LinQ Coding -- (4)[Concat应用]

Concat有两种应用：
1.Enumerable.Concat<(Of <(TSource>)>) 泛型方法应用
此方法是延迟执行的。此方法只返回输入序列中的所有原始元素
应用场景：只有通过直接调用对象的 GetEnumerator 方法来枚举该对象时，才执行此方法表示的查询。

IEnumerable<String> queryWithEnumerable = students.Select(student => student.City).Concat(teachers.Select(teacher => teacher.City));

            Console.WriteLine("Enumerable.Concat<(Of <(TSource>)>) 泛型方法应用");

            foreach (string city in queryWithEnumerable)
                Console.WriteLine(city);

2.QueryableConcat<(Of <(TSource>)>) 泛型方法应用
此方法生成 MethodCallExpression，其实方法本身即作为了构造的泛型方法。
然后，将 MethodCallExpression 传递给 IQueryProvider 的 CreateQuery方法，由 source1 参数的 Provider 属性表示。其实说白了，就是将 source2 中的元素连接到 source1 中的元素以组合成一个新的序列。

 IEnumerable<String> queryWithQueryable = students.AsQueryable().Select(student => student.City).Concat(teachers.Select(teacher => teacher.City));

            Console.WriteLine("QueryableConcat<(Of <(TSource>)>) 泛型方法应用");

            foreach (string city in queryWithEnumerable)
                Console.WriteLine(city);

以下是一个完整的DEMO

public class ChangeDataUsingLinq : Interface
    {
        Interface Members#region Interface Members

        public void invoke()
        {
            // Create the first data source.
            List<Student> students = new List<Student>()
        {
            new Student {First="Svetlana",
                Last="Omelchenko", 
                ID=111, 
                Street="123 Main Street",
                City="Seattle",
                Scores= new List<int> {97, 92, 81, 60}},
            new Student {First="Claire",
                Last="O’Donnell", 
                ID=112,
                Street="124 Main Street",
                City="Redmond",
                Scores= new List<int> {75, 84, 91, 39}},
            new Student {First="Sven",
                Last="Mortensen",
                ID=113,
                Street="125 Main Street",
                City="Lake City",
                Scores= new List<int> {88, 94, 65, 91}},
        };

            // Create the second data source.
            List<Teacher> teachers = new List<Teacher>()
        {                
            new Teacher {First="Ann", Last="Beebe", ID=945, City = "Seattle"},
            new Teacher {First="Alex", Last="Robinson", ID=956, City = "Redmond"},
            new Teacher {First="Michiyo", Last="Sato", ID=972, City = "Tacoma"}
        };
            //连接两个序列
            var peopleInSeattle = (from student in students
                                   where student.City == "Seattle"
                                   select student.Last).Concat(from teacher in teachers
                                                               where teacher.City == "Seattle"
                                                               select teacher.Last);
            Console.WriteLine("City == Seattle");

            foreach (var person in peopleInSeattle)
            {
                Console.WriteLine(person);
            }

            //1.Enumerable.Concat<(Of <(TSource>)>) 泛型方法应用
            IEnumerable<String> queryWithEnumerable = students.Select(student => student.City).Concat(teachers.Select(teacher => teacher.City));

            Console.WriteLine("Enumerable.Concat<(Of <(TSource>)>) 泛型方法应用");

            foreach (string city in queryWithEnumerable)
                Console.WriteLine(city);

            //2.QueryableConcat<(Of <(TSource>)>) 泛型方法应用
            IEnumerable<String> queryWithQueryable = students.AsQueryable().Select(student => student.City).Concat(teachers.Select(teacher => teacher.City));

            Console.WriteLine("QueryableConcat<(Of <(TSource>)>) 泛型方法应用");

            foreach (string city in queryWithEnumerable)
                Console.WriteLine(city);
        }

        #endregion

        class Student
        {
            public string First { get; set; }
            public string Last { get; set; }
            public int ID { get; set; }
            public string Street { get; set; }
            public string City { get; set; }
            public List<int> Scores;
        }

        class Teacher
        {
            public string First { get; set; }
            public string Last { get; set; }
            public int ID { get; set; }
            public string City { get; set; }
        }
    }