Closing modal dialogs

I come across modal dialogs everyday and I see different implementations all the time.

Most modal dialogs have a [X] in the upper right hand corner of the dialog along with a Cancel/Close button.

There are so many options, what is the best practice for closing modal dialogs? Also, are there any studies to reference showing what is preferred?

Implementations I have seen:

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  Cancel/Close button with [X] in the corner(top right/left, bottom right/left?)

  
  


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  Click anywhere on the modal to close

  
  


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  Just an [X] in the corner(top right/left, bottom right/left?)

  
  
  


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  Just a Cancel/Close button

  
  


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  Click anywhere outside the modal

  
  


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  Esc key with or without [X]/Cancel/Close

  
  

Answer

I think the best implementation depends on the type of dialog and desired interaction mode.

Dialogs that require an action shouldn't have an [X] to close them, in my opinion (nor be closed by clicking anywhere other than the action buttons.
Most of those have a "Save" and "Cancel" set of buttons, so it leaves the users thinking (even if for a split second) if "close" means "cancel" or not.

Information dialogs should have both a "close" button and a [X], in my opinion.
If the information is something the user might want to interact with (for example text to be selected and copied), then of course you cannot allow a click inside to close the dialog.
If you want the user to pay attention to the information, I would also disallow the clicking outside to dismiss, since it increases the chance the user has to focus on the dialog to close it (and hopefully read something there).