How do i undo the effect of IntersectClipRect?

Question

Given the following code snippet:

procedure TPicture.PaintLine(_Canvas: TCanvas; _Left, _Top, _Right, _Bottom: Integer);
begin
  IntersectClipRect(_Canvas.Handle, _Left, _Top, _Right, _Bottom);
  try
    _Canvas.MoveTo(_Left - 10, _Top - 10);
    _Canvas.LineTo(_Right + 10, _Bottom + 10);
    // (This is an example only, the actual drawing is much more complex.)
  finally
    SelectClipRgn(_Canvas.Handle, 0); // This does too much
  end;
end;

I want to undo the clipping effected by the call to IntersectClipRect so the previously active clipping becomes active again. In the above code, this is done by SelectClipRgn(...,0) which turns off clipping altogether. This works, kind of, but afterwards there is no clipping active so any drawing that is executed after the above will paint to areas that should not be painted to.

So, what is the correct way to undo only the effect of IntersectClipRect?

EDIT: Removed the unnecessary CreateRectRgn and DeleteObject code after I understood the comment from Sertac, to make the question more readable for others that might stumble upon it later.

Answer 1

IIRC, first store the current clip region using GetClipRgn, and after you're done, SelectClipRgn the stored region again.

Looking at your code, it should be enough to SelectClipRgnyour RGN again, because:

The IntersectClipRect function creates a new clipping region from the intersection of the current clipping region and the specified rectangle.

Answer 2

You can save and restore the state of the DC:

var
  //  RGN: HRGN;
  SavedDC: Integer;
begin
//  RGN := CreateRectRgn(_Left, _Top, _Right, _Bottom);
  SavedDC := SaveDC(_Canvas.Handle);
  try
    IntersectClipRect(_Canvas.Handle, _Left, _Top, _Right, _Bottom);
    _Canvas.MoveTo(_Left - 10, _Top - 10);
    _Canvas.LineTo(_Right + 10, _Bottom + 10);
    // (This is an example only, the actual drawing is much more complex.)
  finally
    RestoreDC(_Canvas.Handle, SavedDC);
  end;
  ...