Auto fill dropdown of Asp.net using C#

Create AutoSuggestBox.css file and place the below css Code
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.CompletionList
{
    background-repeat: repeat-x;
    background-color: #FFFFFF;
    z-index: 10000;
    width: 245px !important;
    max-height: 210px;
    text-align: inherit;
    text-indent: -1;
    list-style: none;
    overflow-y: scroll;
    scrollbar-arrow-color: #B89020;
    scrollbar-base-color: #8E6E1C;
    scrollbar-face-color: #B6C5D4;
    scrollbar-3dlight-color: #8E6E1C;
    scrollbar-highlight-color: #EED47D;
    scrollbar-shadow-color: #959595;
    scrollbar-darkshadow-color: #00337E;
    margin-left: 0px;
    border-bottom: 1px solid #B5C6D4;
    border-left: 1px solid #B5C6D4;
    border-top: 1px solid #B5C6D4;
    margin-top: 0px;
}
.CompletionListItemCssClass
{
    background: none;
    border-collapse: collapse;
    color: #00337E;
    white-space: nowrap;
    text-align: inherit left;
}
.CompletionListHighlightedItemCssClass
{
    background-repeat: repeat-x;
    background-color: #EED47D;
    color: #00337E;
    border-top: 1px solid #FFF8E8;
    border-left: 1px solid #FFF8E8;
    border-bottom: 1px solid #00337E;
    border-right: 1px solid #00337E;
}

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Place this in .aspx file

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<link href="../App_Themes/AutoSuggestBox.css" rel="Stylesheet" type="text/css" />

    <script type="text/javascript">
    function Common_ItemSelected(sender, e)
    {
        var hdnCommonID = $get('<%= hdnCommonID.ClientID %>');
        hdnCommonID.value = e.get_value();
    }
    function Common_ItemPopulated(sender, e)
    {
        var hdnCommonID = $get('<%= hdnCommonID.ClientID %>');
        hdnCommonID.value = '';
    }
    </script>


<asp:Panel ID="panAutoSuggest" runat="server">
<asp:TextBox ID="txtSearch" runat="server" MaxLength="50" OnTextChanged="txtSearch_OnTextChanged"
 AutoPostBack="true" Width="182px"></asp:TextBox>
<cc1:AutoCompleteExtender ID="aceCommonCon" MinimumPrefixLength="1" OnClientPopulated="Common_ItemPopulated"      OnClientItemSelected="Common_ItemSelected" runat="server" TargetControlID="txtSearch" ServiceMethod="GetCommonList" CompletionSetCount="5" Enabled="True" ServicePath="" CompletionListCssClass="CompletionList"
DelimiterCharacters=";, :" CompletionListItemCssClass="CompletionListItemCssClass"
CompletionListHighlightedItemCssClass="CompletionListHighlightedItemCssClass"
ShowOnlyCurrentWordInCompletionListItem="true">
</cc1:AutoCompleteExtender>
<cc1:TextBoxWatermarkExtender ID="txtWatermarkExtender" runat="server" TargetControlID="txtSearch"
WatermarkText="--Select--">
</cc1:TextBoxWatermarkExtender>
<asp:HiddenField ID="hdnCommonID" runat="server" />
</asp:Panel>

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place the below code in code behind - .aspx.cs file
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    [System.Web.Services.WebMethod]
    public static string[] GetList(String prefixText, int count)
    {

        DataTable dt1 = new DataTable();
        //store and get the datatable from session
        dt1 = (DataTable)System.Web.HttpContext.Current.Session["EmpDT"];
        List<String> suggetions = GetSuggestions(prefixText, count, dt1);
        return suggetions.ToArray();
    }

    private static List<String> GetSuggestions(string key, int count, DataTable dt1)
    {
        List<String> suggestions = new List<string>();
        try
        {
            DataRow[] dtSuggestions = dt1.Select(filterExpression);

            foreach (DataRow dr in dtSuggestions)
            {
                string suggestion = AutoCompleteExtender.CreateAutoCompleteItem(dr["NAME"].ToString(), dr["ID"].ToString());
                suggestions.Add(suggestion);
            }
        }
        return suggestions;
    }
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