Avoiding PostBack in asp - asp.net

Question

I want to avoid postback when my button event fired. Due to postback my List gets empty again. Kindly Help me out with this. What should i do to avoid postback. I am using ASP button.
Here is my button method
id = Convert.ToInt32(Request.QueryString["id"].ToString());
venues.Add(id);
foreach (int item1 in venues)
{
if (!hashset.Add(item1))
{
Response.Write("<script>alert('Duplicate Items');</script>");
break;
}
else
{
foreach (var items in venues)
{
con.Open();
SqlCommand cmd = con.CreateCommand();
cmd.CommandType = CommandType.Text;
cmd.CommandText = "select * from Venues where VenueID = '" + items + "'";
cmd.ExecuteNonQuery();
DataTable dt = new DataTable();
SqlDataAdapter ds = new SqlDataAdapter(cmd);
ds.Fill(dt);
foreach (DataRow dr in dt.Rows)
{
Name = dr["Name"].ToString();
Address = dr["Address"].ToString();
Picture = dr["Picture"].ToString();
Price = dr["Price"].ToString();
}
con.Close();
if (Request.Cookies["aa"] == null)
{
Response.Cookies["aa"].Value = Name.ToString() + "," + Address.ToString() + "," + Picture.ToString() + "," + Price.ToString();
Response.Cookies["aa"].Expires = DateTime.Now.AddDays(1);
}
else
{
Response.Cookies["aa"].Value = Request.Cookies["aa"].Value + "|" + Name.ToString() + "," + Address.ToString() + "," + Picture.ToString() + "," + Price.ToString();
Response.Cookies["aa"].Expires = DateTime.Now.AddDays(1);
}
}
}
}
}