from student  in  app.students
                 where  student.ID  >   111
                select  new  ScoreInfo
                {
                    Average  =  student.Scores.Average(),
                    ID  =  student.ID
                };

            Console.WriteLine( " \r\n studentQuery8: select new named type " );
             foreach  (ScoreInfo si  in  studentQuery8)
            {
                Console.WriteLine( " ID = {0}, Average = {1} " , si.ID, si.Average);
            }

             //  Produce a filtered sequence of students who appear on a contact list
             //  and whose average is greater than 85.
            IEnumerable < ContactInfo >  studentQuery9  =
                from student  in  app.students
                 where  student.Scores.Average()  >   85
                join ci  in  app.contactList on student.ID equals ci.ID
                select ci;

            Console.WriteLine( " \r\n studentQuery9: select result of join clause " );
             foreach  (ContactInfo ci  in  studentQuery9)
            {
                Console.WriteLine( " ID = {0}, Email = {1} " , ci.ID, ci.Email);
            }

             //  Keep the console window open in debug mode
            Console.WriteLine( " Press any key to exit. " );
            Console.ReadKey();
            }
        }
     /*  Output
        Query1: select range_variable
        Claire O'Donnell:112
        Sven Mortensen:113
        Cesar Garcia:114

        studentQuery2: select range_variable.Property
        O'Donnell
        Mortensen
        Garcia

        studentQuery3: select range_variable.Method
        [email protected],206-555-0298
        [email protected],206-555-1130
        [email protected],206-555-0521

        studentQuery4: select range_variable[index]
        First score = 75
        First score = 88
        First score = 97

        studentQuery5: select expression
        Adjusted first score = 82.5
        Adjusted first score = 96.8
        Adjusted first score = 106.7

        studentQuery6: select expression2
        Average = 72.25
        Average = 84.5
        Average = 88.25

        studentQuery7: select new anonymous type
        O'Donnell, Claire
        Mortensen, Sven
        Garcia, Cesar

        studentQuery8: select new named type
        ID = 112, Average = 72.25
        ID = 113, Average = 84.5
        ID = 114, Average = 88.25

        studentQuery9: select result of join clause
        ID = 114, Email = [email protected]
*/

（全文完）

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