Not sure what you mean by "validate attachment control by checking other form control/s", however, you can create a variable and use the count function to enumerate how many attachments there are. Then use the variable in the rule for something.

So the integer variable (int Attachment Count) definition could be: count([Form].[Attachments])

Your rule can then reference the variable (int Attachment Count) is greater than zero then do whatever.

I hope this helps you get started

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Thanks for your reply. I want to make sure that either a link is provided(a simple text box inform) or attachment is added( attachment control). I wanted to create a rule that if link field is empty and attachment is alos not attached then show error message.

Its Nintex responsve list item form  not task form in workflow. I was not able to follow steps you mentioned, please let me know if i am missing something.

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@v-tmasenko - is there a way to validate a specific attachment control? E.g. My form has two attachment controls: the default one which is shown at all times, and a second one which is shown conditionally; when that second one is shown, it also needs to be validated. Thanks. 

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Yes this works. Thank you.

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