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"},
            };

            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 };

    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);
            }

------解决方案--------------------

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));

        }
    }
}