Use AutoComplete Extender in C#

<ap:AutoCompleteExtender ServiceMethod="SearchCustomers"  ServicePath="~/Address_WebService.asmx"
    MinimumPrefixLength="2"
    CompletionInterval="100" EnableCaching="false" CompletionSetCount="10"
    TargetControlID="txtContactsSearch"
    ID="AutoCompleteExtender1" runat="server" FirstRowSelected = "false">
</ap:AutoCompleteExtender>

<asp:TextBox ID="txtContactsSearch" runat="server"></asp:TextBox>

[WebMethod]
public static List<string> SearchCustomers(string prefixText, int count)
{
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager
                .ConnectionStrings["Webconfig_Con"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "select * from tblCustomer where CompanyName like '%" + prefixText + "%' ";
            cmd.Connection = conn;
            conn.Open();
            List<string> customers = new List<string>();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    customers.Add(sdr["CompanyName"].ToString());
                }
            }
            conn.Close();
            return customers;
        }
    }
}