Hack #1: Write access to a read-only property

The other day I was faced with the task of making our main application behave better systems with different screen resolutions (or rather pixel density, as in pixels per inch). This is the classic Large Font/Small Font problem, and getting forms and dialogs to scale properly to show readable fonts and text on all displays. There are several things to keep in mind, some of them are covered here - there are more complications due to MDI windows and Form inheritance.

To make my testing easier (and possibly to let the end-user override the default scaling behavior) I decided to let the current screen density (as determined by Screen.PixelsPerInch) be controlled from a setting in the Registry. The built-in Delphi form scaling works reasonably well, and relies on the fact that the form's design-time PixelsPerInch value is different form the run-time Screen.PixelsPerInch value. Now, PixelsPerInch is a read-only, public property of the singleton TScreen class. It is initialized in the TScreen constructor to the number of vertical pixels per inch as returned by the current graphics driver:

  DC := GetDC(0); 
  FPixelsPerInch := GetDeviceCaps(DC, LOGPIXELSY);

For my testing purposes, I wanted to set the value of the PixelsPerInch property without going to the hassle of actually changing my system setup, but to do that I would somehow have to modify the value of the read-only property. Impossible, right? Well, in software, nothing is really impossible. Software is soft, so we can change it :-). Changing the declaration of TScreen to make the property writeable would work, but as Allen has pointed out, making changes in the interface section of RTL and VCL units can have cascading effects, that are often undesirable. Besides, that would not really qualify as a bona-fide hack - it would have been too easy. Nah, lets do something a little more fun ;P. PixelsPerInch is only a public property, so there is no RTTI for it. Lets declare a descendent class, that promotes the property to published:

type 
  TScreenEx = class(TScreen) 
  published 
    property PixelsPerInch; 
  end;

Now, since TScreen indirectly inherits from TPersistent, and TPersistent was compiled in the $M+ mode, published properties in our TScreenEx class will have RTTI generated for them. But PixelsPerInch is still a read-only property - and there is no way our TScreenEx can make it writeable, because the backing field FPixelsPerInch is private, not protected, and so is off-limits for us.

The cunning thing about the RTTI generated for the TScreenEx.PixelsPerInch property, is that it includes enough information about where to find the backing field in the object instance. Open TypInfo.pas and locate the TPropInfo record that describes the RTTI for each property. Included is the GetProc pointer. For backing fields, this contains the offset off the field in the object instance (sans some flag bits). After decoding this offset and adding it to the base address of the object instance, we now can get a pointer to the backing field and thus modify it - voila write-access! Here is the short version:

procedure SetPixelsPerInch(Value: integer); 
begin 
  PInteger(Integer(Screen) + (Integer(GetPropInfo(TScreenEx, 'PixelsPerInch').GetProc) and $00FFFFFF))^ := Value; 
end;

Decoding that is left as an exercisecise for the reader.