我正在编写一个基于 Web 的程序，这是我的身份验证页面。它工作正常，但突然开始出现该错误。
这是我的代码：
else if (LoginAs.SelectedValue == "Student")
{
string tableName = "StudentTable";
String name = "", surname = "", email = "";
string query = "Select level from " + tableName + " where ID='" + idBox.Text + "'";
SqlCommand cmd = new SqlCommand(query, con);
string level = Convert.ToString(cmd.ExecuteScalar());
CreateUser(con, tableName, ref name, ref surname, ref email);
query = "Select program from " + tableName + " where ID='" + idBox.Text + "'";
cmd = new SqlCommand(query, con);
string program = Convert.ToString(cmd.ExecuteScalar());
MyGlobals.student = new Student(Convert.ToInt32(idBox.Text), "Active", email, name, surname, password, level, program);
MyGlobals.currentID = idBox.Text;
query = "Select * from RegisterTable where StudentID='" + idBox.Text + "'";
cmd = new SqlCommand(query, con);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataTable dt = new DataTable();
da.Fill(dt);
foreach (DataRow dr in dt.Rows)
{
query = "SELECT * FROM CourseTable WHERE CourseCode='" + dr["CourseCode"] + "' AND CourseNumber='" + dr["CourseNumber"] + "' AND Term='" + dr["Term"] + "'";
cmd = new SqlCommand(query, con);
SqlDataAdapter da2 = new SqlDataAdapter(cmd);
DataTable dt2 = new DataTable();
da2.Fill(dt2);
DataRow dr2 = dt2.Rows[0]; //ERROR COMES AT HERE
Course course = new Course(dr2["InstructorName"].ToString(), dr2["CourseCode"].ToString(), dr2["CourseNumber"].ToString(), dr2["CourseName"].ToString(), dr2["Term"].ToString(), dr2["CRN"].ToString(), dr2["Level"].ToString(), dr2["Credit"].ToString(), dr2["Description"].ToString(), dr2["Capacity"].ToString());
Register reg = new Register(course, MyGlobals.student);
MyGlobals.student.addToSchedule(reg);
}
int num = (int)Application["OnlineUsers"];
Response.Redirect("Student.aspx");
}
谁能帮我这个？提前致谢。