DropDownList AutoGetBack ??

When we want to retrieve some data from the site we must use HTTP Get method. My DropDownList must works like a data filter, so I need something like AutoGetBack, but DropDownList have only AutoPostBack.
If I set method="get" of the form, and AutoPostBack=true, then URL will be changed but server says IsPostBack=true. Why DropDownList dosn't have AutoGetBack ?? (with AutoGetBack I use PRG-pattern - unnecessary round trip)
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