DropDownlist causing postback on change

hi every one

I have 1Dropdownlist 1 button and 1 datalist in my page i bind my datalist in Page_load event that show all my product from my DB 

users  can select their city from DDL click on button and in datalist just show product that are on selected city.

Problem:

EX:when users select PARIS from ddl in datalist show product from selected city and when users want change their city from ddl before their click on button to show that city's product page load again and it show all product again...

I want when users change ddl.selecteditem in datalist show last result of user's selected not the result that i define in page load

this is my code on page load

 protected void Page_Load(object sender, EventArgs e)

    {
     
            if (!IsPostBack)
            {
                DDLcity1.Items.Add(new ListItem("select your city", ""));
                BindDropDownList(DDLcity1, "Guidcity", "cityname", "ID");

           
        

    }