Validate Infopath form by another sharepoint list

Question

I have 1 Requirement list and 1 Tasks list with infopath 2010 forms. Tasks list having Lookup column as of Requirement Title field. Both lists having Status field. When user save Requirement (infopath form) with Status=Completed i want to check whether there is any corresponding tasks exist in tasks list with Status != Completed. if so i want to show a message (or form validation) to user saying "Corresponding Task is open for this requirement. Please complete the open task in order to complete requirement."

Is it possible?

Answer 1

Concerning comment " you can do it using custom code":

• 1)Custom code is impossible for Infopath sharepoint list form;

  The question is quite evasive and ambiguous on whether the Infopath form in question is of tpe/compatibility Sharepoint List Form...

• 2)There is no need in coding for this;

What is asked is unclear - what is the type/compatibility of Iinfopath form and which version of Infopath?
Why do you need messagebox but not validation rules or conditional formatting to show and hide a section ?