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// Author: Simon Grossenbacher (webmaster@swissdelphicenter.ch)
// Web: http://www.swissdelphicenter.ch

uses 
  ShlObj, ComObj; 

function ChangeWallpaper(aFile: String): Boolean; 
const 
  CLSID_ActiveDesktop: TGUID = '{75048700-EF1F-11D0-9888-006097DEACF9}'; 
var 
  hObj        : IUnknown; 
  ADesktop : IActiveDesktop; 
  str           : String; 
  wstr         : PWideChar; 
begin 
  hObj        := CreateComObject(CLSID_ActiveDesktop); 
  ADesktop := hObj as IActiveDesktop; 
  wstr         := AllocMem(MAX_PATH); 
  try 
    StringToWideChar(aFile, wstr, MAX_PATH); 
    ADesktop.SetWallpaper(wstr, 0); 
    ADesktop.ApplyChanges(AD_APPLY_ALL or AD_APPLY_FORCE); 
  finally 
    FreeMem(wstr); 
  end; 
end;