DropDownList always does full-page postback, the first time it fires

I have an ASP.NET page using the AJAX library. There is a dropdownlist inside an UpdatePanel, which on index changing is supposed to update a different UpdatePanel to modify a grid control.

But after the page first comes up, and you adjust the ddl, the entire page postbacks, followed by a (correct) partial postback. After this one time, every other usage of the ddl performs correctly, trigger partial postbacks.

Both the panel and the ddl are being added from code. The UP is Conditional updates and ChildrenAsTriggers = true. The dropdownlist i开发者_运维知识库s AutoPostBack true and has an event set up on SelectedIndexChanged.

So what is going on? I've tried adjusting every setting I can think of and still the page completely refreshes once, then works fine after that.

I believe I've solved my current woes in regards to this problem, though I'm left feeling a bit dumb by the outcome. When programmatically adding the dropdownlist, I didn't give it an ID, assuming ASP.NET would just assign it some sequential name-container mangled ID anyway. It seems this was the cause. I'm assuming that the initial postback was due to ASP.NET trying to deal with the lack of a proper ID, assigning one, and then using that afterwards for the partial postbacks.

Or maybe it's something totally different at root, but the simple fix was: add an ID to the control.