Use Query String for this:
Default.aspx.cs Page
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
DropDownList str = sender as DropDownList;
Response.Redirect("Default2.aspx?country=" + str.SelectedValue + "", false);
}
Default2.aspx.cs Page
if (Request.QueryString["country"] != null)
{
if (Request.QueryString["country"].ToString().ToUpper() == "INDIA")
{
DropDownList1.Items.Add("Andhrapardesh");
DropDownList1.Items.Add("Assam");
}
else if (Request.QueryString["country"].ToString().ToUpper() == "USA")
{
DropDownList1.Items.Add("Texas");
DropDownList1.Items.Add("California");
}
else if (Request.QueryString["country"].ToString().ToUpper() == "UK")
{
DropDownList1.Items.Add("London");
}
}