Using Datareader value to query database

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C#

C#4.0

Am a new programmer in c# and i want to use the value return in datareader to re-query the database.
This is the part of my code:

string queryscore = "SELECT CourseUnit,Score,MatricNo FROM student_result WHERE CourseCode='" + cmbcoursecode.Text.ToString() + "'";

           SqlCommand cmdscore = new SqlCommand(queryscore, con);

           SqlDataReader readerscore;

           readerscore = cmdscore.ExecuteReader();

           while (readerscore.Read())
           {
               score1 = (int)readerscore["Score"];
               int CourseUnit = (int)readerscore["CourseUnit"];
               if (score1 <= 39)
               {
                   string updateresult = "Update Student_Result SET Grade='" + "F" + "' WHERE MatricNo='" + readerscore["MatricNo"] + "'";
                   SqlCommand cmdupdatescore = new SqlCommand(updateresult, con);
                   cmdupdatescore.ExecuteNonQuery();
                  // MessageBox.Show("F");

               }