ขอรบกวนหน่อยครับท่าน คือผมหางหายจากการเขียนโปรแกรมไปนาน สอบถามเรื่อง combobox

 Private Sub showcombox()
        sb.Remove(0, sb.Length)
        sb.Append("select counter_name from counter")
        Dim slecounter As String = sb.ToString()
        com = New SqlCommand
        With com
            .CommandType = CommandType.Text
            .CommandText = slecounter
            .Connection = con
            dr = .ExecuteReader()
            While dr.Read
                ComboBox1.Items.Add(dr("counter_name"))
            End While
            dr.Close()
        End With
    End Sub

 Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        If ComboBox1.Text <> "" Then
            Dim Da As New SqlDataAdapter
            Dim Ds As New DataSet
            Try
                Da = New SqlDataAdapter("select counter_code from counter where counter_name='" + ComboBox1.SelectedItem.ToString() + "'", con)
                Da.Fill(Ds, "counter")
                dt = Ds.Tables("counter")
            Catch ex As Exception
                MessageBox.Show(ex.Message)
            End Try
            txt_counter.Text = CStr(dt.Rows.Item(0).Item(0))
        End If
    End Sub