RectTransform.rect.Set doesn't work!

I tried to set the position of the buttons automatically at startup. In the documentation, as well as on the Internet, I was prompted to use something like this:

 public float MaxButtonWidth = 200.0f;
 public float MaxButtonHeight = 32.0f;
 public float PanelButtonsYPosition = -4.0f;
 public Button[] ButtonsArray;

 void Start()
 {
     int index;
     for(index = 0; index<ButtonsArray.Length; index++)
     {
         ButtonsArray[index] = ButtonsArray[index].GetComponent<Button>();
         RectTransform MyRectTransform = ButtonsArray[index].GetComponent<RectTransform>();
         MyRectTransform.rect.Set(
             index*MaxButtonWidth+4.0f, PanelButtonsYPosition,
             MaxButtonWidth, MaxButtonHeight
             );
         MyRectTransform.pivot.Set(0, 0);
     }
 }

(i also tried set the pivot position too)

But it doesn't work. I even tried to use this code:

         MyRectTransform.rect = Rect(
             index*MaxButtonWidth+4.0f, PanelButtonsYPosition,
             MaxButtonWidth, MaxButtonHeight
             );

But this does not work either. What did I do wrong?