Sometimes we deal with more than one dropdown that related to each other. It calls Cascading DropDown.
Many ways to create Cascading DropDown with asp .net which are using autopostback that page refresh everytime dropdown item selected, plain asp .net ajax that only reload partially and using CascadingDropDown Ajaxcontroltoolkit that also use ajax process.
The benefit of using CascadingDropDown Ajaxcontroltoolkit are increasing productivity due to a lot of design work has been done in its functionality and also the web page doesnt have to reload a whole page since its process is using ajax.
CascadingDropDown AjaxControlToolkit Description
<ajaxToolkit:CascadingDropDown ID=”CascadingDropDown1″ runat=”server” TargetControlID=”DropDownList1″
Category=”category” PromptText=”….” LoadingText=”[Loading …]”