ช่วยดู Error >> Operation is not valid due to the current state of the object. ด้วยคะ

 Protected Sub btnList_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnList.Click
        prListData()
    End Sub

    Sub prListData()

        With Conn
            If .State = ConnectionState.Open Then .Close()
            .ConnectionString = strConn
            .Open()
        End With

        SQLWrk = "Select M.CustCode,isnull(M.Title,'')+' '+isnull(M.CustName,'')+' '+isnull(M.CustSurname,'') As CustName "
        SQLWrk = SQLWrk & ",M.SaleBy,isnull(E.ThaiFirstName,'')+' '+isnull(E.ThaiLastName,'') As EmpName,M.CreditLitmit "
        SQLWrk = SQLWrk & ",M.Address1,M.Address2,M.Address3,M.Province,M.ZipCode,V.VatPcnt "
        SQLWrk = SQLWrk & "From tbMTCustomer M "
        SQLWrk = SQLWrk & "Left Join tbMTEmployee E On M.CompanyCode = E.CompanyCode And M.SaleBy = E.EmpCode "
        SQLWrk = SQLWrk & "Left Join tbMTVat V On M.CompanyCode = V.CompanyCode And M.VatCode = V.VatCode "
        SQLWrk = SQLWrk & "Where M.CompanyCode='" & Session("gSCompanyCode") & "' "

        If txtCust.Text <> "" Then
            SQLWrk = SQLWrk & "And (M.CustCode like '%" & txtCust.Text & "%' Or M.CustName like '%" & txtCust.Text & "%') "
        End If

        SQLWrk = SQLWrk & "Order By M.CustCode "

        da = New SqlDataAdapter(SQLWrk, Conn)
        da.Fill(ds, "ListData")

        sb.Append("<table border = '0' cellpadding='2' cellspacing='2' align='center' width='99%' >")
        sb.Append("<tr bgcolor=" & gColorH1 & ">")
        sb.Append("<td align='center' class='trHead'><b>รหัสลูกค้า</b></td>")
        sb.Append("<td align='left' class='trHead'><b>ชื่อลูกค้า</b></td>")
        sb.Append("<td align='left' class='trHead'><b>พนักงานขาย</b></td>")
        sb.Append("<td align='center' class='trHead'><b>เครดิต</b></td>")
        sb.Append("</tr>")

        If ds.Tables("ListData").Rows.Count <> 0 Then
            Dim iCnt As Integer = 0
            While iCnt < ds.Tables("ListData").Rows.Count
                If iCnt Mod 2 = 0 Then
                    sb.Append("<tr bgcolor=" & gColor1 & ">")
                Else
                    sb.Append("<tr bgcolor=" & gColor2 & ">")
                End If

                sb.Append("<td align='center'><a href='javascript:fnSelect(" & (iCnt) & ")'>")
                sb.Append(ds.Tables("ListData").Rows(iCnt)("CustCode") & "</a></td>")
                sb.Append("<td align='left'>" & ds.Tables("ListData").Rows(iCnt)("CustName") & "</td>")
                sb.Append("<td align='left'>" & ds.Tables("ListData").Rows(iCnt)("EmpName") & "</td>")
                sb.Append("<td align='center'>" & ds.Tables("ListData").Rows(iCnt)("CreditLitmit") & "</td>")

                sb.Append("<input type='hidden' name='hReturn" & (iCnt) & "' id='hReturn" & (iCnt) & "'")
                sb.Append("runat='server' value='" & Server.UrlEncode(ClsUtility.gfnPadStr(8, ds.Tables("ListData").Rows(iCnt)("CustCode"))))
                sb.Append(ClsUtility.gfnPadStr(288, ds.Tables("ListData").Rows(iCnt)("CustName")))
                sb.Append(ClsUtility.gfnPadStr(80, clsApp.checkNull(ds.Tables("ListData").Rows(iCnt)("Address1"))))
                sb.Append(ClsUtility.gfnPadStr(80, clsApp.checkNull(ds.Tables("ListData").Rows(iCnt)("Address2"))))
                sb.Append(ClsUtility.gfnPadStr(80, clsApp.checkNull(ds.Tables("ListData").Rows(iCnt)("Address3"))))
                sb.Append(ClsUtility.gfnPadStr(40, clsApp.checkNull(ds.Tables("ListData").Rows(iCnt)("Province"))))
                sb.Append(ClsUtility.gfnPadStr(10, clsApp.checkNull(ds.Tables("ListData").Rows(iCnt)("ZipCode"))))
                sb.Append(ClsUtility.gfnPadStr(8, ds.Tables("ListData").Rows(iCnt)("SaleBy")))
                sb.Append(ClsUtility.gfnPadStr(160, ds.Tables("ListData").Rows(iCnt)("EmpName")) & "' />")
                'sb.Append(ClsUtility.gfnPadStr(10, CStr(clsApp.GetFormatCurrency(ds.Tables("ListData").Rows(iCnt)("CreditLitmit"), 2))))
                'sb.Append(ClsUtility.gfnPadStr(10, CStr(clsApp.GetFormatCurrency(ds.Tables("ListData").Rows(iCnt)("VatPcnt"), 2))) & "' />")

                iCnt += 1
            End While
        End If
        sb.Append("</table><hr />")

        display.InnerHtml = sb.ToString()

        Conn.Close()
        ds.Tables("ListData").Clear()
    End Sub