Control Postback Only On Second Attempt After Validation Error

Recently, I encountered a problem whereby my DropDownList in UpdatePanel didn't AutoPostBack after a validator control return failure. The PostBack only happen after I tried to change the SelectedIndex the second time.

Reading from a blog, I learnt that resetting Page_BlockSubmit back to False in client side will resolve the problem.

What is Page_BlockSubmit?

When the user clicks on a button causing a full post back, after running Page_ClientValidate ASP.NET runs another built in function ValidatorCommonOnSubmit. Within Page_ClientValidate, Page_BlockSubmit is set based on the validation. The postback is then blocked in ValidatorCommonOnSubmit if Page_BlockSubmit is true. No matter what, at the end of the function Page_BlockSubmit is always reset back to false.

If a page does a partial postback without running any validation and Page_BlockSubmit has not been reset to false, the partial postback will be blocked. In essence the above function, RunValidation, acts similar to ValidatorCommonOnSubmit. It runs the validation and then returns false to block the postback if needed. Since the built in postback is never run, we need to reset Page_BlockSubmit manually before returning the validation result.