Attachment Validation Not working

Summary:

 I am written attachment validation.

Like user should not allow to delete attachment in custom object.

below is my code

def allattach = Attachment

while(allattach.hasNext())

{

def attachRow = allattach.next()

def docId = attachRow.AttachedDocumentId

if(docId==null)

{

return false

}

}

but it is not working as expect

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