linq 数据类型转换

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Xml.Linq;

namespace WebApplication1
...{
    public partial class Default : System.Web.UI.Page
    ...{
        protected void Page_Load(object sender, EventArgs e)
        ...{
            List<Student> students = new List<Student>()
            ...{
                new Student ...{First="张",
                    Last="三",
                    ID=111,
                    Street="复兴大道338",
                    City="杭州",
                    Scores=new List<int>...{84,97,68,92}},      
                new Student ...{First="张",
                    Last="影",
                    ID=111,
                    Street="复兴大道338",
                    City="云南",
                    Scores=new List<int>...{84,97,68,92}},
                new Student ...{First="张",
                    Last="雨",
                    ID=111,
                    Street="复兴大道338",
                    City="芜湖",
                    Scores=new List<int>...{84,97,68,92}},
            };

            List<Teacher> teachers = new List<Teacher>()
            ...{
                new Teacher ...{First="张",
                    Last="敏",
                    ID=111,
                    City="杭州"},   
                new Teacher ...{First="张",
                    Last="雪",
                    ID=111,
                    City="云南"},
                new Teacher ...{First="张",
                    Last="青",
                    ID=111,
                    City="芜湖"}
            };

            //创建查询
            var peopleInSeattle = (from student in students
                                   where student.City == "芜湖"
                                   select student.First + student.Last)
                                     .Concat(from teacher in teachers
                                             where teacher.City == "芜湖"
                                             select teacher.First + teacher.Last);
            Response.Write("在芜湖的教师和学生:");
            //执行查询
            foreach (var person in peopleInSeattle)
            ...{
                Response.Write(" " + person);
            }
        }

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

        class Teacher//教师类
        ...{
            public string First ...{ set; get; }
            public string Last ...{ set; get; }
            public int ID ...{ set; get; }
            public string City ...{ set; get; }
        }

    }
}

		

List<Student> _list = new List<Student>() 
...{ 
new Student ...{FirstName="Svetlana", LastName="Omelchenko", 
             Unid=111, Scores = new List<int>...{97, 92, 81, 60}},

new Student ...{FirstName="Claire", LastName="O’Donnell", 
             Unid=112, Scores = new List<int>...{75, 84, 91, 39}},

new Student ...{FirstName="Sven", LastName="Mortensen", 
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linq