[قناص المدينة]

Try
' هرض درجـات المواد في الداتا قريدفيو
If Con.State = ConnectionState.Closed Then Con.Open()

Dim dt As DataTable
Using da As New OleDbDataAdapter("SELECT TBL_Student.IDStudent AS رقم, " & _
"TBL_Student.alqayd AS [رقم القيد], " & _
"TBL_Student.NameStudent AS [اســـم الطالـــب], " & _
"TBL_Student.IDClas AS الصــف " & _
"FROM TBL_Student " & _
"WHERE TBL_Student.IDClas = ?", Con)
da.SelectCommand.Parameters.AddWithValue("?", Val(listClas.SelectedItem.ToString()))
dt = New DataTable
da.Fill(dt)
DataGridView1.DataSource = dt
End Using

If Not String.IsNullOrEmpty(CombSemester.Text) AndAlso IsNumeric(CombSemester.Text) Then
Using daCheck As New OleDbDataAdapter("SELECT IDStudent FROM TBL_Final1 WHERE IDSemester = ?", Con)
daCheck.SelectCommand.Parameters.AddWithValue("?", Val(CombSemester.Text))
Dim dtCheck As New DataTable
daCheck.Fill(dtCheck)
If dtCheck.Rows.Count > 0 Then
Using da2 As New OleDbDataAdapter("SELECT S.IDStudent, S.alqayd, S.NameStudent, S.IDClas, F.IDSemester, F.ON" & (CombCourses.SelectedIndex + 1) & " AS اعمال " & _
"FROM TBL_Final1 F RIGHT JOIN TBL_Student S ON F.IDStudent = S.IDStudent " & _
"WHERE F.IDSemester = ? AND S.IDClas = ?", Con)
da2.SelectCommand.Parameters.AddWithValue("?", Val(CombSemester.Text))
da2.SelectCommand.Parameters.AddWithValue("?", Val(listClas.SelectedItem.ToString()))
Dim dt2 As New DataTable
da2.Fill(dt2)
DataGridView1.DataSource = dt2
End Using
End If
End Using
End If

FillDataGridView()

Catch ex As Exception
MessageBox.Show(ex.Message)
Finally
If Con.State = ConnectionState.Open Then Con.Close()
End Try