<p><a href="https://stackoverflow.com/questions/205887/postback-security">Related Article</a></p>

<p>On a similar topic to the above article, but of a more specific note. How exactly do you handle items that are in the viewstate (so they are included on submit), but can also be changed via AJAX. For instance, say we had a dropdown list that was populated through an AJAX web service call (not an update panel). How can I get the page to validate once the dropdownlist's items have been changed?</p>

why not validating onChange even in the dropdownlist?
just add the script manager and add that property to the onchange in the Page_Load event
```
' Creating the javascript function to validate
Dim js As String
js = "function validateDDL1(ddl) { alert(ddl.value); }"

' Adding onChange javascript method
DropDownList1.Attributes.Add("onchange", "validateDDL1(this);")

' Registering the javascript
ScriptManager.RegisterClientScriptBlock(Me, GetType(String), "validateDDL1(ddl)", js, True)
```