Why the count of items in my ComboBox is always 0 although the datasource of this combobox has data !!

<div align="right" dir="rtl">
        <asp:Label ID="lbl_contactListName" runat="server" Text="Menu Name :" CssClass="span"></asp:Label>
        <telerik:RadComboBox ID="ddl_contactList" runat="server" AutoPostBack="True" CausesValidation="False"
            CollapseDelay="0" Culture="ar-EG" ExpandDelay="0" Filter="StartsWith" ItemsPerRequest="10"
            MarkFirstMatch="true" Skin="Outlook" EnableAutomaticLoadOnDemand="True" EmptyMessage="-New Menu-"
            ShowMoreResultsBox="True" OnSelectedIndexChanged="ddl_contactList_SelectedIndexChanged"
            EnableVirtualScrolling="True" DataTextField="list_desc" DataValueField="list_code"
            DataSourceID="ObjectDataSource1" EnableViewState="true" Width="300px">
        </telerik:RadComboBox>
    </div>
        <asp:ObjectDataSource ID="ObjectDataSource1" runat="server" SelectMethod="GetContactListsByDep"
            TypeName="SendSMS_EmailModule.ContactList">
            <SelectParameters>
                <asp:SessionParameter Name="year" SessionField="year" Type="Int32" />
                <asp:SessionParameter Name="main_code" SessionField="main_code" Type="Int32" />
            </SelectParameters>
        </asp:ObjectDataSource>