Delphi dll image stretchdraw errors

Question

I am trying to resize (scale) a bitmap image using a dll function which is below mentioned

{ to resize the image }
function ResizeImg(maxWidth,maxHeight: integer;thumbnail : TBitmap): TBitmap;
var
 thumbRect : TRect;
begin
 thumbRect.Left := 0;
 thumbRect.Top := 0;

 if thumbnail.Width > maxWidth then
  begin
   thumbRect.Right := maxWidth;
  end
 else
  begin
    thumbRect.Right := thumbnail.Width;;
  end;

 if thumbnail.Height > maxHeight then
  begin
   thumbRect.Bottom := maxHeight;
  end
 else
  begin
   thumbRect.Bottom := thumbnail.Height;
  end;
 thumbnail.Canvas.StretchDraw(thumbRect, thumbnail) ;

  //resize image
 thumbnail.Width := thumbRect.Right;
 thumbnail.Height := thumbRect.Bottom;

 //display in a TImage control
 Result:= thumbnail;
end;

It works fine when I use this application call (to feed all the images in my listview):

  //bs:TStream; btmap:TBitmap;
  bs := CreateBlobstream(fieldbyname('Picture'),bmRead);
  bs.postion := 0;
  btmap.Loadfromstream(bs);
  ListView1.Items[i].ImageIndex := ImageList1.Add(ResizeImg(60,55,btmap), nil);

But when I try this application call (to get individual image into my TImage component):

 bs := CreateBlobstream(fieldbyname('Picture'),bmRead);
 bs.postion := 0;
 btmap.Loadfromstream(bs);
 Image1.Picture.Bitmap := ResizeImg(250,190,btmap);

It gives me an error on:

 thumbnail.Canvas.StretchDraw(thumbRect, thumbnail) ;

saying:

 AV at address 00350422 in module 'mydll.dll' Read of Address 20000027

And when I close my executable I get this:

 runtime error 216 at 0101C4BA 

If I define and use the same function (ResizeImg) inside my exe pas file it works completely fine without any errors.

Answer 1

You can't pass Delphi objects between modules unless you take steps to ensure that those modules share the same runtime and memory allocator. Ir appears you have not taken such steps.

The basic problem is that a Delphi object is both data and code. If you naively call a method on an object that was created in a different module then you execute code from this module on data from that module. That typically ends in runtime errors.

You have at least the following options:

1. Use runtime packages. This will enforce a shared runtime.
2. Use COM interop. COM was designed for sharing components across module boundaries.
3. Link all code into a single executable.
4. Pass HBITMAPs between the modules since they can be shared in such a manner.