تعبئة الليست بوكس عند الإختيار من الكومبوكس

كود :
  Public DT_Student As New DataTable

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    Private Sub CmbStudentID_DropDown(sender As System.Object, e As System.EventArgs) Handles CmbStudentID.DropDown, CmbStudentID.DropDown

        Try



            CmbStudentID.DataSource = Nothing

            CmbStudentID.Items.Clear()

            CmbStudentID.Text = vbNullString

            DT_Student.Clear()

            Dim Da As New SqlDataAdapter("Select * From Student Where ClassID = " & CmbClassID.SelectedValue & " ", Con) '

            If Con.State = 1 Then Con.Close()

            Con.Open()

            Da.Fill(DT_Student)

            Con.Close()

            Da = Nothing





        Catch ex As Exception

            Con.Close()

        End Try

        If DT_Student.Rows.Count <> 0 Then

            With CmbStudentID

                .DataSource = DT_Student

                .DisplayMember = "StudentName"

                .ValueMember = "StudentID"

            End With

        End If



    End Sub

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    Private Sub CmbClassID_DropDown(sender As System.Object, e As System.EventArgs) Handles CmbClassID.DropDown

        CmbStudentID.SelectedIndex = -1

    End Sub