Perché non con ... do istruzione e il lavoro rettangolo insieme senza problemi?

Question

Ultimamente, sono stato meno di notare che ogni volta che uso variabile rettangolo con Con ... fare dichiarazione, non funziona affatto per qualche ragione.

Per esempio:

var bounds:=new Rectangle(0,0,0,0);

with bounds do
begin
  X:=1;
  Y:=2;
  Width:=33;
  Height:=44;
end;

I valori bounds' rimangono zeri non che cosa è in con la dichiarazione. Tuttavia, se faccio la seguente, funziona benissimo.

var bounds:=new Rectangle(0,0,0,0);

bounds.X:=1;
bounds.Y:=2;
bounds.Width:=33;
bounds.Height:=44;

C'è qualche motivo per cui lo farebbe.

Answer 1

What Hans Passant is trying to imply is that the "with" statement creates a copy of bounds, works on it, then throws it away. I don't have enough information to verify that, but I feel that it's unlikely - Delphi's assignment operator works by reference, so implicit shallow copies don't actually happen that often.

However, "with" statements intentionally create a special kind of variable scoping hell. You could be grabbing a field inside bounds, or you could be grabbing a field from the containing method, or you could even be grabbing a field from a previous unclosed "with" statement. Automatic refactoring can't touch a with statement. Adding a field to the class the with statement operates on can break your method.

Consider

with myLongNamedComponent.anotherLongNamedChild.pedanticRectangle do
begin
    x:=1;
    y:=2;
    width:=33;
    height:=44;
end;

This is actually better written as

var bounds := new Rectangle(0,0,0,0);
bounds.x := 1;
bounds.y := 2;
bounds.width := 33;
bounds.height := 44;
myLongNamedComponent.anotherLongNamedChild.pedanticRectangle := bounds;

TL:DR; the "with" statement is no longer considered good coding practice.