Question

我尝试加入表格学生和课程，然后使用SID将标记（按下添加按钮时）添加到表格 Student_Course （学生ID）和CID（课程ID）组合框，绑定到学生中的表SID和课程中的CID。

在代码中我试图加入表格，但它给了我一个

＆＃34;未处理的类型＆＃39; System.InvalidOperationException＆＃39;发生在System.Core.dll＆＃34;`错误。我加入表格时出错。

我该如何解决这个问题？还是有另一种方法可以做到这一点吗？

TLDR;

所以我有两个名为CID和SID的组合框，我可以选择课程ID和学生ID。然后我有一个Marks的文本框。我希望能够选择课程ID和学生ID，然后将标记添加到其中。

＆＃13;

private void btnAdd_Click(object sender, EventArgs e)
    {
        using (Entities2 db = new Entities2())
        {
            int SID = Convert.ToInt32(cboSID.Text);
            int CID = Convert.ToInt32(cboCID.Text);                 

            Student_Course marks = (from c in db.Student_Course join s in db.Students on c.SID equals s.SID join o in db.Courses on c.CID equals o.CID where c.SID == SID && c.CID == CID select c).First();
            marks.Mark = Convert.ToInt32(txtMark.Text);

            db.SaveChanges();
            MessageBox.Show("Marks Added");
        }

        using (Entities2 db = new Entities2()) //link combobox and DataGridView to List in Student_Course
        {
            var studentCourse = from c in db.Student_Course select new { SID = c.SID, Mark = c.Mark };

            editDataGridView.DataSource = studentCourse.ToList();
        }

        using (Entities2 db = new Entities2())
        {
            var student = (from s in db.Students select s).ToList();

            cboSID.DisplayMember = "SID";
            cboSID.ValueMember = "SID";
            cboSID.DataSource = student;
        }

        using (Entities2 db = new Entities2())
        {
            var course = (from c in db.Courses select c).ToList();

            cboCID.DisplayMember = "CID";
            cboCID.ValueMember = "CID";
            cboCID.DataSource = course;
        }
    }

＆＃13;

Answer 1

    private void btnAdd_Click(object sender, EventArgs e)
    {
        using (Entities2 db = new Entities2())
        {
            int SID = Convert.ToInt32(cboSID.SelectedItem.ToString());
            int CID = Convert.ToInt32(cboCID.SelectedItem.ToString());  
            int got_marks = Convert.ToInt32(txtMark.Text);          

            Student_Course marks = db.Student_Course.Where(sc => sc.SID == SID && c.CID == CID ).FirstOrDefault();
            if( marks!= null ){
              marks.Mark = got_marks;
            }else{
               db.Student_Course.Add(new Student_Course{
                    SID = SID,
                    CID = CID,
                    Mark = got_marks
               });

            }


            db.SaveChanges();
            MessageBox.Show("Marks Added");
        }

        using (Entities2 db = new Entities2()) //link combobox and DataGridView to List in Student_Course
        {
            var studentCourse = from c in db.Student_Course select new { SID = c.SID, Mark = c.Mark };

            editDataGridView.DataSource = studentCourse.ToList();
        }

        using (Entities2 db = new Entities2())
        {
            var student = (from s in db.Students select s).ToList();

            cboSID.DisplayMember = "SID";
            cboSID.ValueMember = "SID";
            cboSID.DataSource = student;
        }

        using (Entities2 db = new Entities2())
        {
            var course = (from c in db.Courses select c).ToList();

            cboCID.DisplayMember = "CID";
            cboCID.ValueMember = "CID";
            cboCID.DataSource = course;
        }
    }

Answer 2

using (Entities2 db = new Entities2())
{
 int SID = Convert.ToInt32(cboSID.SelectedValue);//Change cboSID.Text to cboSID.SelectedValue
 int CID = Convert.ToInt32(cboCID.SelectedValue);//Change cboCID.Text to  cboCID.SelectedValue               
 Student_Course marks = (from c in db.Student_Course join s in db.Students on c.SID equals s.SID join o in db.Courses on c.CID equals o.CID where c.SID == SID && c.CID == CID select c).First();
 marks.Mark = Convert.ToInt32(txtMark.Text);
 //Delete this line db.SaveChanges();
 MessageBox.Show("Marks Added");
}

如何从两个表中获取信息并将其添加到单独的表中？

2 个答案: