Why should I use Free and not FreeAndNil in a destructor?

Question

I have read A case against FreeAndNil but still don't understand why I cannot use this method in a class destructor ? Can anyone explain.

Update: I think the comment from Eric Grange was most useful for me. The link show that this is not obvious how to deal with it and it is mainly a matter of taste. Also the method FreeAndInvalidate was useful.

Answer 1

The problem with that is that many seem to use FreeAndNil as some magic bullet that will slay that mysterious crash dragon. If using FreeAndNil() in the destructor seems to solve a crash or other memory corruption problems, then you should be digging deeper into the real cause. When I see this, the first question I ask is, why is the instance field being accessed after that instance was destroyed? That typically points to a design problem.

It argues that it hides the real problem you have. It must mean your code is accessing properties/fields/methods of an object that is already destroyed (the destructor is called). So instead of hiding the real problem with FreeAndNil you should really be solving the underlying problem.

This code below would not crash if you would use FreeAndNil PropertyA in the destructor of SomeObject. But it hides the real problem that SomeObject is used after it is destroyed. It is better to solve this design problem (accessing destroyed objects) instead of hiding it.

SomeObject.Free;  // Destructor called
if Assigned(SomeObject.PropertyA) then  // SomeObject is destroyed, but still used
  SomeObject.PropertyA.Method1;  

EDIT

On the other case, one could argue that if FreeAndNil is not used, the code would not crash either. Since even though the object is destroyed, the memory might not be reused and all structures might be in tact. The code above might even run without problems if Free is used to destroy PropertyA instead of FreeAndNil.

And if FreeAndNil was used to destroy SomeObject, you would also see the real problem no matter what the code in the destructor is.

So although I agree with the argument that it could hide the real design flaw and personally do not use FreeAndNil in destructors, it is not some magic bullet to discover such design flaws.