function GetBMPSize(const FileName: string): Types.TSize;
type
  // Union of two different versions of windows bitmap info records
  TBitmapInfo = packed record
    case Byte of
      0: (Core: Windows.TBitmapCoreHeader); // early version
      1: (Ext: Windows.TBitmapInfoHeader);  // later version
  end;
const
  cSignature = $4D42; // bitmap file signature
var
  FileHeader: Windows.TBitmapFileHeader;  // bmp file header record
  BmpInfo: TBitmapInfo;                   // bmp info record union
  FS: Classes.TFileStream;                // stream onto bmp file
  BytesRead: Integer;                     // bytes read from stream
begin
  Result.cx := 0;
  Result.cy := 0;
  if (FileName = '') or not SysUtils.FileExists(FileName) then
    Exit;
  FS := Classes.TFileStream.Create(
    FileName, SysUtils.fmOpenRead or SysUtils.fmShareDenyNone
  );
  try
    // Read file header and check signature
    BytesRead := FS.Read(FileHeader, SizeOf(FileHeader));
    if BytesRead <> SizeOf(FileHeader) then
      Exit;
    if FileHeader.bfType <> cSignature then
      Exit;
    // Read bitmap info record (varies according to bmp type)
    BytesRead := FS.Read(BmpInfo, SizeOf(BmpInfo));
    if BytesRead < SizeOf(DWORD) then
      Exit; // couldn't read length field
    if BytesRead < Integer(BmpInfo.Ext.biSize) then
      Exit; // couldn't read whole info header record
    if BmpInfo.Ext.biSize = SizeOf(BmpInfo.Ext) then
    begin
      // later version
      Result.cx := BmpInfo.Ext.biWidth;
      Result.cy := BmpInfo.Ext.biHeight;
    end
    else
    begin
      // early version
      Result.cx := BmpInfo.Core.bcWidth;
      Result.cy := BmpInfo.Core.bcHeight;
    end;
  finally
    FS.Free;
  end;
end;
//delphi/2245