Linq 入门系列 [Take,Skip,TakeWhile,SkipWhile]篇

/**/ ///////////////////////////////////////////
/ Take,Skip,TakeWhile,SkipWhile的例子和解释 //
/**/ //////////////////////////////////////////

class  Program
     {
        DB#region DB

        private static List<Student> GetStudents()
        {
            List<Student> students = new List<Student> { 
             new Student{ Name="YOUNG", Age=25, Language="Chinese"},
             new Student{ Name="JESSIE", Age=21, Language="Scotland"},
             new Student{ Name="KELLY", Age=18, Language="English"},
             new Student{ Name="JUNE", Age=20, Language="Chinese"},
             new Student{ Name="ADRIAN", Age=22, Language="Italy"},
             new Student{ Name="BRUCE", Age=17, Language="Scotland"},
             new Student{ Name="BRANT", Age=30, Language="Germany"},
             new Student{ Name="BEN", Age=25, Language="Chinese"}
             };
            return students;
        }

        #endregion

        static void Main(string[] args)
        {
            List<Student> studentList = GetStudents();            
            
            //Take(studentList);

            //Skip(studentList);
        }

        Take#region Take

        public static void Take<T>(T obj)
        {
            //GetElementFromCollect();
            //GetElementFromSelectResult(obj);
            //GetPointElement(obj);

            //ForExampleAboutTakeWhileOrSkipWhile(obj);
        }

        private static void GetElementFromCollect()
        {
            Console.WriteLine("<hr>从集合中取固定数量的元素：<br>");
            string[] stringArray = { "Ajax", "Csharp", "Javascript", "Css Desige", "Asp.net", "Gis" };

            //Take将枚举出source中的前count个元素,返回给客户端.
            //Lambda的写法   
            //var books = stringArray.Take(3);

            //Linq的写法
            var books = (from n in stringArray
                        select n).Take(3);
            foreach (var book in books)
            {
                Console.WriteLine(string.Format("<div class='result'>{0}</div>", book));
            }

            //扩展方法写法
            //IEnumerator<String> stu_Lambda = stringArray.Take(3).GetEnumerator();

            //while (stu_Lambda.MoveNext())
            //{
            //    Console.WriteLine(stu_Lambda.Current);
            //}
        }

        private static void GetElementFromSelectResult<T>(T obj)
        {
            Console.WriteLine("<hr>从select结果中取固定数量的元素：<br>");

            //Lambda写法
            //var stu = (RevealChangeType(obj).Where(n=>n.Language == "Chinese").Select(n=>new { n.Name, n.Age })).Take(1);

            //Linq写法
            var stu = (from n in RevealChangeType(obj)
                       where n.Language == "Chinese"
                       select new {n.Name, n.Age}).Take(1);

            foreach (var student in stu)
            {
                Console.WriteLine(string.Format("<div class='result'> name:{0}; age:{1}</div>", student.Name, student.Age));
            }

            //扩展方法写法
            //IEnumerator<Student> stu_ExtraMethod = RevealChangeType(obj).Where(n=>n.Language == "Chinese").Take(1).GetEnumerator();

            //while (stu_ExtraMethod.MoveNext())
            //{
            //    Console.WriteLine(stu_ExtraMethod.Current.Age);
            //}
        }

        private static void GetPointElement<T>(T obj)
        {
            Console.WriteLine("<hr>取满足指定条件元素：<br>");

            //Lambda写法
            //var stu = RevealChangeType(obj).TakeWhile(student => student.Age > 20);

            //Linq写法
            var stu = (from n in RevealChangeType(obj)
                       select n).TakeWhile(n => n.Age > 20);

            foreach (var student in stu)
            {
                Console.WriteLine(string.Format("<div class='result'>{0}</div>", student.Name));
            }

            //扩展方法写法
            //IEnumerator<Student> stu_ExtraMethod = RevealChangeType(obj).TakeWhile(student => student.Age > 20).GetEnumerator();

            //while (stu_ExtraMethod.MoveNext())
            //{
            //    Console.WriteLine(stu_ExtraMethod.Current.Name);
            //}

            //TakeWhile在LINQ中实现的思想是:
            //对数据源进行枚举,从第一个枚举得到的元素开始-->
            //                 调用客户端传入的predicate( c.Name == ""woodyN")-->
            //                 如果这个predicate委托返回true的话,
            //                 则将该元素作为Current元素返回给客户端,并且,继续进行相同的枚举,判断操作.
            //                 但是,一旦predicate返回false的话,
            //                 MoveNext()方法将会返回false,枚举就此打住,忽略剩下的所有元素.
            //所以此方法出来的结果是：
            //<div class='result'>YOUNG</div>
            //<div class='result'>JESSIE</div>
            //因为在KELLYAge=18不符合当前where条件,所以会这样。：）
        }

        #endregion

        Skip#region Skip

        private static void Skip<T>(T obj)
        {
            //GetRemnantElementAfterJumpCollect();
            //GetRemnantElementAfterFilter(obj);
        }

        private static void GetRemnantElementAfterJumpCollect()
        {
            Console.WriteLine("<hr>跳过集合的几个元素后取剩余所有元素：<br>");
            string[] stringArray = { "Ajax", "Csharp", "Javascript", "Css Desige", "Asp.net", "Gis" };

            //Skip则恰好相反,将跳过source中的前count个元素,枚举其余元素.

            //Lambda写法
            //var books1 = stringArray.Skip(3);

            //Linq写法
            var books1 = (from n in stringArray
                         select n).Skip(3);

            foreach (var book in books1)
            {
                Console.WriteLine(string.Format("<div class='result'>{0}</div>", book));
            }

            //扩展方法写法
            //IEnumerator<string> books1_ExtraMethod = stringArray.Skip(3).GetEnumerator();

            //while (books1_ExtraMethod.MoveNext())
            //{
            //    Console.WriteLine(books1_ExtraMethod.Current);
            //}
        }

        private static void GetRemnantElementAfterFilter<T>(T obj)
        {
            Console.WriteLine("<hr>将满足指定条件元素过滤后选择剩余的所有元素：<br>");

            //Lambda写法
            //var stu = RevealChangeType(obj).SkipWhile(student => student.Age > 20);
             
            //Linq写法
            var stu = (from n in RevealChangeType(obj)
                      select n).SkipWhile(n => n.Age > 20);

            foreach (var student in stu)
            {
                Console.WriteLine(string.Format("<div class='result'>{0}</div>", student.Name));
            }

            //扩展方法写法
            //IEnumerator<Student> stu_ExtraMethod = RevealChangeType(obj).SkipWhile(student => student.Age > 20).GetEnumerator();

            //while (stu_ExtraMethod.MoveNext())
            //{
            //    Console.WriteLine(stu_ExtraMethod.Current);
            //}

            //类似的,SkipWhile也对数据源进行枚举,从第一个枚举得到的元素开始,
            //调用客户端的predicate,如果返回true,则跳过该元素,继续进行枚举操作.
            //但是,如果一旦predicate返回为false,则该元素以后的所有元素,都不会再调用predicate,
            //而全部枚举给客户端.
            //所以此方法出来的结果是：
            //<div class='result'>KELLY</div>
            //<div class='result'>JUNE</div>
            //<div class='result'>ADRIAN</div>
            //<div class='result'>BRUCE</div>
            //<div class='result'>BRANT</div>
            //<div class='result'>BEN</div>
            //以上根据这个结果对于linq中TakeWhile,SkipWhile有点认识了。
            //但是我还是建议实际使用时还是要小心使用。
        }

        #endregion

        TakeWhile--SkipWhile#region TakeWhile--SkipWhile

        private static void ForExampleAboutTakeWhileOrSkipWhile<T>(T obj)
        {
            var stu = RevealChangeType(obj).TakeWhile(student => student.Age > 20);
            var stu1 = RevealChangeType(obj).TakeWhile(student => student.Age > 18);
            var stu2 = RevealChangeType(obj).SkipWhile(student => student.Age > 20);
            var stu3 = RevealChangeType(obj).SkipWhile(student => student.Age > 18);

            foreach (var student in stu)
            {
                Console.WriteLine(string.Format("<div class='[TakeWhile(student => student.Age > 20)]result'>{0}</div>", student.Name));
            }
            foreach (var student in stu1)
            {
                Console.WriteLine(string.Format("<div class='[TakeWhile(student => student.Age > 18)]result'>{0}</div>", student.Name));
            }
            foreach (var student in stu2)
            {
                Console.WriteLine(string.Format("<div class='[SkipWhile(student => student.Age > 20)]result'>{0}</div>", student.Name));
            }
            foreach (var student in stu3)
            {
                Console.WriteLine(string.Format("<div class='[SkipWhile(student => student.Age > 18)]result'>{0}</div>", student.Name));
            }

            //Runtime Result
            //<div class='[TakeWhile(student => student.Age > 20)]result'>YOUNG</div>
            //<div class='[TakeWhile(student => student.Age > 20)]result'>JESSIE</div>

            //<div class='[TakeWhile(student => student.Age > 18)]result'>YOUNG</div>
            //<div class='[TakeWhile(student => student.Age > 18)]result'>JESSIE</div>

            //<div class='[SkipWhile(student => student.Age > 20)]result'>KELLY</div>
            //<div class='[SkipWhile(student => student.Age > 20)]result'>JUNE</div>
            //<div class='[SkipWhile(student => student.Age > 20)]result'>ADRIAN</div>
            //<div class='[SkipWhile(student => student.Age > 20)]result'>BRUCE</div>
            //<div class='[SkipWhile(student => student.Age > 20)]result'>BRANT</div>
            //<div class='[SkipWhile(student => student.Age > 20)]result'>BEN</div>

            //<div class='[SkipWhile(student => student.Age > 18)]result'>KELLY</div>
            //<div class='[SkipWhile(student => student.Age > 18)]result'>JUNE</div>
            //<div class='[SkipWhile(student => student.Age > 18)]result'>ADRIAN</div>
            //<div class='[SkipWhile(student => student.Age > 18)]result'>BRUCE</div>
            //<div class='[SkipWhile(student => student.Age > 18)]result'>BRANT</div>
            //<div class='[SkipWhile(student => student.Age > 18)]result'>BEN</div>
        }

        #endregion
    }

     sealed   class  Student
     {
        public int age;
        public int Age
        {
            get { return age; }
            set { age = value; }
        }

        private string name;
        public string Name
        {
            get { return name; }
            set { name = value; }
        }

        private string language;
        public string Language
        {
            get { return language; }
            set { language = value; }
        }
    }

        对于predicate的解释和实际用法，在我的blogs中--》 查找List,Array中元素的其他C#写法 中都有文字介绍和例子的演示。