ASP.Net Webforms with SumoSelect

Question

On "Page_Load" I fill a DropDownList with my data source:

lstBoxGrupAcessID.DataTextField = "Description";
lstBoxGrupAcessID.DataValueField = "ID";
lstBoxGrupAcessID.DataSource = new BLLCompany().SelectLstGroupBox(objCompany.CompanyID);
lstBoxGrupAcessID.DataBind();

After this, I need to set the lstBoxGrupAcess with some pre-selected itens ( specific for each company ) ( just for sample, I will select all the itens on the lstBoxGrupAcessID )

foreach (ListItem itm in lstBoxGrupAcessID.Items)
    itm.Selected = true;

When I execute the page, I got this erro:

Cannot have multiple items selected in a DropDownList.

The inicialization of SumoSelect in my code is:

<script>
    $(document).ready(function () {
        var list = $('#<%=lstBoxGrupAcessID.CompanyID%>');

        list.SumoSelect({
            selectAll: false
        });
    });

    Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
    function EndRequestHandler(sender, args) {
        var list = $('#<%=lstBoxGrupAcessID.CompanyID%>');

        list.SumoSelect({
            selectAll: false
        });
    }
</script>

The declaration of my DropDownList is:

<asp:DropDownList ID="lstBoxGrupAcessID" runat="server" multiple="multiple" CssClass="form-control"></asp:DropDownList>

Answer 1

You must use asp:ListBox for select multiple items, not Dropdown