Hi rakeshkuma,

Use Ajax Timer control inside the UpdatePanel.

Add HiddenField and set the value on DropDownList change.

Then reset the selected value in DropDownList again.

Refer below code.

Database

I have made use of the following table Customers with the schema as follows.

I have already inserted few records in the table.

You can download the database table SQL by clicking the download link below.

Download SQL file

HTML

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
Last Refreshed: <%=DateTime.Now.ToString("hh:mm:ss tt") %>
<hr />
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:DropDownList ID="ddlCustomers" runat="server"
            OnSelectedIndexChanged="OnSelectedIndexChanged">
        </asp:DropDownList>
        <hr />
        <asp:Label ID="lblTime" runat="server" />
        <asp:HiddenField ID="hfCountry" runat="server" />
        <asp:Timer ID="Timer1" runat="server"
            OnTick="TimerTick" Interval="2000" />
    </ContentTemplate>
</asp:UpdatePanel>

Namespaces

C#

using System.Data;
using System.Data.SqlClient;
using System.Configuration;

VB.Net

Imports System.Data
Imports System.Data.SqlClient
Imports System.Configuration

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindDropDownList();
    }
}

protected void TimerTick(object sender, EventArgs e)
{
    this.BindDropDownList();
}

protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
    hfCountry.Value = ddlCustomers.SelectedValue;
}

private void BindDropDownList()
{
    lblTime.Text = DateTime.Now.ToString("hh:mm:ss tt");
    string conString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    string query = "SELECT Country FROM Customers";
    using (SqlConnection con = new SqlConnection(conString))
    {
        using (SqlCommand cmd = new SqlCommand(query))
        {
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                using (DataTable dt = new DataTable())
                {
                    sda.Fill(dt);
                    ddlCustomers.DataSource = dt;
                    ddlCustomers.DataTextField = "Country";
                    ddlCustomers.DataValueField = "Country";
                    ddlCustomers.DataBind();
                    if (!string.IsNullOrEmpty(hfCountry.Value))
                    {
                        ddlCustomers.Items.FindByValue(hfCountry.Value).Selected = true;
                    }
                }
            }
        }
    }
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Me.BindDropDownList()
    End If
End Sub

Protected Sub TimerTick(ByVal sender As Object, ByVal e As EventArgs)
    Me.BindDropDownList()
End Sub

Protected Sub OnSelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)
    hfCountry.Value = ddlCustomers.SelectedValue
End Sub

Private Sub BindDropDownList()
    lblTime.Text = DateTime.Now.ToString("hh:mm:ss tt")
    Dim conString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Dim query As String = "SELECT Country FROM Customers"
    Using con As SqlConnection = New SqlConnection(conString)
        Using cmd As SqlCommand = New SqlCommand(query)
            Using sda As SqlDataAdapter = New SqlDataAdapter()
                cmd.Connection = con
                sda.SelectCommand = cmd
                Using dt As DataTable = New DataTable()
                    sda.Fill(dt)
                    ddlCustomers.DataSource = dt
                    ddlCustomers.DataTextField = "Country"
                    ddlCustomers.DataValueField = "Country"
                    ddlCustomers.DataBind()
                    If Not String.IsNullOrEmpty(hfCountry.Value) Then
                        ddlCustomers.Items.FindByValue(hfCountry.Value).Selected = True
                    End If
                End Using
            End Using
        End Using
    End Using
End Sub

Screenshot