AutoComplete Extender Example

 <asp:TextBox ID="txtPatName" runat="server" AutoPostBack="True" CssClass="textSearch"
                                    OnTextChanged="txtPatName_TextChanged" Width="300px"></asp:TextBox>
                                <div id="divwidth">
                                </div>
                                <cc1:AutoCompleteExtender ID="txtPatName_AutoCompleteExtender" runat="server" Enabled="True"
                                    MinimumPrefixLength="1" ServiceMethod="GetEmrPatientList" ServicePath="~/Service/EMRWebService.asmx"
                                    TargetControlID="txtPatName" CompletionSetCount="20" CompletionListItemCssClass="AutoExtenderList"
                                    CompletionListCssClass="AutoExtender" CompletionListHighlightedItemCssClass="AutoExtenderHighlight"
                                    CompletionListElementID="divwidth" DelimiterCharacters="">
                                </cc1:AutoCompleteExtender>

//Web Service As
private List<String> List(string key)
    {

        List<String> List = new List<string>();
        DataTable dtEmrPat = new DataTable();
        using (cls obj = new cls())
        {
            if (Session["XYZ"].ToString().Equals(""))
            {
                dt = obj.GetList(key);
            }
            else
            {
                dt = obj.GetAllLists(key);
            }

            if (dt != null && dt.Rows != null && dt.Rows.Count > 0)
            {
                Session["EmrPatients"] = dt;
                foreach (DataRow dr in dt.Rows)
                {
                    stringname = dr["Name"].ToString();
                    List .Add(stringname );
                }
            }
            return List ;
        }
    }