Searchable dropdownlist control

If you are not interested in going for JQuery auto complete or have specific need, you have the option of using a text box, which will be used to type the text to search and add a dropdownlist or list control in update panel which will be refreshed while you are typing text in textbox.

this is the code you need in your .aspx

<script type="text/javascript">
    function RefreshUpdatePanel() {
        __doPostBack('txtSearch', '');
    };
</script>

<asp:TextBox ID="txtSearch" runat="server" onkeyup="RefreshUpdatePanel();" AutoPostBack="false" OnTextChanged="txtSearch_TextChanged" ></asp:TextBox>
    <asp:UpdatePanel ID="Update" runat="server">
        <ContentTemplate>
            <asp:DropDownList runat="server" ID="ddlSearch" />
        </ContentTemplate>
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="txtSearch" />
        </Triggers>
    </asp:UpdatePanel>

    <asp:TextBox ID="txt2" runat="server"></asp:TextBox>

Below is the code needed in your code behind.

public List<string> Values { get; set; }
        private void GetValues()
        {

            Values = new List<string>();

            Values.Add("Apple");
            Values.Add("Orange");
            Values.Add("Banana");
            Values.Add("Pear");
            Values.Add("Black Berry");
            Values.Add("Pineapple");
        }

        protected void txtSearch_TextChanged(object sender, EventArgs e)
        {
            var dropDownList = sender as TextBox;
            var options = (from o in Values
                           where o.StartsWith(dropDownList.Text, StringComparison.InvariantCultureIgnoreCase)
                           select o).ToList();

            ddlSearch.DataSource = options;
            ddlSearch.DataBind();
        }

This approach updates the panel contents onkeyup event of the textbox, if you need this on other event e.g. onblur, you can register it from code behind using "RegisterClientScriptBlock".

I would still recommend you to go for JQuery auto complete because it is more cleaner approach.

check this plugin -select2[^]