Using GetModifiedSel method
FileDocumentLib.FileDocument fdoc = new FileDocumentLib.FileDocument();
fdoc.Open(@"c:\temp\file.bin");
fdoc.FillByte(0x35, 100, 20);
fdoc.FillByte(0x36, 200, 30);
FileDocumentLib.IMultiSelection msel = fdoc.CreateEmptySelection();
fdoc.GetModifiedSel(msel);
// msel is now: [100..120) U [200..230)