分数管理界面不能写入课程编号，其余均没问题，怎么回事？

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using 学生成绩管理系统;
//看一眼你代码的对象画红线的,右键 解析 引入命名空间
namespace 学生成绩管理系统
{
    public partial class ScoreMsgFrm : Form
    {
        private List<MySchool.StudentMsg> StudentList;
        private List<MySchool.CourseMsg> CourseList;
        private List<MySchool.ScoreMsg> ScoreList;
        private int current;
        private MySchool.MySchoolDBContext db;

        public ScoreMsgFrm()
        {
            InitializeComponent();
            StudentList = new List<MySchool.StudentMsg>();//初始化当前实体对象集合
            CourseList = new List<MySchool.CourseMsg>();
            ScoreList = new List<MySchool.ScoreMsg>();
            current = 1; //默认的当前图书编号
            string sqlConn = @"Data Source=Win-01412261307;Initial Catalog=MySchool;Integrated Security=true";
            db = new MySchool.MySchoolDBContext(sqlConn);
        }
      
        private void showScore()//显示当前学生信息
        {
            if (current >= 1 && current <= ScoreList.Count)
            {
                txtCourseId.Text = ScoreList[current - 1].CourseId.ToString();
                txtStudentId.Text = ScoreList[current - 1].StudentNo.ToString();
                txtScore.Text = ScoreList[current - 1].Score.ToString();
            }
        }

        private void ScoreMsgFrm_Load(object sender, EventArgs e)
        {
            var queryScore = from score in db.scores select score;  //定义查询语句
            foreach (var score in queryScore)//执行LINQ查询
            {
                ScoreList.Add(score);
            }
            showScore();//显示当前成绩
        }

        private void btnPreviouss_Click(object sender, EventArgs e)
        {
            if (current == 1)
            {
                MessageBox.Show("已经到第一条了", "注意", MessageBoxButtons.OK);
            }
            else
            {
                current--;
                showScore();
            };
        }

        private void btnNext_Click(object sender, EventArgs e)
        {
            if (current == ScoreList.Count)
            {
                MessageBox.Show("已经到最后一条了", "注意", MessageBoxButtons.OK);
            }
            else
            {
                current++;
                showScore();
            };

        }

        private void txtCourseId_TextChanged(object sender, EventArgs e)
        {
            var queryCourse =  //定义LINQ查询，获得指定的课程名称
               from course in db.courses
               where course.CourseId == int.Parse(txtCourseId.Text)
               select course;
            foreach (var course in queryCourse)//执行LINQ查询
            {
            
                txtCourseName.Text = course.CourseName;//显示课程名称
            }
        }

        private void txtStudentId_TextChanged(object sender, EventArgs e)
        {
            var queryStudent =  //定义LINQ查询，获得指定的姓名
               from student in db.students 
               where student.StudentNo == int.Parse(txtStudentId.Text) 
               select student;
            foreach (var course in queryStudent)//执行LINQ查询
            {
                学生成绩管理系统.MySchool.StudentMsg student = new 学生成绩管理系统.MySchool.StudentMsg();
                txtStudentName.Text =student.StudentName;//显示学生姓名
            }
        }

        private void btnAdd_Click(object sender, EventArgs e)
        {
            MySchool.ScoreMsg score = new MySchool.ScoreMsg(int.Parse(txtCourseId.Text),
              int.Parse(txtStudentId.Text), int.Parse(txtScore.Text));//创建ScoreMsg对象，封装用户输入
            db.scores.InsertOnSubmit(score);//将ScoreMsg对象添加到table对象中
            try
            {
                db.SubmitChanges();//将更新结果提交给DMS
                ScoreList.Add(score);//在实体集合中添加新ScoreMsg对象
                current = ScoreList.Count;

                showScore();  //显示新添加的图书
                MessageBox.Show("已经到成功添加目录", "注意", MessageBoxButtons.OK);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "注意", MessageBoxButtons.OK);
            }

        }

        private void btnEdit_Click(object sender, EventArgs e)
        {
            if (current >= 1 && current <= ScoreList.Count)
            {
                var updateScores =      //定义LINQ查询
                    from score in db.scores
                    where (score.CourseId == ScoreList[current - 1].CourseId && score.StudentNo == ScoreList[current - 1].StudentNo)
                    select score;
                foreach (MySchool.ScoreMsg score in updateScores)//执行LINQ查询
                {//把用户输入的新成绩赋值给实体对象的对应属性
                    score.Score = int.Parse(txtScore.Text);
                    //注意，由于课程号和学号是成绩表的主键，因此原则上不能修改
                }
                try
                {
                    db.SubmitChanges();//将更新结果提交给DBMS
                    MessageBox.Show("已经到成功更新记录", "注意", MessageBoxButtons.OK);

                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message, "注意", MessageBoxButtons.OK);
                }
            }
        }

        private void btnDelete_Click(object sender, EventArgs e)
        {
            if (current >= 1 && current <= ScoreList.Count)
            {
                var delScores =      //定义LINQ查询
                    from score in db.scores
                    where (score.CourseId == ScoreList[current - 1].CourseId && score.StudentNo == ScoreList[current - 1].StudentNo)
                    select score;
                foreach (MySchool.ScoreMsg score in delScores)//执行LINQ查询
                {
                    db.scores.DeleteOnSubmit(score);//删除Table对象中的指定对象

                }
                try
                {
                    db.SubmitChanges();//将更新结果提交给DBMS
                    ScoreList.RemoveAt(current - 1);
                    if (current > 0) current--;
                    showScore();

                    MessageBox.Show("已经到成功删除记录", "注意", MessageBoxButtons.OK);

                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message, "注意", MessageBoxButtons.OK);
                }
            }
        }
    }
}