Dialog Box Measurements

Every dialog box template contains measurements that specify the position, width, and height of the dialog box and the controls it contains. These measurements are device independent, so an application can use a single template to create the same dialog box for all types of display devices. This ensures that a dialog box will have the same proportions and appearance on all screens despite differing resolutions and aspect ratios between screens.

Dialog box measurements are given in dialog base units. One horizontal unit is equal to one-fourth of the average character width for the system font. One vertical unit is equal to one-eighth of the average character height for the system font. An application can retrieve the number of pixels per base unit for the current display by using the GetDialogBaseUnits function. An application can convert measurements from dialog base units to pixels by using the MapDialogRect function.

The template must specify the initial coordinates of the upper left corner of the dialog box. Usually the coordinates are relative to the upper left corner of the owner window's client area. When the template specifies the DS_ABSALIGN style or the dialog box has no owner, the position is relative to the upper left corner of the screen. Windows sets this initial position when creating the dialog box, but permits an application to adjust the position before displaying the dialog box. For example, an application can retrieve the dimensions of the owner window, calculate a new position that centers the dialog box in the owner window, and then set the position by using the SetWindowPos function.

The template should specify a dialog box width and height that does not exceed the width and height of the screen and ensures that all controls are within the client area of the dialog box. Although Windows permits a dialog box to be any size, creating one that is too small or too large can prevent the user from providing input, defeating the purpose of the dialog box. Many applications use more than one dialog box when there are a large number of controls. In such cases, the initial dialog box usually contains one or more buttons that the user can choose to display the next dialog box.

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