Validating textbox control within datagrid

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C#

.NET

WinForms

C#4.0

.NET4

I am developing an application where teacher are allowed to enter marks for students, based on selecting a subject they teach. Now I am using a 4 columed datagrid to which I am populating all the student rollnum(1st col) and name (2nd col) and textbox control to enter Test marks(3rd col) and textbox control to enter quiz marks(4th col). I have succeded in populating data to datagrid through datatable. Now my problem is, when teacher enters all the marks and click on submit button, I want my application to check if any of the textboxes are left empty. Unless and untill all the textboxes are filled, my INSERT query(Adding marks to database query) should not execute. This is what I have tried so far

int rowcount = DTUSNName.Rows.Count;//rows count of datatable
            for (int i = 0; i < rowcount; i++)
            { 
                if (dataGridView1.Rows[i].Cells[0].Value != null)
                {
                    if (dataGridView1.Rows[i].Cells[1].Value != null)
                    {
                        var confirmResult = MessageBox.Show("Once the marks are added you wont be allowed to make changes. Do you want to proceed?","Confirm!!",MessageBoxButtons.YesNo);
                        if (confirmResult == DialogResult.Yes)
                        {
                            SqlCommand Test1query = new SqlCommand("INSERT INTO TblStudentReg (Test1, Quiz1) VALUES (@fld1,@fld2) WHERE Code=@code AND Sem = @sem AND CC=@cc AND AcademicYr = @aca", con);
                            Test1query.Parameters.Add(new SqlParameter("@fld1", dataGridView1.Rows[i].Cells[0].Value.ToString()));
                            Test1query.Parameters.Add(new SqlParameter("@fld2", dataGridView1.Rows[i].Cells[1].Value.ToString()));
                            Test1query.Parameters.Add(new SqlParameter("@code", temp));//rollnum
                            Test1query.Parameters.Add(new SqlParameter("@sem", StaffMEDrp1.SelectedItem));subject code
                            Test1query.Parameters.Add(new SqlParameter("@cc", label3.Text));//subject code
                            Test1query.Parameters.Add(new SqlParameter("@aca", StaffMETxt1.Text));//academic yr
                            con.Open();
                            Test1query.ExecuteNonQuery();
                            con.Close();
                        }
                        else
                        {
                        }
                    }
                    else
                    {
                         MessageBox.Show("You have not set Quiz 1 Marks for USN : '" + usn + "'");
                    }
                }
                else
                {
                    MessageBox.Show("You have not set Test 1 Marks for USN : '" + usn + "'");
                }
            }

Thank you.