Question

Microsoft uses dialog length units (DLU) in their guidelines for UI. How can I convert them into pixels?

As I know, DLU depending on system font size. Can you advise some simple method of such conversion in Delphi for Win32?

Answer 1

You should use the MapDialogRect() function.

Pass in a RECT in dialog units, and the equivalent RECT in pixel units is returned. Note that you need a handle to a dialog in order to give MapDialogRect() sufficient context. The function needs to know the font in order to perform the conversion.

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In case you are tempted to use GetDialogBaseUnits(), remember what Raymond Chen said, GetDialogBaseUnits is a crock.

As you can guess from the title of this entry, GetDialogBaseUnits is a
crock. Since there is no HWND parameter to GetDialogBaseUnits, it
doesn’t know which dialog box’s DLUs you want to retrieve. So it
guesses.

And it always guesses wrong.

GetDialogBaseUnits returns the dialog base units for dialog boxes that
use the default system font. But nobody uses the default system font
any more. It screams "old and dorky". But it remains the default for
compatibility reasons. (And therefore so too does GetDialogBaseUnits.)

If you have to calculate pixel dimensions from DLUs, and you don’t have a handle to a dialog, then you must use the method outlined here: How To Calculate Dialog Base Units with Non-System-Based Font

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However, you made it clear in the comments that, for your problem, you do not actually need to convert from DLUs to pixels. You can use Delphi’s built in form scaling to ensure that your forms are sized appropriately for the prevailing font scaling.