Procedure ConCatFiles(Const targetname: String;
  Const Sourcenames: Array of String);
  Var
  i: Integer;
  target, source: TFileStream;
  Begin
  target := TFileStream.Create( targetname, fmCreate );
  try
  For i:= Low(Sourcenames) To High(Sourcenames) Do Begin
  source := TFileStream.Create( Sourcenames[i],
  fmOpenread or fmShareDenyNone );
  try
  target.Copyfrom( source, 0 );
  finally
  source.free;
  end
  End;
  finally
  target.Free;
  end;
  End;

chDir(ExtractFileDir(Application.Exename));
 ConcatFiles('sum.txt', ['project1.dpr','unit1.pas','unit2.pas']);