Function request: Class contracts - require on class level


(Theo) #1

On method level you can check the prerequisites before the method is executed (requires) and the results after the method has executed (ensure).

On Class level, you can only check the results after a method has executed (invariants).
It would be nice to have the possibility to check the prerequisites on class level too (private / public requires).

Example usage scenarios:

  • check if the object has been disposed before executing any method.
  • check if the object is ready to respond to any action (b.e. state not correct because of still executing async code)

(marc hoffman) #2

you can use invariants for that?


(Theo) #3

No, because invariants are executed after a method is executed instead of before.


(marc hoffman) #4

right. but invariants are enforces to always be true. if they were true at the end of one call, they’ll be true before the next one, too?


(Theo) #5

I am not sure if I understand you right …

That is correct.

That is not the case when async code is still running.

A simple example:

  1. call method1 (normal method) - everything is ok
  2. call async method2 - long running method, that sets a flag halfway the execution - as soon as this flag is set, the object is not in a state that it can execute any other method, until this flag has been reset.
  3. call method3 (normal method) while method2 is still running.

With invariants, the flag is only checked when method3 finishes, not before it starts. If the flag has been reset when method3 finishes, there won’t be any assertion. The check should have been done before method3 executes.


(Theo) #6

And with the case of Dispose; the dispose method sets the IsDisposed flag. When I check on it with an invariant, it means that the dispose method itself will fail instead of any method executed after Dispose.


(SH) #7

Good Point actually :face_with_monocle: