求一个linq的写法(join)

C# code

    public class Student    {        public String ID { get; set; }        public String StuName { get; set; }    }    public class Grade    {        public String ID { get; set; }        public String GrideName { get; set; }    }            //测试数据            List<Student> tempStuList = new List<Student>() {                 new Student(){ ID="001001", StuName="a"},                new Student(){ ID="002001", StuName="b"},                new Student(){ ID="002001", StuName="c"},                new Student(){ ID="003001", StuName="d"},                new Student(){ ID="001001", StuName="e"},            };            List<Grade> tempGradeList = new List<Grade>() {                 new Grade(){ ID="001", GrideName = "g1"},                new Grade(){ ID="002", GrideName = "g2"},                new Grade(){ ID="003", GrideName = "g3"},            };

说明一下:这里仅仅是讨论语法如何写,而不是讨论设计合理与否.这里只是举个例子
Stu.ID的前3位是年级ID,所以可以用下面的代码获取学生表与年级表的关联.这是一个很庆幸的例子.但是如果grade.ID是不定长的情况就无能为力了(不要管需求和实现是否合理,我只是寻求linq的语法).
我的问题是在做join的时候,linq能否写成 stu.ID.IndexOf(grade.ID) ==0类似的写法

C# code

            var a = from stu in tempStuList                    join grade in tempGradeList                    on stu.ID.Substring(0, 3) equals grade.ID                    into tempGradeTable                    from tempItem in tempGradeTable                    select new { StuID = stu.ID, StuName = stu.StuName, GradeName = tempItem == null ? "" : tempItem.GrideName };

[解决办法]
你既然使用了JOIN 就要遵守JOIN的语法 使用 equals关键字

如果你一定要使用 stu.ID.IndexOf(grade.ID) ==0 类似这样的写法

你可以使用 WHERE关键字 结合 ANY()方法来实现
[解决办法]
这样是可以的

C# code

    class GradeComparer : IEqualityComparer<string>    {        public bool Equals(string b1, string b2)        {            string stuId = b1.Length < b2.Length ? b2 : b1;            string gradeId = b1.Length < b2.Length ? b1 : b2;            return stuId.StartsWith(gradeId);                    }        public int GetHashCode(string bx)        {            return 1;  //返回同样的值，在Equals中进行真正的比较。可能效率会稍差。如果GetHashCode返回不同的值，Equals不会被调用。        }    }            GradeComparer gradeComparer = new GradeComparer();            var query = tempStuList.Join(tempGradeList,                                s => s.ID,                                                  g => g.ID,                                (s, g) => new { ID = s.ID, Name = s.StuName, GID = g.ID, GName = g.GrideName},                                gradeComparer);            foreach (var obj in query)            {                Console.WriteLine(obj.ID, obj.Name, obj.GID, obj.GName);            }
[解决办法]
C# code
using System;using System.Collections.Generic;using System.Linq;using System.Text;namespace ConsoleApplication1{    class Program    {        public class Student        {            public String ID { get; set; }            public String StuName { get; set; }        }        public class Grade        {            public String ID { get; set; }            public String GrideName { get; set; }        }        static void Main(string[] args)        {            List<Student> tempStuList = new List<Student>() {                 new Student(){ ID="001001", StuName="a"},                new Student(){ ID="002001", StuName="b"},                new Student(){ ID="002001", StuName="c"},                new Student(){ ID="003001", StuName="d"},                new Student(){ ID="003001", StuName="e"},            };            List<Grade> tempGradeList = new List<Grade>() {                 new Grade(){ ID="001", GrideName = "g1"},                new Grade(){ ID="002", GrideName = "g2"},                new Grade(){ ID="003", GrideName = "g3"},            };            var a = from stu in tempStuList                    select new { StuID = stu.ID, StuName = stu.StuName, GradeName = (tempGradeList.Where(x => stu.ID.IndexOf(x.ID) == 0).Count() > 0 ? tempGradeList.Where(x => stu.ID.IndexOf(x.ID) == 0).First().GrideName : "") };            a.ToList().ForEach(x => Console.WriteLine(x));        }    }}