https://supportcenter.devexpress.com/ticket/details/t633432/how-to-check-and-prevent-filtercontrol-filterstring-empty-criteria-value
How to Check and Prevent FilterControl FilterString Empty Criteria Value | DevExpress Support
May 10, 2018 - Hello, I would like to validate if there is any empty value specified by the end user in FilterControl. see this filterstring for instance:
how to check