Iterator on non-existing structure

[PascalDragon]

Well the fix would actually be quite trivial:

Code: Pascal  [Select][+][-]

1. function TFPGListEnumerator.MoveNext: Boolean;
2. begin
3.   if not assigned(FList) then
4.     Exit(False);
5.   inc(FPosition);
6.   Result := FPosition < FList.Count;
7. end;  

This would not help if the list itself from which the operator is queried is Nil which is what the question is about. The only solution - as Martin_fr had written - would be to check Self in TFPGList.GetEnumerator.

[Warfley]

Why shouldn't it work? The for in loop calls GetEnumerator, with self being nil. GetEnumerator then calls:

Code: Pascal  [Select][+][-]

1. Result := TFPGListEnumeratorSpec.Create(Self);

Just passing self (nil) over to the TFPGListEnumerator<T>.Create, which does not touch FList until the first call to the MoveNext. The call to MoveNext then would immediately return false, causing Current to never be queried and thereby never try to access the list.

Also GetEnumerator is not virtual, so it's completely fine to call it with nil

[PascalDragon]

Why shouldn't it work? The for in loop calls GetEnumerator, with self being nil. GetEnumerator then calls:

Gotta admit that I hadn't looked at how the TFPGList.GetEnumerator method is implemented. Nevertheless it's bad style anyway, because someone changing that method to access Self will result in it blowing up again. Explicitly checking for Self should only be done in really constrained circumstances, like TObject.Free.