I am corrected. The DLL is written in c#. It seems that Bitmap and Rectangle are common predefined/usable objects in C#.
Re Bitmap, I found these notes: "Surely you cannot expect C# Bitmap to be binary compatible with Delphi TBitmap? You need to pass around an HBITMAP. On the Delphi side you use ReleaseHandle to get the HBITMAP. On the C# side it would be Image.FromHbitmap"
Re Rectangle, I found nothing interesting :(
However it seems natural to expect that Rectangle it is given by a pointer to a fixed array of 2 points= 4 integers= 16 bytes in all.