I managed to make AJAX Cascading DropDown through this Demo:
http://www.aspsnippets.com/Articles/AJAX-Cascading-DropDown-Example-in-ASPNet.aspx
But when I try to add a submit button which simply call
protected void Button1_Click(object sender, EventArgs e)
{
Response.Write(ddlCountries.SelectedValue + "|" + ddlCities.SelectedValue + "|" + ddlStates.SelectedValue);
}
I got
Invalid postback or callback argument.
Error, how can I get those values?